Major Signaling Pathways

Cells make life-or-death decisions — grow, divide, differentiate, or die — based on signals they receive from their environment. In the previous lesson we surveyed the general logic of cell signaling and the three major classes of receptors. Here we go deeper into the specific pathways that carry out these decisions: the enzyme-linked receptor pathways (RTK-Ras-MAPK, PI3K-Akt, JAK-STAT, and TGF-β-Smad), the proteolysis-dependent pathways (Wnt, Hedgehog, Notch, and NF-κB), and the specialized signaling systems of plants. We also explore how bioinformatics tools classify kinases, predict phosphorylation sites, and map cancer mutations onto signaling networks.

15.3 — Signaling Through Enzyme-Linked Cell-Surface Receptors

Enzyme-linked receptors are transmembrane proteins whose intracellular domains either have intrinsic enzymatic activity or associate directly with cytoplasmic enzymes. The largest class is the receptor tyrosine kinases (RTKs), but this group also includes tyrosine-kinase-associated receptors, serine/threonine kinase receptors, and guanylyl cyclases.

Activated RTKs Recruit Signaling Proteins to the Plasma Membrane

Receptor tyrosine kinases are single-pass transmembrane proteins with an extracellular ligand-binding domain and an intracellular kinase domain. When a growth factor (EGF, PDGF, FGF, insulin, etc.) binds the extracellular domain, two receptor molecules dimerize. Dimerization brings the intracellular kinase domains into close proximity, and they trans-phosphorylate each other on specific tyrosine residues.

These phosphotyrosines serve as docking sites for intracellular signaling proteins that contain SH2 domains (Src homology 2) or PTB domains (phosphotyrosine-binding). Among the proteins recruited are:

• Grb2 — an adaptor protein that links the receptor to the Ras pathway
• PI 3-kinase — a lipid kinase that generates membrane-bound second messengers
• PLCγ — phospholipase C gamma, which produces IP₃ and DAG
• Src — a cytoplasmic tyrosine kinase that amplifies the signal

By recruiting different combinations of signaling proteins, a single RTK can activate multiple downstream pathways simultaneously, enabling a rich and nuanced cellular response.

The Ras-MAP Kinase Signaling Pathway

One of the most important pathways activated by RTKs is the Ras-MAP kinase (MAPK) cascade. The signaling logic proceeds as follows:

1. Activated RTK recruits Grb2, which binds the guanine nucleotide exchange factor SOS
2. SOS activates the small GTPase Ras by promoting exchange of GDP for GTP
3. Ras-GTP recruits and activates Raf (a MAP kinase kinase kinase, or MAPKKK)
4. Raf phosphorylates and activates MEK (a MAP kinase kinase, or MAPKK)
5. MEK phosphorylates and activates ERK (a MAP kinase, or MAPK)
6. ERK enters the nucleus and phosphorylates transcription factors (Elk-1, Fos, Myc), driving gene expression for cell growth and proliferation

Ras functions as a molecular switch: it is active when bound to GTP and inactive when bound to GDP. A GTPase-activating protein (GAP) accelerates GTP hydrolysis, turning Ras off. Mutations that lock Ras in the GTP-bound state produce a constitutively active oncoprotein — one of the most common alterations in human cancer.

The three-tier kinase cascade (MAPKKK → MAPKK → MAPK) is a recurring architectural motif in eukaryotic signaling. It provides signal amplification at each step, opportunities for regulation by phosphatases, and the ability to integrate inputs from multiple upstream activators.

The Ras protein is highly conserved across eukaryotes, from yeast to humans. Its sequence can be compared across species to identify conserved functional domains:

let egfr_kinase = "FKKIKVLGSGAFGTVYKGLWIPEG"
let braf_kinase = "IGDFGLATVKSRWSGSHQFEQLSG"
print("EGFR kinase domain properties:")
print(Struct.protein_props(egfr_kinase))
print("BRAF kinase domain properties:")
print(Struct.protein_props(braf_kinase))

The near-perfect conservation of Ras across mammals underscores the critical importance of every residue in this GTPase switch. Even single amino acid changes — such as the G12V or G12D mutations — can abolish GTPase activity and drive oncogenesis.

PI 3-Kinase Produces Lipid Docking Sites in the Plasma Membrane

A second major branch of RTK signaling involves phosphoinositide 3-kinase (PI 3-kinase, PI3K). When recruited to an activated RTK, PI3K phosphorylates the membrane lipid PIP₂ (phosphatidylinositol 4,5-bisphosphate) to produce PIP₃ (phosphatidylinositol 3,4,5-trisphosphate).

PIP₃ acts as a docking site on the inner leaflet of the plasma membrane, recruiting proteins that contain a PH domain (pleckstrin homology domain). The two most important are:

• Akt (also called protein kinase B, PKB)
• PDK1 (phosphoinositide-dependent kinase 1)

The lipid phosphatase PTEN removes the 3-phosphate from PIP₃, converting it back to PIP₂ and terminating the signal. Loss of PTEN is one of the most frequent events in human cancer.

The PI 3-Kinase–Akt Signaling Pathway

Once recruited to the membrane by PIP₃, Akt is phosphorylated and activated by PDK1 and by mTORC2. Active Akt phosphorylates a wide array of target proteins to promote:

• Cell survival: Akt phosphorylates and inactivates the pro-apoptotic protein Bad and the Forkhead (FOXO) family of transcription factors
• Cell growth: Akt activates mTORC1 (via inhibition of TSC1/TSC2), which stimulates protein synthesis and ribosome biogenesis
• Glucose metabolism: Akt promotes glucose uptake by stimulating GLUT4 transporter translocation to the membrane
• Cell proliferation: Akt phosphorylates and stabilizes cyclin D and Myc

The PI3K-Akt pathway is thus a master regulator of cell survival and metabolism.

RTK-Ras-MAPK and PI3K-Akt Pathways Are Important in Cancer

The RTK-Ras-MAPK and PI3K-Akt pathways are among the most frequently mutated in human cancer:

The fact that so many cancer-driving mutations cluster in these two pathways highlights their central role in controlling cell growth and survival. Targeted therapies such as EGFR inhibitors (erlotinib, osimertinib), BRAF inhibitors (vemurafenib), MEK inhibitors (trametinib), and PI3K inhibitors (alpelisib) have been developed to treat cancers with specific mutations in these pathways.

Tyrosine-Kinase-Associated Receptors Depend on Cytoplasmic Tyrosine Kinases

Not all enzyme-linked receptors have intrinsic kinase activity. Tyrosine-kinase-associated receptors lack an intracellular catalytic domain and instead associate with cytoplasmic tyrosine kinases that phosphorylate downstream targets.

The best-characterized examples are the cytokine receptors, which bind interleukins, interferons, erythropoietin, and other cytokines critical for immune function and blood cell development. Cytokine receptors associate with Janus kinases (JAKs) — a family of four cytoplasmic tyrosine kinases (JAK1, JAK2, JAK3, and TYK2).

Cytokine Receptors Activate the JAK-STAT Signaling Pathway

The JAK-STAT pathway is one of the most direct routes from receptor to gene expression:

1. Cytokine binding induces receptor dimerization
2. Associated JAKs cross-phosphorylate each other and then phosphorylate tyrosine residues on the receptor
3. STAT (Signal Transducer and Activator of Transcription) proteins dock on the phosphorylated receptor via their SH2 domains
4. JAKs phosphorylate the STATs, which then dimerize and translocate to the nucleus
5. STAT dimers bind DNA and activate transcription of target genes

There are seven mammalian STATs (STAT1–4, 5a, 5b, 6), each activated by different cytokines and controlling different gene programs. The JAK-STAT pathway is notable for its speed and directness: there are no intermediate kinase cascades between the receptor and the transcription factor.

Dysregulation of JAK-STAT signaling causes disease: the JAK2 V617F mutation is found in the majority of patients with polycythemia vera (a myeloproliferative neoplasm), and JAK inhibitors such as ruxolitinib are used therapeutically.

Protein Tyrosine Phosphatases Reverse Tyrosine Phosphorylations

Every phosphorylation event must be reversible for signaling to be dynamic. Protein tyrosine phosphatases (PTPs) remove phosphate groups from tyrosine residues, counteracting the kinases. The human genome encodes approximately 107 PTPs, and they are essential for turning off signaling pathways and maintaining homeostasis. Loss of the phosphatase SHP-2 or PTEN can lead to uncontrolled signaling and cancer.

Serine/Threonine Kinase Receptors Activate Smads

The TGF-β superfamily (TGF-β, BMPs, activins) signals through receptor serine/threonine kinases. The receptor complex consists of two type I and two type II subunits. Ligand binding activates the type II receptor, which phosphorylates the type I receptor. The activated type I receptor then phosphorylates receptor-regulated Smads (R-Smads) — Smad2 and Smad3 for TGF-β/activin signaling, and Smad1, Smad5, and Smad8 for BMP signaling.

Phosphorylated R-Smads bind the co-Smad (Smad4) and the resulting complex translocates to the nucleus, where it activates or represses target genes. Inhibitory Smads (Smad6, Smad7) provide negative feedback by competing with R-Smads for receptor binding.

TGF-β signaling has dual roles in cancer: it acts as a tumor suppressor in normal epithelial cells (inducing growth arrest and apoptosis) but can become a tumor promoter in advanced cancers (driving invasion, metastasis, and immune evasion).

Some Receptors Activate a Ubiquitin Ligase

Some enzyme-linked receptors signal by activating ubiquitin ligases rather than kinases. For example, the TGF-β receptor complex can recruit the ubiquitin ligase Smurf, leading to ubiquitination and degradation of signaling components. This mechanism provides a way to regulate protein levels — and thus signal strength — through the ubiquitin-proteasome pathway.

Some Enzyme-Linked Receptors Are Guanylyl Cyclases

A distinct class of enzyme-linked receptors are guanylyl cyclases that produce the second messenger cyclic GMP (cGMP) directly. The best-known example is the receptor for atrial natriuretic peptide (ANP), which regulates blood volume and pressure. When ANP binds, the receptor’s intracellular guanylyl cyclase domain converts GTP to cGMP, which activates protein kinase G (PKG) and leads to smooth muscle relaxation and vasodilation.

Bioinformatics: Kinase and Phosphorylation Analysis

With over 500 kinases in the human genome — collectively known as the kinome — computational tools are essential for classifying kinases, predicting phosphorylation events, and interpreting cancer mutations.

Kinase Family Classification

The Manning kinome tree organizes the ~518 human protein kinases into a hierarchical classification of groups, families, and subfamilies. The KinBase database catalogs all known kinases and their domain architectures, providing standardized nomenclature. The major groups include:

Phosphorylation Site Prediction

Several tools predict which residues in a protein are likely to be phosphorylated:

• NetPhos — uses neural networks trained on known phosphorylation sites to predict serine, threonine, and tyrosine phosphorylation
• GPS (Group-based Prediction System) — predicts kinase-specific phosphorylation sites based on sequence motifs
• PhosphoPredict — integrates multiple features (sequence, structure, evolutionary conservation) for phosphorylation site prediction

These predictions are validated against experimental databases such as PhosphoSitePlus, which curates hundreds of thousands of experimentally verified phosphorylation sites.

Kinase-Substrate Enrichment Analysis (KSEA)

KSEA is a computational method that infers kinase activity from phosphoproteomics data. Given a set of phosphopeptides whose abundance changes between conditions (e.g., drug-treated vs. control), KSEA tests whether substrates of a particular kinase are enriched among the changing phosphopeptides. This allows researchers to identify which kinases are activated or inhibited without measuring kinase activity directly.

Cancer Mutation Mapping to Kinase Domains (OncoKB)

Databases like OncoKB annotate the functional significance of cancer mutations in kinase genes, classifying them as oncogenic, likely oncogenic, or of unknown significance. By mapping mutations onto the three-dimensional structure of kinase domains, researchers can understand how specific alterations — such as BRAF V600E or EGFR L858R — disrupt the autoinhibited conformation and constitutively activate catalytic activity.

Kinase Inhibitor Selectivity Profiling (KINOMEscan)

The KINOMEscan assay measures the binding affinity of a kinase inhibitor against a panel of hundreds of kinases. The resulting selectivity score (S-score) indicates what fraction of the kinome a drug binds. A highly selective inhibitor binds only its intended target, while a promiscuous inhibitor may hit dozens of kinases, explaining both therapeutic effects and side effects.

Kinase domains share a conserved catalytic fold. Let us compare the kinase domain sequences of two key RTKs to see the structural conservation:

let imatinib = "Cc1ccc(NC(=O)c2ccc(CN3CCN(C)CC3)cc2)cc1Nc1nccc(-c2cccnc2)n1"
let gefitinib = "COc1cc2ncnc(Nc3ccc(F)c(Cl)c3)c2cc1OCCCN1CCOCC1"
let erlotinib = "C#Cc1cccc(Nc2ncnc3cc(OCCOC)c(OCCOC)cc23)c1"
print("Imatinib (BCR-ABL inhibitor):")
print(Chem.properties(imatinib))
print("Gefitinib (EGFR inhibitor):")
print(Chem.properties(gefitinib))
let sim = Chem.tanimoto(imatinib, gefitinib)
print("Imatinib vs Gefitinib similarity: " + sim)
let sim2 = Chem.tanimoto(gefitinib, erlotinib)
print("Gefitinib vs Erlotinib similarity: " + sim2)

The conserved GXGXXG motif (the phosphate-binding loop, or P-loop) and the catalytic lysine are visible in the alignment, illustrating why many kinase inhibitors can cross-react with multiple family members.

Exercise: Kinase Domain Analysis

RTKs share a conserved kinase domain but different substrate specificities. Compare the activation loop sequences of EGFR and insulin receptor to understand how minor differences lead to different substrates:

let egfr_loop = "FKKIKVLGSGAFGTVYKGLWIPEG"
let insr_loop = "DIYETDYYRKGGKGLLPVRWMAPE"
print("EGFR activation loop:")
print(Struct.protein_props(egfr_loop))
print("Insulin receptor activation loop:")
print(Struct.protein_props(insr_loop))
// Which kinase is more hydrophobic in its activation region?
let answer = "EGFR"
print(answer)

GC content in kinase-coding regions can vary substantially and influences codon usage, mRNA stability, and translational efficiency — factors relevant when designing synthetic constructs for kinase expression studies.

15.4 — Signaling Pathways That Depend on Regulated Proteolysis

Not all signaling pathways rely on phosphorylation cascades. Several critical developmental pathways transmit signals by controlling the proteolytic destruction of key regulatory proteins. In the absence of signal, these regulators are continuously degraded; signaling blocks their degradation and allows them to accumulate and activate target genes.

The Wnt/β-Catenin Pathway

The Wnt pathway is essential for embryonic development, stem cell maintenance, and tissue homeostasis. Its core logic revolves around the stability of β-catenin:

Without Wnt signal: A destruction complex containing APC (adenomatous polyposis coli), Axin, GSK-3β, and CK1 continuously phosphorylates β-catenin. Phosphorylated β-catenin is recognized by the ubiquitin ligase β-TrCP, ubiquitinated, and degraded by the proteasome. The transcription factor TCF/LEF is bound to co-repressors and target genes remain silent.

With Wnt signal: Wnt binds the receptor Frizzled and the co-receptor LRP5/6. This recruits the scaffold protein Dishevelled, which inhibits the destruction complex. β-catenin accumulates, enters the nucleus, and binds TCF/LEF, converting it from a repressor to an activator of target genes (including MYC, Cyclin D1, and AXIN2).

Cancer connection: Mutations in APC are found in approximately 80% of colorectal cancers (the inherited form is called familial adenomatous polyposis). Loss of APC prevents β-catenin destruction, leading to constitutive activation of Wnt target genes that drive proliferation.

Hedgehog Proteins Act Through a Receptor That Directly Regulates a Transcription Factor

The Hedgehog (Hh) pathway controls cell fate during development and contributes to cancers when misregulated:

Without Hh signal: The receptor Patched (Ptch) inhibits the signal transducer Smoothened (Smo), keeping it inactive. The transcription factor Gli is proteolytically processed into a repressor form (Gli-R) that enters the nucleus and silences Hh target genes.

With Hh signal: Hedgehog ligand (Sonic Hedgehog, Indian Hedgehog, or Desert Hedgehog) binds Patched, relieving its inhibition of Smoothened. Activated Smo prevents Gli processing, allowing full-length Gli to act as a transcriptional activator (Gli-A) that drives expression of target genes involved in proliferation, patterning, and differentiation.

The Hedgehog pathway is notable because the receptor (Patched) is inhibitory — it keeps the pathway off. Signal removes the inhibitor rather than activating an effector. Mutations that inactivate Patched or constitutively activate Smoothened cause basal cell carcinoma, the most common human cancer. The drug vismodegib targets Smoothened and is used to treat advanced basal cell carcinoma.

The Notch Receptor Is a Latent Transcription Regulator

Notch signaling is unique in that the receptor itself becomes the transcription factor:

1. Notch is a single-pass transmembrane receptor
2. A ligand (Delta or Jagged) on an adjacent cell binds Notch
3. Ligand binding triggers two sequential proteolytic cleavages: first by an ADAM metalloprotease (extracellular), then by γ-secretase (within the membrane)
4. The Notch intracellular domain (NICD) is released and travels to the nucleus
5. NICD binds the transcription factor CSL (also called RBPJ), displaces co-repressors, and recruits the co-activator Mastermind (MAML) to activate target genes (e.g., HES and HEY)

Notch signaling is a direct cell-contact signaling system: no second messengers or kinase cascades are involved. It is critical for lateral inhibition during development (e.g., neuronal vs. epidermal fate decisions) and for maintaining stem cell populations in adult tissues. Activating mutations in NOTCH1 drive T-cell acute lymphoblastic leukemia (T-ALL), while loss-of-function mutations contribute to squamous cell carcinoma.

NF-κB Transcription Regulators Are Activated by Regulated Proteolysis

NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells) is a family of transcription factors that controls inflammation, immunity, and cell survival:

Without signal: NF-κB is sequestered in the cytoplasm by its inhibitor, IκB. The IκB protein masks the nuclear localization signal of NF-κB, preventing its entry into the nucleus.

With signal (e.g., TNF, IL-1, pathogen components binding Toll-like receptors): A signaling cascade activates the IκB kinase complex (IKK), which phosphorylates IκB. Phosphorylated IκB is ubiquitinated by the SCF-βTrCP ubiquitin ligase and degraded by the proteasome. Free NF-κB enters the nucleus and activates hundreds of target genes involved in immune responses, inflammation, and cell survival.

NF-κB provides a key example of how proteolysis can serve as a signaling switch: the signal does not modify the transcription factor itself but rather destroys its inhibitor.

Bioinformatics: Signaling Crosstalk and Integration

Real cellular decisions arise from the interplay of multiple pathways. Computational tools help us understand this complexity:

• Pathway crosstalk analysis tools identify shared components and regulatory nodes. For example, GSK-3β is a node in both Wnt and PI3K-Akt pathways: Akt can phosphorylate and inactivate GSK-3β, creating crosstalk between these two pathways
• Multi-pathway modeling tools (CellDesigner, COPASI, BioNetGen) allow researchers to build kinetic models and simulate pathway dynamics, predicting how perturbations in one pathway affect others
• Context-specific pathway activity inference uses transcriptomic data (e.g., from RNA-seq) to infer which pathways are active in a particular cell type or condition, using methods like GSEA (Gene Set Enrichment Analysis) and PROGENy
• Proteolytic cleavage site prediction tools (SignalP, ProP, PROSPER) predict where proteases will cut, which is relevant for understanding Notch processing, β-catenin destruction, and IκB degradation

The Wnt and Notch pathways frequently interact during development. Let us compare conserved regions of the human APC and NOTCH1 proteins to illustrate the diversity of protein-protein interaction domains used in signaling:

let heatmap_data = '[{"row": "ERK", "col": "EGF", "value": 0.95}, {"row": "ERK", "col": "Wnt", "value": 0.1}, {"row": "ERK", "col": "Notch", "value": 0.05}, {"row": "Akt", "col": "EGF", "value": 0.7}, {"row": "Akt", "col": "Wnt", "value": 0.15}, {"row": "Akt", "col": "Notch", "value": 0.1}, {"row": "β-catenin", "col": "EGF", "value": 0.05}, {"row": "β-catenin", "col": "Wnt", "value": 0.9}, {"row": "β-catenin", "col": "Notch", "value": 0.1}, {"row": "NICD", "col": "EGF", "value": 0.05}, {"row": "NICD", "col": "Wnt", "value": 0.05}, {"row": "NICD", "col": "Notch", "value": 0.85}]'
let chart = Viz.heatmap(heatmap_data, '{"title": "Pathway Activation by Ligand", "x_label": "Signal", "y_label": "Effector"}')
print(chart)

The low alignment score between these domains reflects their completely different structural folds — armadillo repeats (APC/β-catenin binding) versus ankyrin repeats (Notch/CSL binding) — despite both serving as platforms for assembling signaling complexes in proteolysis-dependent pathways.

15.5 — Signaling in Plants

Plants, like animals, are multicellular organisms that require sophisticated cell-to-cell communication. However, the molecular machinery they use is often strikingly different from that of animals.

Multicellular Plants and Animals Use Different Signaling Molecules

While animals rely heavily on peptide hormones, neurotransmitters, and steroid hormones, plants use a distinct repertoire of phytohormones:

Plants lack GPCRs and receptor tyrosine kinases — the two largest receptor families in animals. Instead, they have evolved their own signaling strategies built around receptor serine/threonine kinases, histidine kinases, and ubiquitin ligase-based hormone perception.

Receptor Serine/Threonine Kinases Are the Largest Class of Cell-Surface Receptors in Plants

The largest family of cell-surface receptors in plants consists of receptor-like kinases (RLKs) — more than 600 in Arabidopsis thaliana alone, compared to ~58 RTKs in the human genome. Most plant RLKs are receptor serine/threonine kinases with extracellular leucine-rich repeat (LRR) domains for ligand binding. These are structurally analogous to animal RTKs but phosphorylate serine and threonine rather than tyrosine.

Notable examples include:

• BRI1 — the brassinosteroid receptor, which controls cell expansion
• FLS2 — recognizes bacterial flagellin and triggers plant immune responses
• CLV1 — controls stem cell number in shoot apical meristems

The dominance of RLKs in plant signaling represents a case of convergent evolution: plants and animals independently evolved single-pass transmembrane kinase receptors, but the details of kinase specificity and downstream signaling differ substantially.

Ethylene Blocks the Degradation of Specific Transcription Regulators in the Nucleus

The plant hormone ethylene — a simple two-carbon gas (C₂H₄) — uses a signaling pathway that operates by regulated proteolysis, conceptually similar to the Wnt and NF-κB pathways in animals:

Without ethylene: The receptor (a histidine kinase-like protein in the ER membrane) is active and turns on CTR1, a Raf-like kinase. CTR1 phosphorylates EIN2, targeting it for degradation. The transcription factors EIN3/EIL1 are also continuously degraded via ubiquitin-proteasome pathways. Ethylene-responsive target genes remain off.

With ethylene: Ethylene binds the receptor and inactivates it (ethylene receptors are negative regulators — analogous to Patched in Hedgehog signaling). CTR1 is inactivated, EIN2 is stabilized and cleaved, and its C-terminal fragment moves to the nucleus, where it stabilizes EIN3/EIL1. These transcription factors then activate ethylene-responsive genes controlling fruit ripening, leaf senescence, and defense responses.

This inversion of typical receptor logic — where the receptor is active without ligand and inactivated by ligand binding — is a fascinating parallel between ethylene and Hedgehog signaling that arose independently in plants and animals.

Regulated Positioning of Auxin Transporters Patterns Plant Growth

Auxin (indole-3-acetic acid, IAA) is unique among signaling molecules because its distribution is actively controlled by polar auxin transport. The PIN family of auxin efflux carriers is asymmetrically localized on specific faces of the cell, creating directional auxin flow. The regulated repositioning of PIN proteins through endocytic recycling generates auxin concentration gradients that pattern:

• Embryonic axis formation (apical-basal polarity)
• Root gravitropism (auxin accumulates on the lower side of horizontal roots)
• Leaf phyllotaxis (auxin maxima determine where new leaf primordia form)
• Vascular patterning (auxin canalization guides vein formation)

In the nucleus, auxin is perceived by the TIR1/AFB receptor (an F-box protein that is part of an SCF ubiquitin ligase complex). Auxin promotes binding of TIR1 to Aux/IAA repressor proteins, leading to their ubiquitination and degradation. This frees ARF (Auxin Response Factor) transcription factors to activate target genes. Thus, auxin signaling, like Wnt and ethylene signaling, depends on regulated proteolysis of transcriptional repressors.

Bioinformatics: Plant Signaling

Computational tools for plant signaling research address unique challenges:

• Plant-specific kinase classification: The PlantsP and iTAK databases organize plant kinases into families based on sequence and domain architecture, revealing the massive expansion of the RLK family that is unique to land plants
• Phytohormone signaling pathway databases: KEGG Plant, PlantCyc, and the Arabidopsis Information Resource (TAIR) provide curated pathway maps for all major phytohormone signaling cascades
• Plant receptor-ligand interaction prediction: tools that combine structural modeling, co-expression analysis, and phylogenetic profiling to predict novel ligand-receptor pairs in plants
• Comparative signaling analysis between plant and animal pathways: by aligning plant and animal kinase domains, researchers can trace the evolutionary relationships and divergence of kinase-mediated signaling across kingdoms of life

Exercise: Drug Selectivity Through Chemical Similarity

Kinase inhibitors are designed to be selective for specific kinases. Compare the structural similarity of three cancer drugs — do drugs targeting the same pathway resemble each other more?

let imatinib = "Cc1ccc(NC(=O)c2ccc(CN3CCN(C)CC3)cc2)cc1Nc1nccc(-c2cccnc2)n1"
let gefitinib = "COc1cc2ncnc(Nc3ccc(F)c(Cl)c3)c2cc1OCCCN1CCOCC1"
let erlotinib = "C#Cc1cccc(Nc2ncnc3cc(OCCOC)c(OCCOC)cc23)c1"
let sim_egfr = Chem.tanimoto(gefitinib, erlotinib)
let sim_cross = Chem.tanimoto(imatinib, gefitinib)
print("Gefitinib vs Erlotinib (both EGFR): " + sim_egfr)
print("Imatinib vs Gefitinib (different targets): " + sim_cross)
// Which pair of drugs is more similar?
let answer = "EGFR inhibitors"
print(answer)

The partial conservation of the glycine-rich P-loop (GXGXXG motif) between plant BRI1 and animal EGFR kinase domains reveals their shared ancestry in the protein kinase superfamily, despite billions of years of independent evolution and the switch from tyrosine to serine/threonine phosphorylation in plants.

Pathway Integration and Crosstalk

A cell does not activate one pathway at a time. Growth factors may simultaneously engage RTK-Ras-MAPK, PI3K-Akt, and PLCγ-Ca²⁺ signaling. Developmental signals through Wnt and Notch may converge on the same target genes. This pathway crosstalk is both a feature and a challenge:

Understanding these interactions is essential for predicting drug responses in cancer therapy, where targeting one pathway often leads to compensatory activation of another — a major source of therapeutic resistance.

Exercise: Pathway Crosstalk Visualization

Signaling pathways don’t operate in isolation — they cross-talk extensively. Build a heatmap showing how different growth factors activate overlapping downstream effectors:

let data = '[{"row": "ERK", "col": "EGF", "value": 0.9}, {"row": "ERK", "col": "Insulin", "value": 0.3}, {"row": "ERK", "col": "Wnt", "value": 0.1}, {"row": "Akt", "col": "EGF", "value": 0.6}, {"row": "Akt", "col": "Insulin", "value": 0.9}, {"row": "Akt", "col": "Wnt", "value": 0.2}, {"row": "mTOR", "col": "EGF", "value": 0.4}, {"row": "mTOR", "col": "Insulin", "value": 0.8}, {"row": "mTOR", "col": "Wnt", "value": 0.1}, {"row": "β-catenin", "col": "EGF", "value": 0.05}, {"row": "β-catenin", "col": "Insulin", "value": 0.05}, {"row": "β-catenin", "col": "Wnt", "value": 0.9}]'
let chart = Viz.heatmap(data, '{"title": "Pathway Crosstalk", "x_label": "Growth Factor", "y_label": "Effector"}')
print(chart)
// Which effector is activated by both EGF and Insulin (crosstalk)?
let answer = "Akt"
print(answer)

Knowledge Check

Summary

In this lesson you learned:

• RTKs dimerize upon ligand binding, trans-phosphorylate on tyrosine residues, and recruit SH2-domain proteins to activate downstream cascades
• The Ras-MAPK pathway (Ras → Raf → MEK → ERK) drives gene expression for cell growth; oncogenic Ras mutations are found in ~25% of human cancers
• PI3K-Akt signaling promotes cell survival, growth, and metabolism via PIP₃-mediated recruitment of Akt; PTEN opposes PI3K by dephosphorylating PIP₃
• The JAK-STAT pathway provides a direct route from cytokine receptors to gene expression, with no intermediate kinase cascade
• TGF-β receptors are serine/threonine kinases that activate Smad transcription factors with dual roles in tumor suppression and promotion
• Some receptors activate ubiquitin ligases or are guanylyl cyclases producing cGMP
• The kinome (~518 human kinases) is classified using the Manning tree; tools like NetPhos, KSEA, OncoKB, and KINOMEscan support phosphoproteomics and drug discovery
• Wnt signaling controls β-catenin stability via a destruction complex (APC, Axin, GSK-3β); APC loss drives ~80% of colorectal cancers
• Hedgehog signaling is controlled by the inhibitory receptor Patched; Smoothened mutations cause basal cell carcinoma
• Notch is a latent transcription factor — proteolytic cleavage by γ-secretase releases NICD to activate target genes
• NF-κB is held inactive by IκB; signal-induced IκB degradation frees NF-κB for nuclear entry
• Plants use receptor-like kinases (>600 in Arabidopsis), not GPCRs or RTKs, as their main receptor class
• Ethylene signaling uses negative regulation (receptor active without ligand), stabilizing EIN3 transcription factors
• Auxin gradients are established by polarized PIN transporters and signal through SCF-TIR1 ubiquitin ligase–mediated degradation of Aux/IAA repressors
• Pathway crosstalk (e.g., PI3K-Akt phosphorylation of GSK-3β feeding into Wnt) is pervasive and critical for understanding drug resistance in cancer therapy