GPCRs, RTKs, second messengers, MAPK, PI3K-AKT, Wnt/Hh/Notch, TGF-β, mTOR, ubiquitin-proteasome.
Cell Signaling
Signal transduction
Cascade of molecular events converting extracellular signal (ligand) into intracellular response. Receptor → effectors → second messengers…
G-protein coupled receptor (GPCR)
Largest receptor family (~800 in humans): 7-transmembrane-helix receptors coupling ligand binding to heterotrimeric Gα/β/γ activation.…
Second messenger
Small diffusible intracellular molecule amplifying receptor signal: cAMP, cGMP, Ca²+, IP3, DAG, PIP3, NO. Concept introduced by Sutherland…
Receptor tyrosine kinase (RTK)
Single-TM receptors that dimerise on ligand binding, trans-autophosphorylate cytoplasmic Tyr residues recruiting SH2-domain effectors.…
MAPK cascade
Three-tier kinase cascade: MAPKKK → MAPKK → MAPK (e.g. Raf → MEK → ERK). Downstream of RTKs via Ras; drives proliferation, differentiation.…
PI3K-AKT pathway
PI3K phosphorylates PIP2→PIP3; AKT/PKB bound via PH domain, phosphorylated by PDK1 + mTORC2. Drives survival, growth, metabolism. PTEN…
Wnt pathway (canonical)
Wnt ligand binds Frizzled + LRP5/6, inhibits GSK3β-mediated β-catenin degradation. β-catenin accumulates, enters nucleus, partners with…
Notch pathway
Juxtacrine signalling: Delta/Jagged ligand on neighbour cell activates Notch receptor by triggering S2 (ADAM) + S3 (γ-secretase) cleavages.…
TGF-β pathway
TGF-β superfamily (TGF-β, BMP, activin) binds type-II/I Ser/Thr-kinase receptors; phosphorylates R-SMADs which partner with Co-SMAD4 →…
mTOR pathway
mTOR kinase forms mTORC1 (rapamycin-sensitive, growth/metabolism via S6K + 4E-BP1) + mTORC2 (rapamycin-resistant, cytoskeleton + AKT).…
Ubiquitin-proteasome system
E1 → E2 → E3 cascade appends ubiquitin to substrate (K48-polyUb) for 26S proteasomal degradation. Regulates cell cycle, signalling, MHC-I…