Lecture 4: Enzyme-linked Receptors

Enzyme-linked receptors Enzyme-coupled receptors Enzyme-coupled receptors have enzymatic activity or are associated with enzymes. Most of them are kinases or associate with kinases Enzyme-linked receptors Are often in cell growth and movement Ligands are typically growth factors (short peptides) Some ligands are attached to the cell surface over which the target cell is crawling Cytosolic receptor domain is either an enzyme or directly associates with an enzyme Genetic defects in these receptors can lead to uncontrolled cell proliferation (cancer) Six classes of enzyme-linked receptors Tyrosine kinases Tyrosine kinase-associated Tyrosine phosphatases Serine/Threonine kinases Histidine-associate kinase Guanylyl cyclase Examples of Receptor Tyrosine Kinases (RTKs) Most ligands of RTKs are secreted peptide growth factors and peptide hormones EGF - Epidermal growth factor NGF - Nerve growth factor PDGF - Platelet-derived growth factor FGF - Fibroblast growth factor M-CSF - Macrophage colony stimulatory factor Other ligands are bound to the cell surface upon which the target cell crawls Ephrins = ligands Ephrin receptors (EphRs) are RTKs Important for development, including axon guidance Ephrin receptor - ephrin interaction is involved in contact-dependent signaling Eph receptors are RTKs The largest class of membrane-bound ligands, the most numerous RTKs Help guide migration of cells and axons; important in development Act both ways: activated Eph receptors are transmitting the signal back to signaling cell Behavior of both signaling cell and target cell are altered This bidirectional signaling is important to prevent cells from different tissues from mixing Which of the following usually goes both ways (signaling cell-target cell and target cell-signaling cell)? • A. Any receptor tyrosine kinase signaling • B. Signaling through Ephrins • C. Signaling through GPCRs • D. All of the above Receptor Tyrosine Kinases (RTKs) Extracellular ligand-binding domain In most cases a signal transmembrane domain Intracellular tyrosine kinase domain Activation of the receptor induces a conformational change, which causes receptors or dimerize (induced proximity) Two kinase domains brought together by dimerization activate each other (transautophosphorylation) Phosphorylation activates kinase activity of RTK Cross-phosphorylation stimulates kinase activity and creates high affinity docking sites for other proteins Scaffold proteins prevent cross-talk between parallel MAP kinase modules The role of scaffold proteins is just to bring the kinases into close proximity Scaffold proteins help tethering signaling components and localizing them to specific areas of the cell (plasma membrane, cytoplasm, nucleus, Golgi, endosomes, and the mitochondria) Sometimes an activated (phosphorylated) receptor can function as a scaffold protein Proteins with SH2 or PTB interaction domains bind to phosphorylated tyrosines at RTKs SH2 domain is most common It can recognize phosphorylated tyrosines in the context of different flanking amino through its two binding surfaces SH2 domain can be inserted anywhere in the protein without disturbing its structure and function The assembly of signaling complexes depends on interaction domains Interaction domains of signaling proteins are highly conserved They are compact They bind to certain structural motifs Some signaling proteins only function as adaptors Common interaction domains Bind to phosphorylates tyrosines: SH2 domain - Src homology PTB domain - Phosphotyrosine binding Bind to proline-rich sequences: SH3 - Src homology Bind to the charged head group of phosphoinositides: PH domain - pleckstrin homology Interaction domains help propagate the signal due to _____ • A. Kinase activity • B. Conformational changes that occur within interaction domains • C. Attaching signaling molecules to each other Important signaling protein Ras is coupled to RTKs through adaptor proteins Ras is a monomeric GTPase Ras superfamily consists of many proteins, but only Rho and Ras relay signals from cell-surface receptors Ras and Rho can spread signals along several distinct signaling pathways, acting as signaling hubs Ras contains several lipid groups, which anchor it to cytoplasmic face of the membrane Ras often regulates cell proliferation and differentiation 30% of tumors have hyperactive forms of Ras Ras and Ras regulatory proteins are highly conserved in evolution; were studied in C. elegans and Drosophila Regulatory proteins GEFs and GAPs regulate activity of Ras GTPases RTKs activate Ras by coupling to GEFs (guanine nucleotide exchange factors). Activation is usually indirect, through SH2 domains of other proteins GAP - GTPase-activating proteins drive Ras into “off” state. A mutation which makes Ras insensitive to GAPs results in its permanent activation and leads to cancer A mutate version of Ras that is overactive and promotes continuous proliferation ______ • A. Is resistant to GAPs • B. Is resistant to GEFs Ras activation signal is relayed to the nucleus by MAP kinase module Ras activation by RTK is quickly reversed by tyrosine phosphatases and GAPs The short-term signaling event should be converted to a long-lasting signal to proliferate or differentiate, and relayed to the nucleus to alter gene expression A mitogen-activated protein kinase module (MAP kinase module) relays the signal This module is conserved from yeast to humans It is involved in regulation of gene expression, cell division, cell differentiation, and cell survival/apoptosis Which of the following is membrane-associated? • A. GPCR • B. RAS • C. G protein • D. All of the above Which of the kinases can translocate to the nucleus? • A. MAPKKK • B. MAPKK • C. MAPK