Signal Response and Amplification Signal Transduction Signal Recognition Extracellular domain Signal Transmission Transmembrane domain Signal Response and Amplification Intracellular domain

Biosignaling (Signal Transduction) A signaling molecule interacts with the extracellular side of a cell surface receptor (a transmembrane protein). The receptor undergoes a conformational change, is activated and interacts with other intracellular components. A second signal is produced or the activity of a specific cellular protein is affected, further affecting downstream cellular events. 4. The receptor returns to the pre-stimulated conformation.

Features of Signal Transducing Systems

Six General Types of Signal Transducers

G Protein Receptor Activation

Activation of Adenylate Cyclase

G protein structure

Coordination of Mg2+ is necessary for GTP binding to G proteins

Structure of G-alpha

Active vs Inactive G-alpha Figure 13.7

Active vs Inactive G-alpha

GTPase mechanism

Signal Transduction - Insulin Signal Recognition Extracellular domain Signal Transmission Transmembrane domain Signal Response and Amplification Intracellular domain Tyrosine Kinase

Receptor Tyrosine Kinases

Receptor Tyrosine Kinases

Insulin Signaling

Insulin is synthesized as an inactive prohormone

Insulin Signaling

Extracellular side of insulin receptor Transmembrane portion of insulin receptor (structure not solved) Intracellular side of insulin receptor (unphosphorylated and phosphorylated)

Insulin Signaling

Insulin Signaling

SH2 and SH3 domains in Grb2

SH2 domain

SH3 domain binds to a polyproline Figure 13.28

SH3 domain peptide complex

Insulin Signaling

G protein (Ras) structure

GTP bound to Ras A Mg2+ is coordinated to the β and γ phosphates of GTP and links GTP to Ras. The Mg2+ is also coordinated to Ser and Thr side chain oxygens and two water molecules. Ras is prenylated (geranylgeranyl), anchoring it to the membrane surface.

Insulin Signaling

Raf and MEK Raf and MEK are Protein Kinases that phosphorylate target proteins. Raf (bound to Ras for activation) phosphorylates MEK on two serine residues. MEK (activated when phosphorylated by Raf) phosphorylates ERK on a threonine and a tyrosine.

Insulin Signaling In unstimulated cells, most Ras is in the inactive form with bound GDP; binding of insulin to the insulin receptor leads to formation of the active Ras·GTP. Activated Ras binds to the N-terminal domain of Raf, a serine/threonine kinase. Raf binds to and phosphorylates MEK on two serines, activating it. MEK is a dual-specificity protein kinase that phosphorylates MAP kinase (ERK) on threonine and tyrosine residues separated by a single amino acid. Phosphorylation at both sites is necessary for activation of ERK. ERK phosphorylates many different proteins, including nuclear transcription factors, that mediate cellular responses.

ERK Structure Structures of ERK in its inactive, unphosphorylated form (a) and active, phosphorylated form (b) Phosphorylation of ERK by MEK at tyrosine 185 (pY185) and threonine 183 (pT183) leads to a marked conformational change in the phosphorylation lip (red). This change promotes dimerization of ERK and binding of its substrates, ATP and certain proteins. The dimeric form of ERK (but not the monomeric form) can be translocated to the nucleus where it regulates the activity of a number of nuclear localized transcription factors , increasing production of mRNAs that encode specific proteins.