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VII. Signal Transduction: conversion of signals carrying
information from one form to another. Different cell types
have different reasons for needing to do this, but they all
need a way to sense the environment or communicate with
each other.
- (^) Find nutrients/prey
- (^) Sense light and dark
- (^) Avoid poisons/predators
- (^) Identify conjugation/mating partners - (^) Coordination of cells - (^) Development and differentiation - (^) Growth, maintenance, and death - (^) Physiology and behavior Fig 16-
A. General principles of signal transduction
- Some general definitions a. The signaling cell produces a particular type of signaling molecule.
b. The target cell detects the signal with a receptor
protein. c. Signal transduction is when the target cell receives the signal and converts it to an intracellular response
Contact dependent signaling controls nerve cell
production in Drosophila (figure 16-4)
3) Response to a particular signal depends on the
cell receiving it (fig 16-5).
a. Must have a receptor protein, which can differ from
cell to cell and usually only responds to one type of
signal (specific ligand).
b. Can trigger different intracellular signaling
molecules which
c. Activate different effector proteins in different cells.
5) Responses to signaling can be fast or slow (fig. 16-
- There are two general classes of signal receptors (fig 16-8)
a. Bind to molecules that
can’t cross the lipid bilayer
Signals are proteins, peptides, small hydrophilic molecules
b. Signal molecules can go
through the lipid bilayer
- (^) Steroid hormones
- (^) Nitric oxide
- Dissolved gases (e.g, NO) cross the plasma membrane and activate intracellular enzymes directly (fig. 16-11). Viagra blocks the enzyme that degrades cGMP, prolonging the NO signal. cGMP signals the cell’s response by activating protein kinase G, which leads to increased blood flow.
C. Signaling molecules that cannot cross the plasma
membrane bind cell surface receptors relay extracellular
signals via intracellular pathways (figure 16-13)
- (^) Reception: Primary transduction
- (^) Relay: Secondary transduction
- (^) Amplify
- (^) Integrate: Signals from other pathways converge.
- (^) Distribute to Coordinate the final response Possible Responses
b. GTP-binding proteins switch from an active and inactive state depending on whether they have a GTP bound to them. Their intrinsic GTPase activity shuts them off.
2) Cell-surface receptors fall into three main classes (fig
Already covered (chapter 12)
- Activation of heterotrimeric G-proteins (fig 16-17). (^) GDP-inactive
- (^) Ligand binds to GPCR and causes a conformation change producing exchange of GDP for GTP.
- (^) Active GTP and dissociated -active subunits interact with target proteins associated with the PM (^) Either or both can have affects on target proteins Tethered to plasma membrane
-Turning off the G-protein (fig 16-18).
• Target protein is affected
as long as G protein
subunits are in contact
with it.
GTP self hydrolyzes.
GDP dissociates from
target and reassociates
with
Movie 16.
- (^) Membrane bound enzymes (e.g., adenylyl cyclase and phospholipase C) that produce “second messengers” (e.g., cAMP, DAG and IP 3 ), allowing signal to be amplified. Fig. 16-
- (^) G-protein activates (Gs) or inhibits (Gi) adenylyl cyclase, which makes cAMP
- (^) cAMP phosphodiesterase (blocked by caffeine) turns off signal by converting cAMP to AMP -Second messengers Figure 16-
