Cell signaling (bio 172 exam 3)

categories of short-range signaling

1) paracrine signaling

2) juxtacrine signaling —- even closer, directly attached

3) autocrine signaling—— signal is on same cell

long-range signaling

1) electric signaling— along axons of nerve cells—essential for nervous system to function

2) endocrine signaling— essential for physiology homeostasis

signaling molecules and receptors

1) small hydrophobic signals

2) large hydrophillic signals

small hydrophobic signals definition

lipid solule and molecule can cross the plasma membrane to enter the cell = intracellular receptor

large hydrophilic signals definition

lipid-insuluble and they cannot cross the plasma membrane= cell-surface receptor

intracellular receptors:

• steroid hormone receptor

cell surface receptor:

• ligand gated ion channels

• G-protein coupled receptors

• receptor tyrosine kinases

what determines whether a cell responds to a signal

if it has a receptor it will respond, if it doesn’t the cell won’t respond

what determines how a cell responds to a signal

response depends on the type of proteins present in the responding cell

steroid hormone release

• signal travels through the circulatory system

• binding proteins transport the hormone through the blood stream

• steroid hormones are lipids (hydrophobic)

steroid hormone binds the receptor

• steroid hormone diffuses across the plasma membrane and binds intracellular receptor in the responding cell

• the receptor changes its conformation and the receptor- hormone complex enters the nucleus

conformational change in protein changes its activity

steroid hormone in the nucleus

• steroid hormone receptors are regulatory tanscription factors

• in the nucleus steroid hormone receptors bind to enhancers to regulate transcription

cell surface receptor 1: ligand gated ion channels

no ligand: channel closed

ligand present: opens channel, ions move along concentration gradient

common theme: conformation change in protein changes its activity

cell surface receptor 2: G protein-coupled receptor signaling

• signaling molecule binds to receptor

• receptor changes conformation and binds to G protein, G-protein replaces GDP with GTP

• active G-protein binds to and activates effector protein effector produces secondary messenger and triggers response

  • inactive when GDP bound

  • active when GTP bound

conformation change in protein changes its activity

second messenger: inside cell

• G-protein-activated enzymes catalyze the production of many small nonprotein signaling molecules called second messengers that diffuse rapidly and spread the signal message throughout the cell

• same second messages can trigger different events

termination of G proteins

• signaling molecule detaches. receptor is inactivated

• G-protein hydrolyzes GTP and GDP and is inactivated

• secondary messengers are degraded

cell surface receptor 3: receptor tyrosine kinases

unbound: inactive

signal bound: dimerization autophophorylation of receptor receptor is activated

signal through small G-protein Ras

• signal arrives and binds recptor

• receptor dimerizes and is phosphorylated

• receptor binds Ras, Ras exchanges GDP and GTP and is activated

• GTP bound Ras activates a kinase and triggers a kinase cascade

• a phosphorylation cascades results, each protein phosphorylates another until response is triggered

Termination of receptor tyrosine kinase signaling

• signaling molecule detaches. receptor is inactivated

• phosphorylated proteins must be dephosphorylated by phosphotases

• Ras hydrolyzes GTP to GDP and is inactivated

kinase signaling

addition of non-hydrolizable form of ATP inhibits signaling

G-protein signaling

addition of non-hydrolyzed form of GTP activates signaling