principles of cell signaling

Question 1

extracellular signals can travel long distances or short distances. give examples of local and long distance signals

Answer 1

short: contact dependent, paracrine
long: synaptic, endocrine

Question 2

which part of signaling actually has the information and determines what the effects are?

Answer 2

receptors. the signal has little info, it just activates receptors. The same signaling molecule can have different effects on different target cells

Question 3

what are the three major classes of cell surface receptor proteins?

Answer 3

Ion channel coupled receptors
G protein coupled receptors
enzyme coupled receptors

Question 4

what does GAP do?

Answer 4

GAP drives proteins to the inactivated “off” state by increasing hydrolysis of GTP to GDP

Question 5

what does GEF do?

Answer 5

GEF activates proteins “on” by promoting release of GDP and allowing GTP to bind.

Question 6

does signaling always activate pathways with fast changes?

Answer 6

no. slow changes are also triggered when signaling leads to altered protein SYNTHESIS. when protein FUNCTION is altered, that is fast.

Question 7

what are GPCRs? structure?

Answer 7

G protein coupled receptors. metabotropic plasma membrane receptors that couple to heterotrimeric G proteins. also known as Seven transmembrane receptors or Serpentine 7 receptors because they have 7 transmembrane alpha helices

Question 8

what percent of all human genes encode GPCRs? what percent of drugs target GPCRs?

Answer 8

4% of human genes

40-50% of drugs

Question 9

what are the subunits of heterotrimeric G proteins?

Answer 9

alpha subunit: molecular switch that changes shape depending on GTP or GDP bound. has lipid membrane anchor
beta subunit: forms dimer with gamma
gamma subunit: forms dimer with beta and has lipid membrane anchor

Question 10

where does GTP/GDP bind on a heterotrimeric G protein?

Answer 10

in the AH domain within the alpha subunit

Question 11

how does GDP binding impact affinity of G protein subunits for each other?

Answer 11

the alpha subunit has a high affinity for the beta/gamma dimer when GDP bound, it is inactive in this trimeric state

Question 12

what causes the subunits to dissociate?

Answer 12

a GPCR is activated by a signaling molecule and is able to induce alpha to release GDP and bind GTP. alpha-GTP has decreased affinity for beta/gamma and it dissociates

Question 13

GPCRs act as what to alpha subunit?

Answer 13

GEFs. the induce release of GDP and binding GTP, activating the G protein

Question 14

what is the concentration of GTP vs GDP in the cytosol?

Answer 14

GTP: around 0.5 mM
GDP: around 10 times lower
This makes it so the G protein can easily bind GTP as soon as GDP is released

Question 15

what interacts with downstream effector proteins?

Answer 15

either the activated alpha or beta/gamma subunits. they may activate or inhibit effector proteins

Question 16

do alpha and beta/gamma always dissociate?

Answer 16

no. in some cells they remain bound but shift relative positions so interaction sites become available.

Question 17

what is an effector antagonist?

Answer 17

a downstream effector protein that blocks signaling by physically blocking either alpha or beta/gamma. Ex. GRK2 binds beta/gamma

Question 18

what are GRKs? what is their purpose? (Arrestin)

Answer 18

GPCR kinases that phosphorylate GPCRs and desensitize them. some proteins, like Arrestin, bind to phosphorylated GPCRs and block coupling to G proteins. Arrestin is an adaptor protein for clathrin and leads to endocytosis of the GPCR

Question 19

four processes that block/reduce signaling through GPCRs

Answer 19

Inactivation: GPCRs are covalently modified to prevent them from acting as GEFs
Sequestration: GPCRs are endocytosed in to recycling Endosomes and are not exposed to agonist. may be recycled
Desensitization: same mech as inactivation
Down regulation: GPCRs are endocytosed and destroyed in lysosomes

Question 20

what is yeast mating factor? what is the response to mating factor?

Answer 20

yeast mating factor is a secreted peptide that acts as an agonist for a GPCR on yeast PM. Haploid yeast that are ready to mate secrete mating factor peptide and other yeast respond by extending in that direction, called Shmooing

Question 21

alpha subunits hydrolyze GTP slowly, what helps them accelerate this process?

Answer 21

GAPs accelerate GTP hydrolysis up to 1000 fold. for the alpha subunit specifically, Regulators of G-protein Signaling (RGS) is used as a GAP. PLCbeta1 acts as a downstream effector protein and functions as a GAP for alpha subunits (specifically Gaq/11). PLCB1 is negative feedback loop

Question 22

there are many types of G protein alpha subunits in humans. name three

Answer 22

alpha S: activates adenylyl cyclase
alpha i/o: inhibits adenylyl cyclase
alpha q/11: activates phospholipase C

Question 23

what does Cholera do to alpha subunit?

Answer 23

Cholera toxin can modify GalphaS to prevent GTP hydrolysis and result in constitutive signaling by alphaS until cell down refulates

Question 24

what does Pertussis do to the alpha subunit?

Answer 24

Pertussis toxin can modify Galphai to block activation by GPCRs and causes whooping cough