Advanced Molecular Cell Biology - Lecture 5

cells use approximately how many different families of receptor proteins to detect and respond to a myriad of chemical and physical stimuli?

25

What is the most diverse family of receptor proteins to detect and respond to a myriad of chemical and physical stimuli?

GPCRs

describe the structural features of GPCRs:

• The characteristic 7 TMs (α helices) of all GPCRs are packed in a similar way

• 3 of the loops linking TMs are located extracellularly and 3 are located intracellularly

• TM3 is centrally located next to the binding pocket, crucial for ‘transduction’ of ligand binding

• Other TMs and extracellular N-terminus also
  contribute to ligand binding

• C-terminal is where the G-protein binds; it is in the lipid bilayer; the N-terminal is where the ligand binds

GPCR ‘classes’ are distinguished by structural features of the extracellular domains defining the ‘_____’ _______ ____. This is linked directly to the huge diversity of the stimuli GPCRs can detect.

ligand, binding site

what type of receptor are Protease-activated receptors (PAR) in platelets:

GPCR

how does the PAR work?

the N-terminus gets cleaved and becomes it’s own agonist

how many independent stimuli activate platelets?

3

what are the 3 types of GPCRS on platelets?

integrin (fibrinogen receptor), ADP and thrombin

describe what happens when there is damage to the skin:

the basal lamina becomes exposed, causing integrin to bind to collagen in basal lamina, there is then activation of ADP in the platelet which in turn activates the receptors on other platelets and allows them to cluster together
thrombin receptors also get activated leading to platelet activation

in the resting state where is the receptor and the G protein located?

receptor is in the membrane and the G protein is anchored to the membrane

what are the 3 subunits of the G-protein?

alpha, beta and gamma

upon ligand binding what happens to the GPCR?

there is a conformational change allowing G protein binding to the receptor, the alpha subunit exchanges GDP for GTP, which is where is gets it’s energy from and is now active, it then dissociates, the G-protein then moves from the receptor and the alpha moves away from the other 2 subunits, it can them move through the membrane to interact with it’s effectors

what are G proteins?

• Guanine nucleotide-binding proteins – belong to the GTPase family

• Act as molecular switches inside the cell to transmit signals from extracellular
  stimuli

• Regulated by ability to bind and hydrolyse GTP (‘on’) to GDP (‘off’)

• Exist as heterotrimeric complexes made up of ⍺, β, and 𝛾 subunits

in it’s inactive state what is bound to the alpha subunit of the G protein?

GDP

duration of signalling by an activated trimeric G protein is regulated by?

the rate of GTP hydrolysis by the Galpha subunit

what protein stimulates GTPase activity in the alpha subunit?

RGS (regulators of G-protein signalling)

how many different families of G-proteins exist?

6

what are the different families of G-proteins?

G(i)alpha, G(s)alpha, G(q)alpha, G(13)alpha, G(t)alpha, G(olf)alpha

name the receptors associated with G(i)alpha:

alpha-adrenergic, acetylcholine, chemokines, various neurotransmitters

name the receptors associated with G(s)alpha:

beta-adrenergic amines, hormones (corticotropin, glucagon, parathyroid, thyrotropin, others)

name the receptors associated with G(q)alpha:

alpha-adrenergic amines, acetylcholine, various neurotransmitters

name the receptors associated with G(13)alpha:

thrombin and others

name the receptors associated with G(t)alpha:

rhodopsin, which absorbs light

name the receptors associated with G(olf)alpha:

odorant receptors

what are the 3 layers of diversity that determine the biological response in G(olf)alpha?

specific ligand, specific subunit and cell type

what is the effector for G(i)alpha?

inhibits adenylyl cyclase, opens potassium channels, and closes calcium channels

what is the effector for G(s)alpha?

stimulates adenylyl cyclase to produce cAMP receptor kinase

what is the effector for G(q)alpha?

activates phospholipase C(beta) to produce IP3, which releases Ca2+

what is the effector for G(13)alpha?

Rho and others

what is the effector for G(t)alpha?

Activate cGMP phosphodiesterase to break down cGMP, receptor kinase

what is the effector for G(olf)alpha?

activate adenylyl cyclase, receptor kinase

effectors of Galpha subunits include ______ that create 2nd messengers and ____ ________ whose gating is regulated either directly (beta gamma subunits) or indirectly by 2nd messengers and their effectors

enzymes, ion channels

G-proteins can directly _________ an ion-channel, similar mechanism as ligand-gated channels, it is slow to open or close, and they stay open or closed for ______ (minutes rather than seconds)

activate, longer