G-protein-coupled Receptors

GPCRs are often referred to as seven-pass transmembrane receptors because they have:

a polypeptide chain that crosses the lipid bilayer seven times

Which subunit of a G-protein is not tethered to the plasma membrane by a short lipid tail?

Beta subunit

Which G-protein component or components have GTPase activity?

Alpha subunit

Which component or components can, on its own, activate target proteins in an intracellular signaling pathway?

Alpha and Beta subunits

Signaling via a GPCR ceases when the alpha subunit of an activated G-protein hydrolyzes its bound GTP to GDP.

True

Cholera and whooping cough are both caused by bacteria that:

Alter the activity of a G-protein

The proteins targeted by either alpha subunit of the beta-gamma complex of a G-protein are either enzymes or ion channels in the plasma membrane.

True

Which of the following is not a small intracellular signal molecule? A) Ca2+ B) Cyclic AMP C) Diacylgylcerol D) Inositol E) Phospholipase C

E

Small intracellular signal molecules are often called:

Second messengers

When activated by a GPCR, adenylyl cyclase:

Converts ATP to cAMP

The enzyme cyclic AMP phosphodiesterase helps terminate a response mediated by an increase in cyclic AMP by:

Converting cyclic AMP to AMP

The cyclic AMP pathway can both activate enzymes and turn on genes.

True

When activated phospholipase C chops the head off a inositol phospholipid, it produces two small signaling molecules that relay the signal onward. Which of these stays in the plasma membrane?

Diacylglycerol (DAG)

How does IP3 function in inositol phospholipid pathway?

It binds to and opens Ca2+ channels that are embedded in the ER membrane, releasing Ca2+ into the cytosol.

How does diacylglycerol function in the inositol phospholipid pathway?

Together with Ca2+, it recruits PKC from the cytosol to the plasma membrane and activates it

Ca2+ is an intracellular messenger involved with:

Initiation of muscle contraction, release of neurotransmitters, secretion of hormones, and triggering of embryonic development after fertilization

The concentration of free Ca2+ in the cytosol of an unstimulated cell is kept low compared with its concentration in both the extracellular fluid and the endoplasmic reticulum. What does not help maintain the difference?

Ca2+ destroying enzymes in the cytosol

When activated by the binding of Ca2+, calomodulin relays the Ca2+ signal onward by:

Binding to Ca2+/calomodulin-dependent protein kinases, which then phosphorylate other intracellular proteins

Signaling cascades associated with GPCRs always operate at the same speeds irrespective of external stimuli.

False

Small intracellular signaling molecules diffuse rapidly through the cytosol, thereby amplifying and spreading the intracellular signal.

False

What class of cell-surface receptor often triggers the production of cyclic AMP?

G-protein-coupled receptors

Because cyclic AMP diffuses quickly through the cytosol and a cyclic AMP phosphodiesterase rapidly converts cyclic AMP to AMP:

The effects of increasing cyclic AMP can be rapid or slow

The signal molecules that bind to GPCRs all have a similar structure.

False

What represents the order of signaling events triggered by a GPCR that activates expression of a target gene via the production of cyclic AMP?

Activation of G-protein, activation of adenylyl cyclase, production of cyclic AMP, activation of PKA, activation of transcription regulator

Acetylcholine stimulates cells in the pancreas to secrete enzymes that aid in digestion of sugars. It does so by activating GPCRs that stimulate the membrane-bound enzyme phospholipase C (PLC).

PLC then triggers which of these downstream events?

An elevation of cytosolic Ca2+ concentration, which leads to the activation of protein kinase C (PKC).

In photoreceptor cells in the retina, light is sensed by a GPCR called rhodopsin. Which of the following represents the events that take place after the activation of rhodopsin?

Activation of G protein, activation of phosphodiesterase, breakdown of cyclic GMP, closing of ion channels, change in membrane potential

Some G-proteins inhibit the production of cyclic AMP.

True