In this question, all of the answer choices will be true except for one. Therefore, we'll need to consider each answer choice, one by one, in order to determine whether it is a true statement.

First, let's briefly recall the basics of G protein-coupled receptors (GPCRs). These receptors are located on the cell membrane, and work by binding to ligands on the extracellular side. This binding induces a conformational change in the receptor, which then activates a G protein on the inner membrane. This activated G protein becomes active by shedding the GDP it has bound, and in exchange it binds to a new GTP molecule. This newly activated G protein then goes on to activate another enzyme found within the cell membrane, called adenylyl cyclase. This enzyme, when activated this way, catalyzes the transformation of ATP into cyclic AMP (cAMP). This cAMP, in turn, acts as a second messenger and, in doing so, activates protein kinase A (PKA). PKA then goes on to phosphorylate a wide range of target proteins, which ultimately leads to a cellular response.

In addition, there are other types of GPCRs that lead to a different kind of signal transduction cascade. This alternative pathway involves the signaling molecules inositol triphosphate (IP3), diacylglycerol (DAG), and protein kinase C (PKC). But for the purposes of this question, we will focus on the one explained above.

G Protein cascades are one type of transduction pathway, and just like any other biological process, it is important that it is regulated. Therefore, when turn on, there needs to also be mechanisms in place to turn it off.

One of the ways in which the cascade is turned off is through the innate GTPase activity of the G proteins themselves. After a certain amount of time has passed, these G Proteins are able to hydrolyze their bound GTP into GDP, and in doing so, they become inactivated.

Another method used to turn off the pathway is through the combined action of beta-adrenergic receptor kinase and beta-arrestin. In this case, beta-adrenergic receptor kinase phosphorylates the receptor. This, in turn, attracts beta-arrestin to the receptor, which then physically blocks the receptor from interacting with G Proteins.

Additionally, the pathway can be turned once the extracellular ligand has fallen to a low concentration.

Yet another way that the pathway can be turned off is by the reduction in the second messenger cAMP. This is accomplished by the activity of a class of enzymes called the phosphodiesterases.

And lastly, phosphatases are enzymes which remove phosphate groups from their targets. As was mentioned above, the phosphorylation of GPCRs results in their inactivation. Therefore, if phosphate groups were to be removed from them, this would tend to have the opposite effect of activating them. Therefore, the action of phosphatases on GPCRs would not turn them off.

G protein-linked receptors are, indeed, the most commonly found in prokaryotes, and their polypeptide chain crosses the membrane seven times -- hence the alternate name, serpentine receptor. The G-protein is bound to the interior of the plasma membrane, sometimes, at least, in a complex with the receptor. G proteins do have three subunits, , , and . However, it is  that releases GDP and binds GTP, dissociating the protein, with two resulting parts, an  and a  complex.

When a G-protein is activated (in its ATP bound state), the alpha subunit dissociates from the beta and gamma subunits and binds to the effector enzyme for further activation and signal amplification downstream. For example, when adrenaline binds to the beta-andrenergic receptor, the alpha subnit dissociates from the beta+gamma subunits and activates adenylyl cyclase, which then produces cAMP, signaling for downstream protein targets to be phosphorylated. 

In its unactivated state, a G protein is present as a heterotrimer consisting of an alpha, a beta, and a gamma subunit.  This heterotrimer is bound to GDP.  Upon activation by conversion of GDP to GTP, the G protein will dissociate into an alpha subunit separated from the beta and gamma unit (these two are still connected).

In its unactivated state, a G protein is present as a heterotrimer consisting of an alpha, a beta, and a gamma subunit. This heterotrimer is bound to GDP. Upon activation by conversion of GDP to GTP, the G protein will dissociate into an alpha subunit separated from the beta and gamma unit (these two are still connected).

The alpha subunit of a G protein has intrinsic GTPase activity that, although slow, will automatically convert GTP back to GDP when the action of the G protein has finished.