Question

Activation of signal transduction pathways controlled by G-protein coupled Receptors (GPCRs) has been implicated in many forms of cancer. One part of the pathway that seems to be particularly sensitive to mutation are trimeric G proteins.

a. Inactivation of the Ga subunit can occur 100 times faster in the presence of a GAP (GTPase Activating Protein). Explain the function of a GAP and why it will deactivate the Ga subunit of a trimeric G protein being sure to account for specific nucleotides present and change in protein conformation of the G protein? Rubric (4): correct explanation of GAP effects on Ga. Note the changes that occur (nucleotide=2pts), protein structure (2pts) as the protein is turned off.

Answer 1

For controlling cellular functions, the activity of G proteins are regulated by GAP(GTPase Activating Protein) and GEFs (Guanine Exchange Factor). GAPs are best known for downregulating the activity of G protein.

The G protein when bound to GTP, is considered as in 'on' state to proceed further cellular signaling where as when they are bound to GDP are considered in 'off' state. Naturaly, the G protein has the activity of hydrolysing its GTP very slowly into GDP to turn themselves off.

But, when GAP binds to GTP bound G protein, it changes the conformation of the G protein. The new conformation is more susceptible to nucleophilic attack causing GTP to hydrolyse rapidly into GDP. Thus a water molecule attacks the bond between beta and gamma (farthest) phosphate in GTP breaking it to GDP. This turns the G protein inactive and hence terminates the downstream reactions.