Question

Which answer is FALSE. The Ras protein, a prototype for all G proteins:

  

A. must bind a GTPase activator protein for hydrolysis of GTP to GDP

   

B. has two critical residues that H-bond with the γ-phosphate of GTP

   

C. holds the γ-phosphate of GTP in the P loop

Answer 1

Ras is a member of small GTPase protein family. It has only one subunit as compared to trimeric G proteins. It acts as a molecular switch, activating proteins when on. It has two domains; one GDP/GTP binding domain and a membrane targetting domain. It binds to GTP on activation by GTPase activating proteins.

A) True

The G domain has five motifs. The G1( also called the P loop) motif binds to the beta phosphate of GTP/GDP .The two switch motifs( G2 and G3) are the main part of the protein. The G2 motif binds to the gamma phosphate(terminal) of GTP. The threonin35 and gly60 residue in the G2 motif binds to the gamma phosphate of GTP. The G3 motif provides specificity for GTP and also play role in hydrolysis of GTP.

B) True

The gamma phosphate of the GTP does not bind to the P loop. It binds into the G2 motif of the Ras protein.

C) False