Mutant enzymes challenge all assumptions

Abstract

Enzymes called Rab GTPases that carry so-called "activating" mutations may never become activated at all.

Keywords: Membrane traffic; Rab GTPase; nucleotide exchange factor.

Figure 1.. The switch II glutamine residue plays diverse roles in Rab1.

(A) When the Rab1b enzyme (shown in grey) interacts with the GAP protein TBC1D20 (light blue; PDB 4HLQ), the switch II glutamine residue (Q67) is oriented away from the active site (which is at the centre of the figure). This means that it is not directly involved in the GAP-catalysed hydrolysis of GTP, but is still required for the interaction between the enzyme and the GAP protein (Gavriljuk et al., 2012). (B) When Rab1b interacts with the GAP protein LepB (PDB: 4I1O), the switch II glutamine residue is oriented toward the active site by residues that belong to the enzyme and the GAP protein (Gazdag et al., 2013; Mishra et al., 2013). (C) When the Ypt1p enzyme (which is the yeast equivalent of Rab1b) interacts with the guanine nucleotide exchange factor (GEF) called TRAPP (PDB: 3CUE), the switch II glutamine residue is oriented away from the nucleotide binding site of the enzyme, which is near a lysine residue (K21). This residue is stabilized by an interaction with a glutamic acid residue (E192) on the Bet3p subunit of the exchange factor, leading to the displacement of GDP from the nucleotide binding site. (D) When the Rab1b enzyme interacts with the exchange factor DrrA (PDB: 3JZA), the switch II glutamine residue is oriented toward the lysine residue. The interaction between these glutamine and lysine residues is further stabilized by interaction with an aspartate residue (D63) on Rab1b. GAP: GTPase-activating protein.