GTPase activity of bacteriophage T4 sheath protein

We show by nuclear magnetic resonance studies that, following GTP hydrolysis during phage T4 sheath contraction, GDP remains bound to the sheath protein (gp18), whereas orthophosphate is released. gp18 in the contracted state has GTPase activity and can hydrolyse exogenous GTP; the reaction is calcium-dependent and displays high substrate specificity. The process comprises two steps: (1) displacement of GDP from gp18 by exogenous GTP, and (2) GTP hydrolysis proper. The first step appears to be rate-limiting and to be accelerated when the nucleotide-protein interaction is mechanically disrupted by sonication.