Ribosome-independent GTPase activity of translation initiation factor IF2 and of its G-domain

In the absence of ribosomes, Bacillus stearothermophilus translation initiation factor IF2 (Mr = 82 kDa) and its GTP-binding domain (i.e. the G-domain, Mr = 41 kDa) promote barely detectable hydrolysis of GTP. Upon addition of some aliphatic alcohols, however, the rate of nucleotide cleavage is substantially increased with both IF2 and G-domain, the highest stimulation being observed with 20% (v/v) ethanol. Under these conditions, the rates of ribosome-independent GTP hydrolysis with both IF2 and G-domain are approximately 30-fold lower than the corresponding rates obtained in the presence of ribosomes, while the Km for GTP is approximately the same in all cases. These results indicate that, as with the other two prokaryotic G proteins involved in translation (i.e. elongation factors EF-Tu and EF-G), also in the case of IF2, the GTPase catalytic center resides in the factor and, more specifically, in its G-domain.