The Escherichia coli GTPase CgtA E Cofractionates with the 50S Ribosomal Subunit and Interacts with SpoT, a ppGpp Synthetase/Hydrolase

Abstract

ABSTRACT CgtA E /Obg E /YhbZ is an Escherichia coli guanine nucleotide binding protein of the Obg/GTP1 subfamily whose members have been implicated in a number of cellular functions including GTP-GDP sensing, sporulation initiation, and translation. Here we describe a kinetic analysis of CgtA E with guanine nucleotides and show that its properties are similar to those of the Caulobacter crescentus homolog CgtA C . CgtA E binds both GTP and GDP with moderate affinity, shows high guanine nucleotide exchange rate constants for both nucleotides, and has a relatively low GTP hydrolysis rate. We show that CgtA E is associated predominantly with the 50S ribosomal subunit. Interestingly, CgtA E copurifies with SpoT, a ribosome-associated ppGpp hydrolase/synthetase involved in the stress response. The interaction between CgtA E and SpoT was confirmed by reciprocal coprecipitation experiments and by two-hybrid assays. These studies raise the possibility that the ribosome-associated CgtA E is involved in the SpoT-mediated stress response.