WHEN AN ATPASE IS NOT AN ATPASE - AT LOW-TEMPERATURES THE C-TERMINAL DOMAIN OF THE ABC TRANSPORTER CVAB IS A GTPASE

ATP-binding cassette (ABC) transporters belong to a large superfamily of proteins which share a common function and a common nucleotide-bind ing domain. The CvaB protein from Escherichia coli is a member of the bacterial ABC exporter subfamily and is essential for the export of th e peptide antibiotic colicin V. Here me report that, surprisingly, the CvaB carboxyl-terminal nucleotide-binding domain (BCTD) can be prefer entially cross-linked to GTP but not to ATP at low temperatures. The c ross-linking is Mg2+ and Mn2+ dependent. However, BCTD possesses simil ar GTPase and ATPase activities at 37 degrees C, with the same kinetic parameters and with similar responses to inhibitors, Moreover, a poin t mutation (D654H) in CvaB that completely abolishes colicin V secreti on severely impairs both GTPase and ATPase activities in the correspon ding BCTD, indicating that the two activities are from the same enzyme . Interestingly, hydrolysis activity of ATP is much more cold sensitiv e than that of GTP: BCTD possesses mainly GTP hydrolysis activity at 1 0 degrees C, consistent with the cross-linking results. These findings suggest a novel mechanism for an ABC protein-mediated transport with specificity for GTP hydrolysis.