Evidence that SecB enhances the activity of SecA

In Escherichia coli, SecA is a critical component of the protein transport machinery which powers the translocation process by hydrolyzing ATP and rec ognizing signal peptides which are the earmark of secretory proteins. In co ntrast, SecB is utilized by only a subset of preproteins to prevent their p remature folding and chaperone them to membrane-bound SecA. Using purified components and synthetic signal peptides, we have studied the interaction o f SecB with SecA and with SecA-signal peptide complexes in vitro. Using a c hemical cross-linking approach, we find that the formation of SecA-SecB com plexes is accompanied by a decrease in the level of cross-linking of SecA d imers, suggesting that SecB induces a conformational change in SecA. Furthe rmore, functional signal peptides, but not dysfunctional ones, promote the formation of SecA-SecB complexes. SecB is also shown to directly enhance th e ATPase activity of SecA in a concentration-dependent and saturable manner . To determine the biological consequence of this finding, the influence of SecB on the signal peptide-stimulated SecA/lipid ATPase was studied using synthetic peptides of varying hydrophobicity. Interestingly, the presence o f SecB can sufficiently boost the response of signal peptides with moderate hydrophobicity such that it is comparable to the activity generated by a m ore hydrophobic peptide in the absence of SecB. The results suggest that Se cB directly enhances the activity of SecA and provide a biochemical basis f or the enhanced transport efficiency of preproteins in the presence of SecB in vivo.