Mutation of a key residue in the type II secretion system ATPase uncouples ATP hydrolysis from protein translocation

Membrane-associated ATPase constitutes an essential element common to all secretion machineries in Gram-negative bacteria. How ATP hydrolysis by these ATPases is coupled to secretion process remains unclear. Here we identified R286 as a key residue in the type II secretion system (T2SS) ATPase XpsE of Xanthomonas campestris that plays a pivotal role in coupling ATP hydrolysis to protein translocation. Mutation of R286 to alanine made XpsE hydrolyse ATP at a rate five times that of the wild-type XpsE. Yet the mutant XpsE(R286A) is non-functional in protein secretion via T2SS. Detailed analyses indicated that the mutant XpsE(R286A) lost the ability co-ordinating the N- and C-domain of XpsE. Without significantly influencing XpsE binding affinity with ATP or its oligomerization, R286A mutation however, caused XpsE lose the ability to associate with the cytoplasmic membrane via XpsL(N). As a consequence, ATP hydrolysis by XpsE was uncoupled from protein secretion. Because R286 is highly conserved among members of the secretion NTPase superfamily, we speculate that its equivalent in other homologues may also play a critical energy coupling role for T2SS, type IV pilus assembly and type IV secretion system.