Direct interaction of the EpsL and EpsM proteins of the general secretion apparatus in Vibrio cholerae

The general secretion pathway of gram-negative bacteria is responsible for extracellular secretion of a number of different proteins, including protea ses and toxins. This pathway supports secretion of proteins across the cell envelope in two distinct steps, in which the second step, involving transl ocation through the outer membrane, is assisted by at least 13 different ge ne products. Two of these components, the cytoplasmic membrane proteins Eps L and EpsM of Vibrio cholerae, have been purified and characterized. Based on gel filtration analysis, both purified EpsM((His)6) and wild-type EpsL p resent in an Escherichia call Triton X-100 extract are dimeric proteins. Ep sL and EpsM were also found to interact directly and form a Triton X-100 st able complex that could be precipitated with either anti-EpsL or anti-EpsM antibodies. In addition, when the L and M proteins were coexpressed in E. c oli, they formed a stable complex and protected each other from proteolytic degradation, indicating that these two proteins interact in vivo and that no other Eps protein is required for their association. Since EpsL is predi cted to contain a large cytoplasmic domain, while EpsM is predominantly exp osed on the periplasmic side, we speculate that these components might be p art of a structure that is involved in bridging the inner and outer membran es. Furthermore, since EpsL has previously been shown to interact with the autophosphorylating cytoplasmic membrane protein EpsE, we hypothesize that this trimolecular complex might be involved in regulating the opening and c losing of the secretion pore and/or transducing energy to the site of outer membrane translocation.