The role of SecA2 in targeting substrates to the Sec-dependent protein translocase

Abstract

The Sec pathway is the major route for secretion of proteins by bacteria. Its major components are: (i) the translocation channel comprising the SecYEG proteins; (ii) SecA: a chaperone/targeting/molecular motor that drives the movement of proteins across the membrane with participation of SecB or SRP (signal recognition particle) chaperones. Following translocation, proteins are folded to their mature conformation by folding factors such as PrsA. When compared with B. subtilis, the secretory translocase of B. anthracis contains homologues of several Sec pathway components: (i) two homologues of SecA (SecA1 and SecA2), (ii) two homologues of SecY (SecY1 and SecY2), (iii) three homologues of the PrsA foldase (PrsAA, PrsAB and PrsAC). In previous studies, SecA2 was shown to be specific for secretion of S-layer proteins: Sap and EA1, while SecY2 has not been shown to have any substrate specificity. Instead, it seems to ensure high levels of protein secretion in later phases of the growth cycle. A combination of approaches was used to continue the analysis of functioning of the SecA2 secretion pathway. They involved analysis of deletion mutants of B. anthracis, complementation studies, gene expression analysis, protein interaction investigation. We found that a novel protein BA0881 facilitates secretion of Sap and EA1, but is not essential for the processes, and was renamed SecH. Moreover, protein interaction and complementation studies revealed putative interactions between (i) SecA2 and EA1, (ii) SecA2 and SecH, (iii) EA1 and SecH, (iv) SecA1 and SecA2, raising the possibility that SecA2 and SecA1 form a dimer, which might be a functional entity for the secretion of Sap and EA1, with SecH having a role in enhancing interaction between SecA1/SecA2 dimer and its substrates. The role of PrsA-like foldases on secretion was also investigated. PrsAB was found to show substrate specificity for Sap and EA1, while PrsAA showed substrate specificity for penicillin binding proteins. Analysis of cell morphology suggests that PrsAB and SecA2 may also have substrates other than those of Sap and EA1 as their null-mutants show changes in cell length and shape. Lastly, analysis of gene expression showed that the deletion of genes encoding elements of the translocation sytem: SecA2, SecH, PrsAA, PrsAB, PrsAC, SecY2 leads to changes in the level of expression of sap and eag.