Export and processing of MalE-LacZ hybrid proteins in Escherichia coli.

Five classes of MalE-LacZ hybrid proteins have previously been characterized. These proteins differ in the amount of the maltose-binding protein (MBP) that is attached to beta-galactosidase. Although none of these proteins is secreted into the periplasm, the four larger classes of hybrid proteins, those that include an intact MBP signal peptide, are inserted into the cytoplasmic membrane, suggesting that the secretion process has at least been initiated. In this study, we demonstrated that some portion of the four larger hybrid proteins can be translocated across the cytoplasmic membrane, thus permitting processing of the signal peptide. We have found that hybrid proteins that include only a small portion of the mature MBP are inefficiently recognized as exported proteins, and translocation and processing of these appear to be relatively slow, posttranslational events. In marked contrast, hybrid proteins that include a substantial portion of the mature MBP are efficiently recognized, and translocation and processing of these occur very rapidly, possibly cotranslationally. Our results complement other studies and very strongly suggest a role for the mature MBP in the export process.