33 AMINO-ACIDS OF THE MATURE MOIETY OF AN UNPROCESSED MALTOSE-BINDINGPROTEIN ARE SUFFICIENT FOR EXPORT IN ESCHERICHIA-COLI

Maltose-binding protein (MBP) is translocated across the cytoplasmic m embrane of Escherichia coli; successful export depends on information in both the signal peptide and the mature moiety of the protein. To de termine the shortest portion of the mature region that would maintain detectable entry of MBP into the export pathway, we took advantage of the properties of an MBP species with proline substituted in the +1 po sition relative to the cleavage site (MBP27-P). This protein efficient ly crosses the cytoplasmic membrane but is not processed and acts as a competitive inhibitor of signal peptidase 1 (leader peptidase). Expor t of MBP27-P is measured by the inhibition of processing of other prot eins, such as ribose-binding protein (RBP). A series of truncated deri vatives of MBP27-P were tested for the ability to inhibit processing o f RBP. An MBP27-P species with only 33 amino acids of the mature moiet y inhibited processing of RBP, indicating that this truncated polypept ide was probably exported and interacted nith signal peptidase I.