A carboxyl-terminal four-amino acid motif is required for secretion of the metalloprotease PrtG through the Erwinia chrysanthemi protease secretion pathway.

PrtG is an extracellular metalloprotease secreted by the Gram-negative bacterium Erwinia chrysanthemi through a signal peptide-independent secretion pathway. Previous studies showed that the PrtG secretion signal is COOH-terminal and located in the last 56 residues of PrtG. We have now performed a deletion and elongation mapping of a short secretion competent COOH-terminal peptide CterG. This approach allowed us to show that: (i) the smaller COOH-terminal sequence containing the information necessary to promote the secretion of a small polypeptide is contained in the last 29 residues of PrtG; (ii) a low but significant level of secretion can be promoted by the last 15 residues of PrtG when fused to the COOH terminus of a non-secreted PrtG derivative; (iii) the extreme COOH-terminal sequence Dxxx, where xs are hydrophobic residues, is a conserved motif in all constructs that are secreted through the E. chrysanthemi transporter. (vi) This motif has to be COOH terminally exposed since addition of even one amino acid impairs the secretion of CterG. The extent of the secretion defect observed with the COOH terminally extended variants correlates with the length of the extension. These results indicate a key role for the COOH-terminal exposition of the last four amino acids in the secretion of PrtG.