ROLE OF THE PROPILIN LEADER PEPTIDE IN THE MATURATION OF F-PILIN

F-pilin maturation and translocation result in the cleavage of a 51-am ino-acid leader sequence from propilin and require LepB and TraQ but n ot the SecA-SecY secretion pathway, The unusual propilin lender peptid e and the dependence of its cleavage on TraQ suggested that TraQ recog nition may be specific for the leader peptide, An in vitro propilin cl eavage assay yielded propilin (13 kDa), the pilin polypeptide (7 kDa), and a 5.5-M)a protein as the traA products. The 5.5-kDa protein comig rates with the full-length 51-amino-acid leader peptide, and [C-14] pr oline labeling confirmed its identity since the only proline residues of propilin are found within the leader peptide. The in vitro and in v ivo propilin-processing reactions proceed similarly in a single polype ptide cleavage step, Furthermore, TraQ dependence is a property of F-p ilin maturation specifically rather than a property of the leader pept ide. A propilin derivative with an amino-terminal signal sequence gene rated by deleting codons 2 to 28 required TraQ for processing in vivo. On the other hand, a chimeric protein with the propilin wild-type lea der peptide fused to the mature portion of beta-lactamase was processe d in a TraQ-independent manner, Thus, despite its unusual length, the propilin leader peptide seems to perform a function similar to that of the typical amino-terminal signal sequence. This work suggests that T raQ is not necessary for the proteolysis of propilin and therefore is likely to act as a chaperone-like protein that promotes the translocat ion of propilin.