ProOmpA is stabilized for membrane translocation by either purified E. coli trigger factor or canine signal recognition particle

We have isolated large amounts of E. coli outer-membrane protein A precursor (proOmpA). Purified proOmpA is active in membrane assembly, and this assembly is saturable with respect to the precursor protein. A proOmpA-Sepharose matrix allows affinity isolation of trigger factor, a soluble, 63,000 dalton monomeric protein that stabilizes proOmpA in assembly competent form. Comparison of trigger factor's amino-terminal sequence with those in a computer data bank and with those encoded by sec genes, as well as groEL and heat shock gene dnaK, suggests that trigger factor is encoded by a previously undescribed gene. Trigger factor and proOmpA form a 1:1 complex that can be isolated by gel filtration. Purified canine signal recognition particle (SRP) can also stabilize proOmpA for membrane insertion. This postribosomal activity of SRP suggests a unifying theme in protein translocation mechanisms.