Alterations in the extracellular domain of M13 procoat protein make its membrane insertion dependent on secA and secY.

The products of genes secA and secY (SecA and SecY) are putative components of a bacterial protein export machinery and are required for the export of many periplasmic and membrane proteins. Only a few proteins, among them the M13 procoat protein, insert independently of SecA and SecY. To investigate the reason why the procoat protein inserts independently of sec functions, various hybrid proteins were constructed. By in-frame gene fusions the central procoat region, which translocates across the membrane, was extended in size. Fragments of the ompA gene ranging from 522-294 bp were ligated with the procoat gene. The hybrid proteins were inserted into the membrane and processed normally, but only in the presence of functional SecA and SecY.