A lower size limit exists for export of fragments of an outer membrane protein (OmpA) of Escherichia coli K-12.

The ompA gene codes for a 346 residue precursor of a 325 residue protein of the outer membrane of Escherichia coli K-12. Internally and/or COOH-terminally deleted genes were constructed that encode 123, 116, 88, 72 or 68 residue precursors. The former three were processed and localized to the periplasmic space; the latter two were not processed and remained cytosolic. These data suggest that the signal sequence has to interact with a component of the export apparatus (the Sec pathway) before translation is finished. Comparison of these results with others obtained for prokaryotic and eukaryotic systems shows that: (1) a very similar lower size limit exists for membrane translocation of the 147 residue chicken prelysozyme or the 229 residue bovine preprolactin; (2) precursors smaller than those reported here can be translocated in both systems; (3) the latter translocation, in contrast to, for example, the ompA gene products, does not depend on the cellular export machinery but most likely requires folding of the precursors into an export-competent conformation. In general, at least two quite different, not necessarily mutually exclusive, mechanisms for translocation of a protein across or assembly into a membrane appear to exist.