Export and sorting of the Escherichia coli outer membrane protein OmpA

Results of studies, mostly using the outer membrane, 325 residue protein OmpA, are reviewed which concern its translocation across the plasma membrane and incorporation into the outer membrane of Escherichia coli. For translocation, neither a unique export signal, acting in a positive fashion within the mature part of the precursor, nor a unique conformation of the precursor is required. Rather, the mature part of a secretory protein has to be export-compatible. Export-incompatibility can be caused by a stretch of 16 (but not 8 or 12) hydrophobic residues, too low a size of the polypeptide (smaller than 75 residue precursors), net positive charge at the N-terminus, or lack of a turn potential at the same site. It is not yet clear whether binding sites for chaperonins (SecB, trigger factor, GroEL) within OmpA are important in vivo. The mechanism of sorting of outer membrane proteins is not yet understood. The membrane part of OmpA, encompassing residues 1 to about 170, it thought to traverse the membrane eight times in antiparallel beta-sheet conformation. At least the structure of the last beta-strand (residues 160-170) is of crucial importance for membrane assembly. It must be amphiphilic or hydrophobic, these properties must extend over at least nine residues, and it must not contain a proline residue at or near its center. Membrane incorporation of OmpA involves a conformational change of the protein and it could be that the last beta-strand initiates folding and assembly in the outer membrane.