Role of a small cytoplasmic domain in the establishment of serine chemoreceptor membrane topology.

The Escherichia coli serine chemoreceptor takes on a simple membrane topology with two transmembrane segments separating cytoplasmically disposed N and C termini from a central periplasmic domain. We investigated the role of the small N-terminal cytoplasmic domain in membrane insertion using alkaline phosphatase gene fusions. Mutations eliminating the positive charge of the domain altered insertion dramatically, with reciprocal effects on hybrids with periplasmic and C-terminal cytoplasmic fusion junctions. Efficient export of the normally cytoplasmic C-terminal domain required that, in addition to the N-terminal changes, a short amphiphatic sequence at the beginning of the C-terminal domain be also absent. These findings document the importance of the positive character of the N-terminal domain in chemoreceptor membrane insertion and imply that partially redundant sequence information controls the orientation of the second transmembrane segment.