Multiple SecA protein isoforms in Escherichia coli.

To define the anti-SecA-LacZ antiserum, immunoprecipitates produced with either whole anti-SecA-LacZ rabbit antiserum or affinity-purified antibodies were used to analyze nondenatured lysates of Escherichia coli. The antiserum contains antibodies that recognize different proteins. Antibody purified by preadsorption to the SecA-LacZ hybrid protein precipitated only the SecA protein from extracts. In contrast, antibody purified from the intact SecA protein precipitated several additional proteins with SecA protein. Ribosomal protein L7L12 is one of the polypeptides coprecipitated with SecA protein by antibody purified by immunoadsorption to the intact SecA protein as well as by unfractionated anti-SecA-LacZ antiserum. Two-dimensional gel electrophoresis of the SecA protein immunoprecipitated by either antiserum or purified antibody indicated that the SecA protein exists in at least two, and probably four, isoforms. Only one of the SecA isoforms is present in a ribosomal preparation.