Effect of mutations in the general secretory pathway on outer membrane protein and surface layer assembly in Aeromonas spp.

Abstract

Mutations in the exeC-N operon of Aeromonas hydrophila, which block extracellular protein secretion, also result in large decreases in the level of the major outer membrane porin, protein II. Immunoblot analysis demonstrated that the porin missing from the outer membrane of the mutant was not accumulating elsewhere in the cell. Pulse-chase and immunoprecipitation analyses showed that the porin was as stable in the mutant as in the wild type, but that far less porin was synthesized in the exe mutants. The relationship between extracellular secretion involving the exe genes and the assembly of other outer membrane and surface proteins was also examined. Both the wild type and exeE mutants of A. hydrophila were capable of assembling the protein I and protein III porins under inducing conditions. Aeromonas sohria As9071, which contains a surface array protein, could secrete A. hydrophila aerolysin and required homologues of the A. hydrophila exe genes to do so; however, an exe− derivative of this bacteria was unaffected in its ability to assemble its surface array. These results demonstrate that the exe genes are not required for general outer membrane protein assembly in these bacteria, but that the synthesis of protein II is specifically downregulated in the exe mutants.Key words: extracellular secretion, outer membrane assembly, surface layer, Aeromonas.