SecA2 is distinct from SecA in immunogenic specificity, subcellular distribution and requirement for membrane anchoring in Streptococcus parasanguis

A secA2 gene is present in the genomes of a wide variety of Gram-positive bacteria. In Streptococcus parasanguis, a primary colonizer of the tooth surface, secA2 is involved in the secretion of a small group of proteins including the fimbrial adhesin, Fap1. Although the substrate specificity is different, SecA2 is predicted to be similar to SecA in structure and function based on the homology between these two proteins. In this study, polyclonal antibodies against SecA2 and SecA did not cross-react with each other, indicating that these two proteins possessed distinct immunogenic epitopes. Fractionation analysis demonstrated that SecA2 was not evenly distributed between the cytoplasmic membrane and the cytoplasm as was noted for SecA. SecA2 was associated with the membrane in the wild type and in secA2 mutants with different regions deleted. The subcellular distribution of SecA2 was not dependent on secY2, suggesting that the membrane association is not through SecY2. These data suggested that SecA2 is distinct from SecA in many respects such as substrate specificity, immunogenic specificity, subcellular distribution and requirement for membrane anchoring.