pH-regulated secretion of a glyceraldehyde-3-phosphate dehydrogenase from Streptococcus gordonii FSS2: Purification, characterization, and cloning ofthe gene encoding this enzyme

Streptococcus gordonii and other viridans streptococci (VS) are primary eti ologic agents of infective endocarditis, despite being part of the normal o ral microflora. Recently, a surface-bound glyceraldehyde-3-phosphate dehydr ogenase (GAPDH) has been found on the cells of all tested streptococcal spe cies, where it has been implicated as a virulence factor. In contrast, we o bserved that a soluble extracellular GAPDH was the major secreted protein f rom S. gordonii FSS2, an endocarditis strain. The biochemical properties an d gene sequence of S. gordonii GAPDH are almost identical to those of other streptococcal GAPDHs. Growth at defined pHs showed that secretion of GAPDH is regulated by environmental pH. GAPDH was primarily surface-associated a t growth pH 6.5 and shifted to > 90% secreted at growth pH 7.5. Others have identified S. gordonii promoters that are upregulated by a pH shift simila r to that experienced by organisms entering the blood stream (neutral) from the oral cavity (slightly acid). Analysis of our results suggests that sec retion of GAPDH may be a similar adaptation by S. gordonii.