Generation and Surface Localization of Intact M Protein inStreptococcus pyogenesAre Dependent onsagA

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Abstract
The M protein is an important surface-located virulence factor ofStreptococcuspyogenes, the group A streptococcus (GAS). Expression of M protein is primarily controlled by Mga, a transcriptional activator protein. A recent report suggested that thesaglocus, which includes nine genes necessary and sufficient for production of streptolysin S, another GAS virulence factor, is also needed for transcription ofemm, encoding the M protein (Z. Li, D. D. Sledjeski, B. Kreikemeyer, A. Podbielski, and M. D. Boyle, J. Bacteriol. 181:6019–6027, 1999). To investigate this in more detail, we constructed an insertion-deletion mutation insagA, the first gene in thesaglocus, in the M6 strain JRS4. The resulting strain, JRS470, produced no detectable streptolysin S and showed a drastic reduction in cell surface-associated M protein, as measured by cell aggregation and Western blot analysis. However, transcription of theemmgene was unaffected by thesagAmutation. Detailed analysis with monoclonal antibodies and an antipeptide antibody showed that the M protein in thesagAmutant strain was truncated so that it lacks the C-repeat region and the C-terminal domain required for anchoring it to the cell surface. This truncated M protein was largely found, as expected, in the culture supernatant. Lack of surface-located M protein made thesagAmutant strain susceptible to phagocytosis. Thus, althoughsagAdoes not affect transcription of the M6 protein gene, it is needed for the surface localization of this important virulence factor.

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