SfbII protein, a fibronectin binding surface protein of group A streptococci, is a serum opacity factor with high serotype-specific apolipoproteinaseactivity

Serum opacity factor (SOF) is produced by group A streptococci belonging to certain M types. SOF cleaves the apolipoprotein component of the high dens ity lipoprotein fraction of serum rendering it insoluble which in turn lead s to serum opacity. SfbII protein, a fibronectin binding surface protein cl oned from group A streptococci, was obtained from a strain of M75. Here we show that this protein has a second functional domain responsible for SOF a ctivity. The fibronectin binding region was located in the C-terminal end o f the protein. Deletion analysis showed that the remainder of the protein w as required for SOF activity. Sequence analysis of SfbII, when compared wit h the published sequence of SOF22, showed 99% identity with a difference of only four amino acids. In spite of this high homology, SOF from M75 was ty pe-specific and antibody evoked specifically inhibited only SOF produced by M75. Antibodies found in human serum following natural infection also inhi bited the SOF of SFbII in a type-specific manner. The results showed that t he SfbII protein from M75 is SOF with a high serotype-specific enzyme activ ity. (C) 1999 Federation of European Microbiological Societies. Published b y Elsevier Science B.V. All rights reserved.