The Opacity Factor of Group-A Streptococci

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Abstract
Cell-bound opacity factor (OF) was extracted with sodium dodecyl sulfate (SDS) to yield stable extracts with titers of > 20,000. The MW distributions of extracellular and SDS-extracted OF, determined by ultrafiltration or chromatography on Sepharose 4B, suggested that the high MW (1 .times. 106) of extracellular OF is due to aggregation, because cell-bound and extracellular OF in the presence of SDS had an average MW of only 2 .times. 105. At least 4 apparent multiple-molecular forms (MW 7.4-12.0 .times. 104) of Of were detected by SDS polyacrylamide-gel electrophoresis. Evidently, these were due to aggregation rather than the existence of different stable conformations. To explain the molecular size distribution, the subunit would have to be as small as 1 .times. 104; this was supported by the detection of OF after passing through a dialysis membrane, provided that its substrate, .alpha.1-lipoprotein, was present on the other side. Thus, of may be associated with a carrier molecule. The isoelectric-focusing profiles of OF were complex and differed markedly with the method used to prepare OF. Extracellular OF had a simple profile with an isoelectric point of 4.0; Triton-extracted OF was the most complex and formed 3 peaks, the position of which varied depending on whether the detergent was present or absent during focusing runs.