The opacity factor of group-A streptococci.

Cell-bound opacity factor (OF) was extracted with sodium dodecyl sulphate (SDS) to yield stable extracts with titres of greater than 20 000. The mol.-wt distributions of extracellular and SDS-extracted OF, determined by ultrafiltration or chromatography on Sepharose 4B, suggested that the high mol. wt (1 x 10(6)) of extracellular OF is due to aggregation, because cell-bound and extracellular OF in the presence of SDS had an average mol. wt of only 2 x 10(5). At least four apparent multiple-molecular forms (mol. wt 7.4-12.0 x 10(4)) of OF were detected by SDS polyacrylamide-gel electrophoresis. It seemed more probable that 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 x 10(4) but this was supported by the finding that OF can be detected after passing through a dialysis membrane provided that its "substrate", alpha 1-lipoprotein, is present on the other side. This raises the possibility that OF is 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, whereas Triton-extracted OF was the most complex and formed three peaks, the position of which varied depending on whether the detergent was present or absent during focusing runs.