Novel complex formed between a nonproteolytic cell wall protein of group A streptococci and alpha 2-macroglobulin.

Binding of 125I-labeled alpha 2-macroglobulin (alpha 2M) to streptococci belonging to serological groups A, B, C, and G was studied. Streptococci of groups A and G interacted only with native alpha 2M, and those of group C reacted only with alpha 2M-trypsin complex. Binding of alpha 2M to group A streptococci was saturable and reversible. The dissociation constant was 2.02 X 10(-7) M, and the number of binding sites was calculated to be 18,000 per streptococcus. The alpha 2M-binding protein could be solubilized by treatment of group A streptococci with a murolytic enzyme and subsequently purified by affinity chromatography and high-pressure liquid chromatography. The purified protein was homogeneous on sodium dodecyl sulfate-polyacrylamide gel electrophoresis and had a molecular weight of 78,000. It possessed no proteolytic activity and interacted with native alpha 2M in Western blots (immunoblots). Interaction of purified binding protein with alpha 2M led to a change in the conformation of alpha 2M similar to that obtained by alpha 2M-protease complexes. Reversible binding of a nonproteolytic streptococcal component of alpha 2M is thus a novel feature of alpha 2M reactivity.