Some strains of Streptococcus pyogenes isolated from infected human tissues were shown to bind laminin, a major component of basement membranes. Binding of 125I-laminin to bacteria was time dependent and functionally irreversible. Of several unlabeled proteins tested in competition experiments, laminin and fibrinogen inhibited binding of the radiolabeled protein. The inhibitory effect exerted by fibrinogen was apparently not caused by a binding to the laminin receptors. The number of receptors available for laminin on cells of the strain examined ranged from 0 to 10(3) depending on the media used to grow the bacteria and an apparent KD of 4 X 10(-8)M was calculated for the reaction. Bacterial cells incubated with proteolytic enzymes lose the ability to bind laminin, and a trypsin digest contained active receptors capable of competing with intact cells for 125I-laminin. Active receptors may be adsorbed on a column of laminin-Sepharose but not on Sepharose gels substituted with fibrinogen or fibronectin. After radiolabeling the proteins in the trypsin digest a laminin-binding 125I-labeled protein (Mr greater than 10(6] was isolated by affinity chromatography from a receptor positive strain. Similar components could not be isolated from a strain apparently lacking laminin receptors. Therefore, this protein was tentatively identified as a laminin receptor of streptococci.
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