Purification of a Streptococcus mutans protein that binds to heart tissue and glycosaminoglycans

Abstract

Proteins of Streptococcus mutans MT703 were isolated by differential filtration from chemically defined culture medium following growth of the bacteria. Incubation of this preparation with cryostat-cut sections of fresh rabbit cardiac muscle resulted in deposition of streptococcal components on basement membranes of sarcolemmal sheaths and capillary walls, as indicated by indirect immunofluorescence assay. Binding of radioiodinated streptococcal proteins to heart in vitro was time dependent and saturable. Unlabeled S. mutans proteins competitively inhibited 72% of heart binding by the radiolabeled proteins, indicating a high level of binding specificity. A selection of components common to tissue basement membranes was tested for their abilities to inhibit the binding of streptococcal proteins to heart tissue. Of the glycosaminoglycans, heparin was the most effective inhibitor, followed by heparan sulfate and chondroitin sulfate. Hyaluronic acid was not inhibitory. Of the glycoproteins tested, laminin and collagen type IV were weakly inhibitory, whereas fibronectin was ineffective. A single polypeptide was purified to homogeneity by affinity chromatography on a column of heparin-agarose. Gel filtration chromatography of the purified protein under nondissociating conditions showed a single component at 31 kilodaltons (kDa), whereas in sodium dodecyl sulfate-polyacrylamide gel electrophoresis one band appeared at 8 kDa. This indicates that the tissue-binding protein may either be a linear polypeptide or be released into the environment by the bacterium as a tetramer of the 8-kDa polypeptide. The purified protein had an isoelectric point of 9.5 and showed binding activity for basement membranes in thin sections of heart. Chemical analyses of the purified binding protein showed it to have high contents of lysine and alanine and to be devoid of half-cystine, methionine, tyrosine, histidine, and both neutral and amino sugars.