Several bacterial pathogens including Neisseria gonorrhoeae bind the human serum glycoprotein vitronectin. We aimed at defining the gonococcal receptor for vitronectin. Ligand blots demonstrated that vitronectin bound specifically to the heparin-binding outer-membrane protein OpaA, but that coating OpaA with the sulphated polysaccharide heparin was required for the interaction to occur. Bound vitronectin could be dissociated from OpaA–heparin–vitronectin complexes by the addition of excess heparin, indicating that sulphated polysaccharides provided the main linkage between the two proteins. Binding assays with intact micro-organisms substantiated the requirement of sulphated polysaccharides such as heparin and dextran sulphate for the efficient binding of vitronectin to OpaA+ gonococci. This was underscored by the increased binding of vitronectin to gonococci that had been preincubated with saturating concentrations of dextran sulphate, as opposed to the inhibition of vitronectin binding observed when bacteria were incubated simultaneously with vitronectin and saturating concentrations of dextran sulphate. Binding assays with dextran sulphates of various sizes indicated that vitronectin binding correlated with the size of the polysaccharide rather than with the amount of OpaA produced by the bacteria. The inability of zero-length cross-linking agents to couple vitronectin to OpaA provided further evidence that sulphated polysaccharides formed the linkage between vitronectin and OpaA. Infection experiments demonstrated that proteoglycan-deficient Chinese hamster ovary cells efficiently internalized dextran sulphate/vitronectin-coated gonococci, suggesting that soluble sulphated polysaccharides could substitute for cell surface glycosaminoglycans in the internalization process. On the basis of our results, we propose a novel mechanism of vitronectin binding in which sulphated polysaccharides act as molecular bridges, linking the glycosaminoglycan-binding sites of vitronectin and gonococcal OpaA.

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