Discoidin I-membrane interactions II. Discoidin I binds to and agglutinates negatively charged phospholipid vesicles

The endogenous lectin of Dictyostelium discoideum, discoidin I, binds to multilamellar phosphatidylcholine vesicles containing a net negatively charged lipid constituent. This constituent can be a phospholipid or a fatty acid. The amount of binding observed correlates directly with the magnitude of the net charge on the negatively charged constituent and its mole fraction in the vesicles. The binding exhibits a time course and a specificity similar to those observed previously (Bartles J.R. and Frazier, W.A. (1982) Biochim. Biophys. Acta 687, 121-128) for binding to the I sites on fixed D. discoideum cells. The best inhibitors of binding are those components, such as NaCl and polyelectrolytes, which serve to increase the effective ionic strength of the buffer and thereby attenuate the electrostatic interaction between positively charged domains on the discoidin I tetramer and the negatively charged vesicle surface. Binding to the vesicles is multivalent, exhibits apparent positive cooperativity with respect to discoidin I, and appears to be modulated allosterically by hapten sugars of discoidin I. Discoidin I can cause the agglutination of sonicated negatively charged phospholipid vesicles. Vesicle agglutination requires a concentration of discoidin I above a certain threshold, which is dictated by vesicle composition and concentration. Vesicle agglutination is not observed with equivalent or higher concentrations of concanavalin A or bovine serum albumin. The rate and extent of vesicle agglutination are reduced by inhibitors of discoidin I binding to the phospholipid vesicles. Thus, discoidin I has the ability to bind to and to agglutinate negatively charged membranous structures by an electrostatic mechanism that does not rely on the carbohydrate binding activity of the lectin.