Discoidin I-membrane interactions I. Discoidin I binds to two types of receptor on fixed Dictyostelium discoideum cells

Under physiological buffer conditions (17 mM P_i, pH 6.3), the endogenous lectin of Dictyostelium discoideum, discoidin I, binds to two types of receptors on the surface of glutaraldehyde-fixed, wild-type (NC-4) D. discoideum cells. We have designated these two types of receptors the carbohydrate or C sites and the ionic or I sites. Binding to the C sites is saturable with respect to discoidin I and is inhibited by hapten sugars (such as $\text{N-}\text{acetyl}\text{-}\text{d}\text{-}\text{galactosamine}$), but not by increasing buffer ionic strength with NaCl or polyelectrolytes. The number of C sites increases about 4-fold during the first 8.5 h of suspension differentiation, reaching a capacity for about 2–10⁴ discoidin I tetramers per cell. The binding activity of the C sites is reduced about 50% by sequential NaIO₄ oxidation/NaBH₄ reduction of the fixed cells, but it is not reduced by CHCl₃-CH₃OH extraction of the fixed cells. In marked contrast, binding to the I sites appears nonsaturable with respect to discoidin I, and it is inhibited by increasing buffer ionic strength with NaCl or polyelectrolytes (such as poly-l-glutamic acid or heparin), but not by hapten sugars. The I sites are present on both vegetative and differentiated fixed cells and can bind more than 10⁶ discoidin I tetramers per cell. The binding activity of the I sites on fixed cells is not reduced by sequential NaIO₄ oxidation/NaBH₄ reduction, but is reduced 70 to 90% by CHCl₃-CH₃OH extraction. The data suggest that the I sites represent ionic lipids that bind discoidin I electrostatically.