Effects of valency on thermodynamic parameters of specific membrane interactions

We have measured the equilibrium binding of dioleoylphosphatidylcholine vesicles (800-A diameter) containing various densities of incorporated palmitoyl-alpha-bungarotoxin (PBGT) to acetylcholine receptor (AchR) enriched microsac membranes. We have previously shown that these PBGT vesicles bind specifically to the microsacs mediated by direct interactions with the AchRs [Grant, S. W., Babbitt, B. P., West, L. K., & Huang, L. (1982) Biochemistry 21, 1274-1279]. The percent binding of liposomal lipid and associated PBGT to excess AchR sites, as well as the inhibition of binding by pretreatment of microsacs with excess alpha-bungarotoxin (alpha BGT), was strongly dependent upon the protein/lipid molar ratio of the vesicles. In addition, there existed a threshold level of approximately six PBGT molecules per vesicle at which the binding increased dramatically. The apparent association constant, KAapp, for lipid vesicle-microsac membrane binding increased approximately 4800-fold (from 3.95 X 10(4) to 1.90 X 10(8) M-1) due to an increase of 20-fold in the vesicle-associated PBGT surface density. Direct competition for binding to microsac membranes between vesicles with different PBGT/lipid molar ratios indicated that multivalent binders could easily replace binders of lower valency when receptor sites were limited. Measurement of the temperature dependence of the KAapp indicated that weak (low valency) and medium strength (intermediate valency) PBGT vesicle binders bound to microsacs in a fashion similar to the binding of alpha BGT and PBGT to detergent-solubilized AchRs. Strong PBGT vesicle binders (high valency) appear to bind by a somewhat different mechanism. All results are discussed in terms of the effects of ligand (PBGT) valency on the binding strength of vesicles to microsac membranes.