Partitioning of a-factor analogues into membranes: analysis of binding and importance for biological activity

Analogues of the a-factor mating pheromone of the yeast Saccharomyces cerevisiae were used to measure interactions of the pheromones with lipid vesicles and with isolated yeast membranes. The binding of the analogues of a-factor to vesicles and to membranes was best described as a partitioning of the pheromones into the lipid phase. The partitioning was enhanced by the negative surface potential of the membranes and was well described by the Gouy-Chapman theory of diffuse double layers. From the analysis of the binding of the pheromones to synthetic vesicles of known surface potential, effective charges and intrinsic partition coefficients were obtained for the pheromones. The information was used in subsequent experiments with yeast membranes to determine the intrinsic partition coefficients of the a-factor analogues and the charge density of the yeast membranes. Derivatives of a-factor with different alkyl chains in place of the normal C-terminal farnesyl displayed biological activity that paralleled the degree of partitioning of the pheromones into vesicles. Demethylation of the C-terminus decreased the partition coefficient by 6-fold and decreased the biological activity of the pheromone by greater than 2500-fold. The results show that a-factor can effectively partition into membrane bilayers and that the partitioning is probably involved in the subsequent recognition of the pheromone by the a-factor receptor.