Interaction of alpha-agglutinin and a-agglutinin, Saccharomyces cerevisiaesexual cell adhesion molecules

alpha -Agglutinin and a-agglutinin are complementary cell adhesion glycopro teins active during mating in the yeast Saccharomyces cerevisiae. They bind with high affinity and high specificity: cells of opposite mating types ar e irreversibly bound by a few pairs of agglutinins. Equilibrium and surface plasmon resonance kinetic analyses showed that the purified binding region of alpha -agglutinin interacted similarly with purified a-agglutinin and w ith a-agglutinin expressed on cell surfaces. At 20 degreesC, the K-D for th e interaction was 2 x 10(-9) to 5 x 10(-9) M, This high affinity was a resu lt of a very low dissociation rate (approximate to 2.6 x 10(-4) s(-1)) coup led with a low association rate (= 5 x 10(4) M-1 s(-1)), Circular-dichroism spectroscopy shelved that binding of the proteins was accompanied by measu rable changes in secondary structure. Furthermore, when binding was assesse d at 10 degreesC, the association kinetics were sigmoidal, with a very low initial rate, An induced-fit model of binding with substantial apposition o f hydrophobic surfaces on the two ligands can explain the observed affinity , kinetics, and specificity and the conformational effects of the binding r eaction.