Invariant proteins associated with guinea-pig Ia antigens

Analysis of guinea-pig Ia immunoprecipitates by two-dimensional gel electrophoresis demonstrated the specific association of Ia molecules with several types of invariant proteins. These include a 33,000 mol. wt basic protein homologous to murine invariant chain (Ii), and a set of 34,000-36,000 mol. wt proteins more acidic than Ii (acidic invariant chain). Two 23,000-25,000 mol. wt non-polymorphic proteins with pIs of 6.0 and 6.5 were also observed in association with Ia, as was a basic protein of mol. wt 42,000. Pulse/chase studies using [35S]methionine demonstrated that Ii, but not acidic invariant chain, was associated with newly synthesized Ia molecules. The amount of 35S-Ii decreased greatly throughout the chase period. 35S-acidic invariant chain was clearly present in Ia precipitates by 30 min after Ia synthesis, but was not detected 4 hr after synthesis. Only acidic invariant chain was associated with mature Ia antigens bound by the lectin Ricinus communis I. Our results indicate that guinea-pig invariant proteins are differentially bound by Ia molecules during maturation of Ia alpha- and beta-chains, and suggest that acidic invariant chain could be a processed form of Ii.