Immunogenic Ia-binding peptides immobilize the Ia molecule and facilitate its aggregation on the B cell membrane. Control by the M1s-1 gene.

Aggregation (e.g., through cross-linkage) of cell surface molecules is in various biologic systems a necessary event in cellular activation. Examining the Ia molecule on B cells we found that aggregation is a function of the surface Ag mobility; the higher the fraction of immobile molecules on the plane of the membrane, the better Ia forms aggregates and patches. We identify two factors that control Ia mobility and aggregability. One factor is the M1s-1a gene product; the other factor is an Ia-reactive immunogenic peptide. Both factors increase Ia aggregability and reduce the MHC Ag mobility.