Intracellular phosphotyrosine induction by major histocompatibility complex class II requires co-aggregation with membrane rafts.

Cross-linking MHC class II molecules human leukocyte antigen (HLA-DR) on the surface of THP-1 cells was found to induce their entry into the glycolipid-enriched membrane fraction of the plasma membrane. At the cellular level, this resulted in the synergistic co-aggregation of class II with cholera toxin, a marker of membrane rafts. The accompanying induction of intracellular protein tyrosine phosphorylation could be inhibited by treating cells with methyl-beta-cyclodextrin, a drug that chelates membrane cholesterol and thereby disperses membrane rafts. Signaling could also be inhibited by treating cells with the Src-family kinase inhibitor PP1. Together, these results show that the induced association of class II molecules with membrane rafts can contribute to their aggregation on the cell surface and mediate an association with intracellular protein-tyrosine kinases.