Lipid rafts mediate beta1 integrin endocytosis in monocytes

Endocytosis is the mechanism by which proteins on the plasma membrane are sorted into vesicles for the purpose of intracellular trafficking. The primary machinery for endocytosis involves protein sorting via clathrin coats. However, recently glycolipid-enriched membrane microdomains or "lipid rafts" are emerging as a lipid-sorting based alternative to clathrin-mediated endocytosis. Here we demonstrate for the first time that clustering alpha4beta1 and alpha5beta1 integrins on monocytes results in rapid internalization via a cholesterol- and lipid raft-dependent mechanism. Cross-linking fibronectin also promotes cholesterol-dependent internalization. Since clustering the raft marker GM1 was sufficient to induce endocytosis we conclude that monocyte rafts contain machinery for endocytosis which can be activated by lipid aggregation. Lipid raft-mediated endocytosis requires the activation of a serene/threonine kinase and cytoskeletal reorganization. We show that tetraspanin CD81 expression affects the amount of beta1 integrin endocytosis indicating CD81 is a specific modulator of raft-based endocytosis.