Actin-dependent clustering of insulin receptors in membrane microdomains.

Recent evidence suggests that, after binding insulin, insulin receptors (IR) interact with specialized, cholesterol-containing, membrane microdomains and components of the actin cytoskeleton. Using single particle tracking techniques, we examined how binding of insulin, depletion of membrane cholesterol and disruption of actin filaments affect the lateral diffusion of individual quantum dot-labeled native IR on live rat basophilic leukemia 2H3 cells. We also examined the effects of similar treatments on IR clustering and multivalent insulin binding on these cells using both photon counting histogram analysis and polarization-based fluorescence resonance energy homo-transfer imaging. Our analyses indicate that binding of insulin to IR on these cells is multivalent, involving at least two insulin molecules per IR as labeling concentrations approach 1μM. Insulin binding also reduces lateral diffusion of IR and the size of membrane compartments accessed by IR. For IR that have not bound insulin, lateral diffusion of IR and the size of membrane compartments accessed by IR increase after disrupting actin filaments or depleting membrane cholesterol. However, clustering of insulin-occupied IR is reduced only by disrupting actin filaments or by fixing cells with paraformaldehyde prior to exposure to insulin, but not by depleting membrane cholesterol. Thus, it appears that, although restriction of IR lateral diffusion on these cells is sensitive to both actin filament dynamics and membrane cholesterol content, clustering of insulin-occupied IR primarily involves an actin-dependent mechanism.