Polka-dotted vesicles: lipid bilayer dynamics and cross-linking effects.

We have investigated the effects of cross-linking perturbations on lipid phase-domain coalescence. Our model system explores cross-linking in the fluid-disordered phase of two-phase vesicles. Here, we quantify the vesicle population shift from the expected predominance of two-domain, two-phase configuration to a multidomain vesicle majority. We have found that the increase in multidomain vesicles is a distinct outcome from the cross-linking of biotinylated lipids and avidin. Analysis of our cross-linking data suggests that avidin forms clusters on the surface of the fluid-disordered domains, resulting in a large immobile fraction and restricted diffusion. In cellular membranes, receptor concentrations are similar to our experimental model, and we expect similar cluster formations, leading to nonideal mixing and lateral heterogeneity. We have induced and quantified a global response by cross-linking only a small percentage of lipids in our system, similar to receptor-ligand interactions on the cell membrane. Common activities, such as ligand-receptor coupling, contribute to lateral heterogeneity and membrane protein clustering, adding to cell membrane complexity. Fundamental studies into subtle shifts such as cross-linking events, which induce global cellular response, are pertinent to understanding membrane activities and effects of external stimuli.