Membrane adhesion is a vital component of many biological processes. Heterogeneities in lipid and protein composition are often associated with the adhesion site. These heterogeneities are thought to play functional roles in facilitating signalling. Here we experimentally examine this phenomenon using model membranes made of a mixture of lipids that is near a phase boundary at room temperature. Non-adherent model membranes are in a well-mixed, disordered-fluid lipid phase indicated by homogeneous distribution of a fluorescent dye that is a marker for the fluid-disordered (L d ) phase. We specifically adhere membranes to a flat substrate bilayer using biotin-avidin binding. Adhesion produces two types of coexisting heterogeneities: an ordered lipid phase that excludes binding proteins and the fluorescent membrane dye, and a disordered lipid phase that is enriched in both binding proteins and membrane dye compared with the non-adhered portion of the same membrane. Thus, a single type of adhesion interaction (biotin-avidin binding), in an initially-homogeneous system, simultaneously stabilizes both ordered-phase and disordered-phase heterogeneities that are compositionally distinct from the non-adhered portion of the vesicle. These heterogeneities are long-lived and unchanged upon increased temperature.
Membrane-targeting domains play crucial roles in the association of signaling molecules to the plasma membrane. For most peripheral proteins, the proteinto-membrane interaction is transient; but after proteins dissociate from the membrane they are observed to rebind following a brief excursion in the bulk solution. These membrane hops can have broad implications on protein dynamics. We study the diffusion of membrane-targeting domains using single-molecule tracking in supported lipid bilayers. The ensemble-averaged mean square displacement (MSD) exhibits superdiffusive behavior. However, the time-averaged MSD of individual trajectories is found to be linear with respect to lag time, as in Brownian diffusion. These observations are explained in terms of bulk excursions that introduce jumps with a heavy-tail distribution, rapidly increasing the area explored. Furthermore, this type of motion is characterized by a pronounced scattering in the time-averaged MSD of individual trajectories.
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