Plasma membrane heterogeneities play an important role in various cellular functions such as signal transduction, trafficking and cell polarization. These membrane heterogeneities are thought to be regulated by dynamical and organizational couplings between the two monolayers of the bilayer, the electrostatic and mechanical coupling between the cytoskeleton and lipid membrane, and protein-protein interactions. Because plasma membranes have a very complex composition/organization with a wide variety of lipids, embedded membrane proteins, and the attached cytoskeleton, understanding the effect of individual interactions and the role of particular membrane components on membrane organization or dynamics is not easy to predict in a cellular environment. The general goal of this study is to address these interactions by studying them in sophisticated polymer-tethered and polymer-sandwiched lipid model membrane systems of well defined compositions. This study explores the intermonolayer coupling by showing that raft-like liquid-ordered (l o ) domains in one leaflet of the bilayer can induce corresponding domains in the opposite leaflet, thus, supporting a biophysical mechanism of raft-mediated transbilayer signaling. This work also shows that mechanical coupling of non-membrane-spanning lipid-anchored polymers can provide point obstructions in both leaflets of the lipid membrane by inducing local bilayer deformations. In addition, experiments show that membrane-associated actin filaments (F-actin) may interact electrostatically with particular bilayer lipids leading to obstructions in membrane dynamics and organization. This study also shows that molecular crowding of membrane proteins is strongly coupled to lipid membrane dynamics, both when proteins interact electrostatically with the bilayer and when flexible polymers are linked to lipid anchors causing point obstructions. Polymer-tethered lipid membranes were also used to explore uPAR-integrin interactions by measuring the lateral diffusion of uPAR in membranes containing integrin. These uPAR-integrin experiments confirm for the first time under well-defined conditions complexation between these two types of membrane proteins, as was previously predicted.
