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Abstract:
Cell function apparently requires that cell membranes have both dynamic and static properties. The insertion of newly synthetized proteins into the plasma membrane and their distribution on the cell surface or the uptake of material by pinocytosis and the subsequent recycling of membrane components are only two examples of dynamic events in membranes. The maintenance of size, shape, polarity, and permanent surface specializations, for instance, reflect membrane statics. This chapter will show that the duality of membrane dynamics is apparent in protein lateral mobility and makes it necessary to visualize cell membranes as fluid-solid composites. We assume that this architectural principle pertains to both major membrane coordinates: in-plane and perpendicular to the plane. The plasma membrane of many cells may feature on its cytoplasmic face a relatively stable and static protein network, the membrane skeleton. A more fluid lipid bilayer containing embedded proteins, attached to the membrane skeleton on its external face, is an element common to most cellular membranes. The membrane is completed, in many cases, by a layer of carbohydrate, e.g., the glycocalix of eukaryotes whose dynamic properties, however, have been little studied. The membrane bilayer – rather than a two-dimensional solution of integral proteins in a fluid lipid bilayer – probably is a patchwork made up from solid and fluid domains. Solid domains, mainly composed of protein oligomers, may alternate with more fluid lipo-protein bilayer regions. Phase segregation in the bilayer regions may contribute occasionally to the fluid-solid composite character
