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Abstract

Fluorescence microphotolysis is a versatile method for diffusion measurements in single cells and other microscopic systems. A recent development, continuous fluorescence microphotolysis, extends diffusion measurements to ensembles of a few hundred fluorophores. Cell surface membranes are frequently considered as 2-dimensional fluids. Membrane proteins, dispersed in a fluid bimolecular lipid layer, are expected to be highly mobile in the membrane plane. Such expectations are met by artificial bilayers but not always by cell membranes. As first observed for erythrocytes and subsequently for many other cells translational mobility of membrane proteins can be severely restricted. Frequently, integral membrane proteins have mobile and immobile fractions. In order to account for this duality it has been suggested that cell surface membranes are made up of two layers with opposing dynamic properties: a fluid lipidprotein bilayer, the “membrane skin”, and a rather static protein network, the “membrane skeleton”.