Fluorescence Spectroscopic Studies on Phase Heterogeneity in Lipid Bilayer Membranes

There is a growing interest in functional membrane heterogeneity on the mesoscopic (several tens to hundreds of molecular dimensions) scale. However, the physical-chemical basis for this sort of heterogeneity in membranes is not entirely clear. Unambiguous methods to demonstrate that the cell plasma membrane and other cellular membranes are in fact heterogeneous on the mesoscopic level are also not generally available. Fluorescence techniques do, however, provide excellent tools for this purpose. In particular, the emerging techniques of scanning near-field optical microscopy and single-molecule fluorescence microscopy hold a great deal of promise for the near-future. All these methods require the use of fluorescent probes (lipids and/or proteins) and a clear definition of how these probes partition between domains of coexisting membrane phases. The development of the concept of membrane heterogeneity over the years since the first proposal of the fluid mosaic model is reviewed briefly. The use of lipid-binding proteins in experimental protocols for the labeling of membranes with fluorescent lipid amphiphiles as monomers in aqueous solutions at concentrations well above their critical aggregation concentrations is discussed. The methods of fluorescence spectroscopy available to the cell biologist for determining probe partition coefficients for partitioning between coexisting membrane phases are reviewed in some detail, as is the relevant theoretical and experimental work reported in the literature.