Plasma membranes of many cells appear to be divided into domains, areas whose composition and function differ from the average for an entire membrane. We have previously used fluorescence photo-bleaching and recovery to demonstrate one type of membrane domain, with dimensions of micrometers (Yechiel, E., and M. Edidin. 1987, J. Cell Biol. 105: 755-760). The presence of membrane domains is inferred from the dependence of the apparent mobile fraction of labeled molecules on the size of the membrane area probed. We now find that by this definition classical class I MHC molecules, H-2Db, are concentrated in domains in the membranes of K78-2 hepatoma cells, while the nonclassical class I-related molecules, Qa-2, are free to pass the boundaries of these domains. The two proteins are highly homologous but differ in their mode of anchorage to the membrane lipid bilayer. H-2Db is anchored by a transmembrane peptide, while Qa-2 is anchored by a glycosylphosphatidylinositol (GPI) anchor. A mutant class I protein with its external portion derived from Qa-2 but with transmembrane and cytoplasmic sequences from a classical class I molecule shows a dependence of its mobile fraction on the area of membrane probed, while a mutant whose external portions are a mixture of classical and nonclassical class I sequences, GPI-linked to the bilayer, does not show this dependence and hence by our definition is not restricted to membrane domains.

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