Differential lateral mobility of IgM and IgG receptors in mouse B lymphocyte membranes

Anti-Ig induced redistribution of different Ig subclasses was studied as a function of temperature and correlated with membrane phase transitions as revealed by electron spin resonance spectroscopy. Fluorescein isothiocyanate-coupled anti-IgG2 and anti-IgM antibodies induced patching and capping that proceeded with increasing rates from 2° to 40° (measured at 2° intervals). Characteristic temperatures marked the onset of discontinuities in such rate changes. IgG2-bearing lymphocytes displayed discontinuities at 14°, 22°, 28°, and 36°, whereas IgM-bearing lymphocytes displayed discontinuities at 18°, 24°, 32°, and 38°. Electron spin resonance spectroscopy studies using the spin label 2,2-dimethyl 4-butyl-4-pentyl-N-oxyloxazolidine, a nitroxide-substituted decane, indicated that these temperatures are a function of hydrocarbon phase separations in the B lymphocyte membrane. With a glucosamine derivative [2-(10-carboxydecyl)-2-hexyl-4,4-dimethyl 3-oxazolidinyloxyl glucosamide] as a probe restricted to the outer monolayer of the plasma membrane, the temperatures 14° and 28° denoted the onset and end, respectively, of a fluidizing process in the outer monolayers of IgG2-bearing lymphocytes. Temperatures of 18° and 32° denoted these boundaries in IgM-bearing lymphocytes. Inner monolayer transitions are associated with the remaining temperatures. The authors conclude that membranes of IgM-bearing lymphocytes are less fluid than those of IgG-2 bearing lymphocytes.