Intramembrane particles and the organization of lymphocyte membrane proteins

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Abstract
An experimental system was developed in which the majority of all lymphocyte cell-surface proteins, regardless of antigenic specificity, could be cross-linked and redistributed in the membrane to determine whether this would induce a corresponding redistribution of intramembrane particles (IMP). Mouse spleen cells were treated with p-diazoniumphenyl-.beta.-D-lactose (lac) to modify all exposed cell-surface proteins. Extensive azo-coupling was achieved without significantly reducing cell viability or compromising cellular function in mitogen- or antigen-stimulated cultures. When the lac-modified cell-surface proteins were capped with a sandwich of rabbit antilactoside antibody and fluorescein-goat anti-rabbit Ig, freeze-fracture preparations obtained from these cells revealed no obvious redistribution of IMP on the majority of fracture faces. Detailed analysis showed a statistically significant 35% decrease (P < 0.01) in average IMP density in the E face of the lac-capped spleen cells compared with control cells; a few E-face micrographs showed intense IMP aggregation. There was no significant alteration of P-face IMP densities or distribution. Apparently, the majority of E-face IMP and virtually all P-face IMP do not present accessible antigenic sites on the lymphocyte surface and do not associate in a stable manner with surface protein antigens. IMP, as observed in freeze-fracture analysis, may not comprise a representative reflection of lymphocyte transmembrane protein molecules and complexes. Evidence supporting this conclusion includes the fact that at least some common lymphocyte surface antigens are exposed portions of transmembrane proteins and that aggregation of molecules of any surface antigen results in altered organization of contractile proteins at the cytoplasmic face of the membrane.