Protein disposition in biological membranes

Intracellular localization and immunogenic capacities of phenotypic products of mouse histocompatibility genes

The transplantation antigens are the phenotypic products of genes which control histocompatibility in vertebrate species. The products of major histocompatibility locus of the mouse, H-2, have been studied as a model. The H-2 transplantation antigens are expressed on cellular membranes in all tissues examined. These gene products have been isolated from cells associated with subcellular membranes. These membranes have been assayed both for their antigen content (antigenicity) and for their capacities to induce a primary humoral and a cell-mediated response (immunogenicity). In all tissues examined, the H-2 antigens (products of the K and D regions of H-2) were found expressed in high concentration on cell surface membrane. However, immunogenic activity was observed only with spleen and thymus preparations, consisting mainly of intracellular membranes (MLP). Immunogenic MLP was also isolated from lymphoblast and fibroblast cells, and again was derived mainly from endoplasmic reticulum. In other tissues, such as liver, kidney, and erythrocytes, H-2 antigens were found only on surface membrane and in an antigenic but nonimmunogenic form. A novel method for tagging surface membrane of mammalian cells is presented. It consists of binding, to whole cells in a covalent linkage, purified preparations of the beta-galactosidase of E. coli. The bound enzyme has proved to be an unambiguous marker for surface membrane. With this marker, the stability of surface membrane to shear forces during homogenization could be assessed. A number of considerations suggest that immunogenicity of transplantation antigens may be due to factor(s) present on the membranes in addition to the H-2 antigenic determinants. There are indications that these factors may be controlled by the I region of the H-2 complex. It is interesting to note that normal tissues which have Ia antigens on their surface membranes yield immunogenic MLP (spleen and thymus), whereas those without Ia surface antigens yield an antigenic MLP that has no immunogenic capacity (liver, kidney, and erythrocytes).