The binding to F-actin of several crystalline rabbit muscle enzymes of glycogenolysis and glycolysis, was investigated in vitro. Under the conditions chosen, no binding occurs in the case of glycogen phosphorylase, phosphoglucomutase, glycerate phosphomutase and enolase. Aldolase, pyruvate kinase and triosephosphate dehydrogenase are strongly bound to F-actin, whereas lactate dehydrogenase and phosphoglycerate kinase are bound to a lesser degree. The quantitative differences in the affinity to F-actin are expressed by differences in the determined binding constants. Analytical evaluation of the binding characteristics suggests two binding sites in the case of aldolase and pyruvate kinase. Analytical ultracentrifugation studies and density gradient centrifugation in sucrose revealed a complex formation between aldolase and G-actin monomer. An average ~2 0 ,~ value of 9.2 S was obtained for the complex. A t higher aldolase concentration, a gelation of the G-actin was observed which resembles the action of a-actinin.Several glycolytic enzymes, especially fructosediphosphate aldolase and triosephosphate dehydrogenase have been shown to be bound reversibly to structure proteins of muscle.

Detergent and papain solubilized murine histocompatibility (H-2) antigens have been compared by gel exclusion chromatography, ultracentrifugation, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and aminoacid sequence analysis. From these data, we propose a molecular model for the H-2 antigens that includes the size and arrangement of the subunits on the cell surface and in solution, and we provide evidence for the orientation of these molecules on the cell surface. Detergent solubilized H-2 antigens (molecular weight 116,000) consist of two disulfidelinked heavy chains (46,000 daltons) and two noncovalently associated light chains (12,000 daltons). Alkylation with iodoacetamide prior to extraction prevented the formation of a disulfide linkage between the two heavy chains. A water-solle 51,000-dalton molecule (Fe) consisting of a 39,000-dalton fragment (FH) of the heavy chain and one intact light chain was obtained by papain digestion of cells or detergent extracts. Therefore, the disulfide linkage between the heavy chains is located in the remaining membrane-associated portion (Fm). Amino-acid sequence analysis of the FH fragment of H-2Kb by radiochemical techniques showed that it is identical to the detergent solubilized H-2Kb heavy chain in eight positions for the three amino acids tested. These data indicate that the fragment FH derives from the amino-terminus of the heavy chain and suggest that it projects outward from the cell surface, while the carboxyl-terminal region is associated with the plasma membrane. The described amino-terminal sequence data have been found constant in H-2Kb, H2Kd, 1H-2Kk, and shown to be homologous to the constant regions of immunoglobulin light and heavy chains, raising the possibility of an evolutionary relationship between the immunoglobulins and the histocompatibility antigens (8, 9). In support of this hypothesis recent reports indicate that both H-2 and HL-A antigens resemble immunoglobulins in that they appear to be molecules composed of two heavy chains and two light chains (10, 11).We have carried out an analysis of the molecular organization of H-2 antigens on the cell surface and their physicochemical properties in solution. Partial amino-terminal sequences of heavy chains of different haplotypes were compared in order to obtain some insight into the nature and organization of the antigenic polymorphism. On the basis of these and other studies, we propose a model for the H-2 antigens which includes the arrangement of the polypeptide chains, the location of interactions between the chains, and the orientation of these proteins on the cell surface. The structural homologies found in the amino-terminal sequences of five different H-2 heavy chains allow conclusions about the origin and the diversity of histocompatibility antigens. MATERIALS AND METHODSSolubilization of H-2 Antigens. Splenic lymphocytes (12) from inbred mice strains (BALB/c, DBA/2J, AKR/J, C57BL/10J, and C57BL/6J obtained from Jackson Laboratories) and lymphoma cells (P388, EL4, and...

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