H-protein and X-protein. Two new components of the thick filaments of vertebrate skeletal muscle.
With a view to obtaining a more complete view of the composition and structure of the thick filaments of vertebrate skeletal muscle, we have isolated and characterized two new myofibrillar components, H-protein and X-protein. These were purified by hydroxyapatite column chromatography of an impure C-protein preparation itself made from impure myosin extracted from rabbit back and leg muscles. H-protein is the protein responsible for band H on sodium dodecyl sulphate/polyacrylamide gel electrophoresis of crude myosin. X-protein, although present in such preparations in significant quantities, was not detected previously since it is difficult to resolve from C-protein by sodium dodecyl sulphate/polyacrylamide gel electrophoresis. Physical-chemical parameters have been determined for the new proteins and compared with those of C-protein. The apparent chain weight of H-protein estimated by sodium dodecyl sulphate/polyacrylamide gel electrophoresis is 69,000, whereas that of X-protein (152,000) is only slightly greater than that of C-protein (140,000). The molecular weights of H- and X-proteins determined by sedimentation equilibrium centrifugation show that the molecules contain only a single polypeptide chain. The circular dichroism spectra indicate that the proteins have low alpha-helical contents. Both proteins, particularly H-protein, have a high proline content. Although X-protein is of similar chain weight to C-protein, the two show distinct differences in other properties. The sedimentation coefficient of X-protein is markedly lower than that of C-protein, suggesting X-protein is a more asymmetrical molecule. The amino acid compositions, although broadly similar, also show clear differences. Antibodies to H-protein, X-protein and C-protein have been raised in goats and shown not to cross-react.
D000596:Amino Acids / Q000032:analysis; D000818:Animals; D002352:Carrier Proteins; D002942:Circular Dichroism; D004591:Electrophoresis, Polyacrylamide Gel; D005779:Immunodiffusion; D008970:Molecular Weight; D009124:Muscle Proteins / Q000276:immunology / Q000302:isolation & purification*; D009132:Muscles / Q000032:analysis*; D009210:Myofibrils / Q000032:analysis; D009218:Myosins; D011817:Rabbits; D012995:Solubility; D013056:Spectrophotometry, Ultraviolet; D014461:Ultracentrifugation; D000099163:Myosin Binding Protein C
D000596:Amino Acids; D002352:Carrier Proteins; D009124:Muscle Proteins; D009218:Myosins; D000099163:Myosin Binding Protein C; C039727:H-Protein, Oryctolagus cuniculus; C039728:X-Protein, muscle, Oryctolagus cuniculus