Low molecular weight GTP-binding proteins in cardiac muscle. Association with a 32-kDa component related to connexins

Low molecular weight GTP-binding proteins and their cellular interactions were examined in cardiac muscle. Heart homogenate was separated into various subcellular fractions by differential and sucrose density gradient centrifugation. Various fractions were separated by sodium dodecyl sulfate-gel electrophoresis, blotted to nitrocellulose, and GTP-binding proteins detected by incubating with [alpha-32]GTP. Three polypeptides of M(r) 23,000, 26,000, and 29,000 were specifically labeled with [alpha-32P]GTP in all the fractions examined and enriched in sarcolemmal membranes. The 23-kDa polypeptide was labeled to a higher extent with [alpha-32P]GTP than the 26- and 29-kDa polypeptides. A polypeptide of M(r) 40,000 was weakly labeled with [alpha-32P]GTP in the sarcolemmal membrane and tentatively identified as Gi alpha by immunostaining with anti-Gi alpha antibodies. Cytosolic GTP-binding proteins were labeled with [alpha-32P]GTP and their potential sites of interaction investigated using the blot overlay approach. A polypeptide of 32 kDa present in sarcolemmal membranes, intercalated discs, and enriched in heart gap junctions was identified as a major site of interaction. The low molecular weight GTP-binding proteins associated with the 32-kDa polypeptide through a complex involving cytosolic components of M(r) 56,000, 36,000, 26,000, 23,000, and 12,000. A monoclonal antibody against connexin 32 from liver strongly recognized the 32-kDa polypeptide in heart gap junctions, whereas polyclonal antibodies only weakly reacted with this polypeptide. The low molecular weight GTP-binding proteins associated with a 32-kDa polypeptide in liver membranes that was also immunologically related to connexin 32. These results indicate the presence of a subset of low molecular weight GTP-binding proteins in a membrane-associated and a cytoplasmic pool in cardiac muscle. Their association with a 32-kDa component that is related to the connexins suggests that these polypeptides may be uniquely situated to modulate communication at the cell membrane.