H36-ALPHA-7 IS A NOVEL INTEGRIN ALPHA-CHAIN THAT IS DEVELOPMENTALLY REGULATED DURING SKELETAL MYOGENESIS

H36 is a 120,000-D membrane glycoprotein that is expressed during the differentiation of skeletal muscle. H36 cDNA clones were isolated from a lambda UniZapXR rat myotube cDNA library and sequenced. The deduced amino acid sequence demonstrates that H36 is a novel integrin alpha chain that shares extensive homology with other alpha integrins that includes: (a) the GFFKR sequence found in all alpha integrins; (b) a single membrane spanning region; (c) conservation of 18 of 22 cysteines; and (d) a protease cleavage site found in the non-I region integrin alpha chains. The cytoplasmic domain of H36 is unique and additional regions of nonhomology further indicate H36 is distinct from all other alpha chains. In keeping with current nomenclature we designate this alpha chain alpha-7. Northern blots demonstrate that expression of H36-alpha-7 mRNA is regulated both early in the development of the myogenic lineage and later, during terminal differentiation. Detection of H36-alpha-7 mRNA coincides with conversion of H36- myogenic precursor cells to H36+ cells. H36-alpha-7 mRNA is present in replicating myoblasts: expression increases upon terminal differentiation and is markedly reduced in developmentally defective myoblasts. In addition, H36-alpha-7 mRNA is not detected in C3H10T1/2 cells. It is in myotubes derived from myoblasts obtained by treatment of 10T1/2 cells with azacytidine or transfection with MRF4. Immunoblots and immunofluorescence demonstrate that the H36-alpha-7 chain is associated with integrin beta-1. Affinity chromatography demonstrates that H36-alpha-7-beta-1 selectively binds to laminin. The expression of H36-alpha-7 on secondary myoblasts during the development of the limb in vivo corresponds with the appearance of laminin in the limb, with the responsiveness of secondary myoblast proliferation to laminin, and with the onset of increased muscle mass, suggesting that H36-alpha-7 modulates this stage in limb development. We conclude that H36-alpha-7 is a novel alpha integrin laminin binding protein whose expression is developmentally regulated during skeletal myogenesis.