THE A-VARIANT AND B-VARIANT OF THE ALPHA-3 INTEGRIN SUBUNIT - TISSUE DISTRIBUTION AND FUNCTIONAL-CHARACTERIZATION

The alpha subunits of the laminin-binding integrins alpha 3 beta 1, al pha 6 beta 1, and alpha 7 beta 1 have homologous sequences and are sim ilar in structure. Two cytoplasmic variants, A and B, have been identi fied for each of these alpha subunits, although the alpha 3B splice va riant has been detected only at the mRNA level. We prepared a panel of mouse monoclonal antibodies specific for the A and B variants of the alpha 3 subunit to study their tissue distribution. Four monoclonal an tibodies react with alpha 3A, one of which recognizes only the nonphos phorylated form; of the three anti-alpha 3B antibodies, one cross-reac ts with alpha 6B. Reverse transcriptase-PCR analysis of various human tissues revealed the presence of alpha 3B mRNA in brain, heart, and sk eletal muscle. Moreover, the alpha 3B protein was detected by immunobl otting in brain and heart tissue but not in skeletal muscle. In contra st, alpha 3A mRNA and protein were present in all tissues studied. Thu s, the expression of (alpha 3B in adult tissues is more restricted tha n that of alpha 3A. Immunohistochemical studies showed that in brain t issue, both variants are exclusively expressed on small blood-vessel e ndothelium, whereas in heart tissue their distribution patterns differ markedly. Although (alpha 3A is strongly expressed on vascular smooth muscle cells, alpha 3B is detected only on endothelial cells of veins . Expression of the two variant forms of alpha 3 in K562 cells reveale d that the ligand-binding specificities of alpha 3A beta 1 and alpha 3 B beta 1 are identical: both bind human laminin-2 and -4, laminin-5, a nd laminins isolated from bovine kidney, but not bovine laminin-2 and -4, mouse laminin-1, or human fibronectin. In addition, adhesion media ted by both integrins is induced to the same extent by phorbol 12-myri state 13-acetate. The alpha 3A, but not the alpha 3B subunit, is phosp horylated; and phosphorylation of alpha 3A increases after phorbol 12- myristate 13-acetate stimulation. Thus, we found no differences betwee n the adhesion functions of the A and B variants of alpha 3.