Characterization of the cell surface heterodimer VLA-4 and related peptides

A monoclonal antibody (B-5G10) was produced which specifically recognizes the M(r) 150,000/130,000 VLA-4 complex on the surface of human cells. Crosslinking studies indicated that the M(r) 150,000 α4 subunit of VLA-4 is in noncovalent 1:1 associated with the M(r) 130,000 VLA β subunit. In the absence of crosslinking, the VLA-4 α⁴β subunit complex was easily dissociated, especially in Nonidet P-40 detergent, or at elevated pH (above 8.0). Studies of dissociated subunits showed that B-5G10 recognize an epitope on the M(r) 150,000 α⁴ subunit of VLA-4, whereas the β subunit is immunologically identical to the M(r) 130,000 β subunit common to all VLA heterodimes. VLA-4 is widely distributed on hematopoietic cells, including thymocytes, peripheral blood lymphocytes, monocytes, activated T cells, T and B lymphoblastoid cell lines, and myeloid cell lines. However, VLA-4 is only weakly expressed on most adherent cell lines tested. Immunoprecipitates of VLA-4 often contain additional proteins of M(r) 80,000 and M(r) 70,000. These are probably derived from the M(r) 150,000 α⁴ subunit because: 1) they are both recognized by anti-α⁴ sera, but not anti-β sera; 2) the sum of their sizes is equal to the size of α⁴; 3) they are selectively coexpressed with α⁴ and not other VLA α subunits; 4) the M(r) 80,000 protein has an identical NH₂-terminal sequence to α⁴; 5) like α⁴, the M(r) 70,000 and 80,000 peptides can variably associate with the VLA β subunits; and 6) trypsin appears to cleave the M(r) 150,000 α⁴ subunit into products of M(r) 70,000 and 80,000.