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The human intraepithelial lymphocyte marker HML‐1 is an integrin consisting of a β7 subunit associated with a distinctive α chain

The membrane antigen defined by the monoclonal antibody (mAb) HML‐1 is abundantly expressed on, and largely restricted to, the T cells which populate the intestinal epithelium. We show that the mature form of the antigen is a heterodimer comprising a 150‐kDa α chain and a 120‐kDa β chain. Direct sequencing of tryptic peptides cleaved from the purified β chain identified this polypeptide with the integrin β7 isotype. cDNA clones coding for the β7 chain have recently been isolated from T cell cDNA libraries, but the β7 chain had not been identified at the protein level. No information is available concerning the primary structure of the HML‐1 α chain. We show that this subunit is synthesized as a precursor form that undergoes, like several other integrin α subunits, a post‐translational cleavage of a peptide bond. Among the 11 human integrin α chains previously identified, 10 have biochemical features and/or a distribution different from those of HML‐1 α. One, VLAα4 (CD49d), has a molecular mass of 150 kDa and is expressed on HML‐1+ cells but is not recognized by HML‐1 mAb. We conclude that HML‐1 is a novel member of the integrin family made of the β7 chain and of an as‐yet‐undescribed human α chain characterized by the post‐translational cleavage of a 10‐kDa peptide. HML‐1 is, thus, probably the human counterpart of the mouse antigen M290.