Previous studies with the rat monoclonal antibody KMI6 had localized its antigen in vivo to a discrete subpopulation of marrow stromal cells. The KMI6 antigen has now been identified as the murine homolog of integrin β1 by amino acid sequence analysis and by cross-reactivity with antiserum to the avian integrin β1. The relative tissue abundance of murine integrin β1 was determined by Western blot. Although immunoperoxidase staining of fixed murine hematopoietic tissues demonstrated an abundance of intracellular β1, few primary-derived cells of lymphohematopoietic origin were surface positive as assessed by immunofluorescence and flow cytometry. Fetal erythroblasts provided the only exception. In contrast, the antigen was readily-detected on the surface of several cultured cell lines in association with a variety of α chains. The biochemical properties of the surface labeled murine integrin β1 were similar to those of its human counterpart, exhibiting an altered electrophoretic migration under reduced conditions or following N-glycanase treatment. The antibody recognition of the protein was insensitive to glycosylation state, presence of divalent cations, detergents, or transfer to a nitrocellulose membrane. However, on Western blot, the epitope was lost on reduction of the protein, suggesting that it is conformation dependent. These data indicate that although KMI6 epitope is widely distributed, its surface expression in vivo may be restricted within lymphohemopoietic tissues.
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