IDENTIFICATION OF AN 80KD PROTEIN ASSOCIATED WITH THE ALPHA(3)BETA(1)INTEGRIN AS A PROTEOLYTIC FRAGMENT OF THE ALPHA(3) SUBUNIT - STUDIES WITH HUMAN KERATINOCYTES

We have characterised a protein of approximately 80kD previously obser ved to co-immunoprecipitate with the alpha(3) beta(1) integrin in lysa tes of surface labelled human epidermalkeratinocytes. The 80kD protein only appeared when keratinocytes were harvested with trypsin/EDTA pri or to lysis and a protein of similar molecular mass could be immunopre cipitated from human dermal fibroblasts following treatment of the cel ls with trypsin/EDTA. N terminal sequencing established that the 80kD protein had homology with the alpha(3) integrin subunit. Peptide-mass fingerprinting was used to confirm that the protein comprised the amin o terminus of alpha(3) and established that the site of cleavage was a fter amino acid 629. The 80kD fragment could be coimmunoprecipitated w ith alpha(3) beta(1) using an antibody to the cytoplasmic domain of th e alpha(3) subunit, showing that the fragment remained complexed with intact alpha(3) beta(1). When antibodies to the cytoplasmic and extrac ellular domains of alpha(3) were used to label human epidermis by immu nofluorescence, the staining patterns were indistinguishable and there is therefore no evidence that proteolysis of alpha(3) plays a role in keratinocyte detachment from the basement membrane during terminal di fferentiation. Whether the 80kD fragment has any effects, positive or negative, on alpha(3) beta(1)-mediated adhesion remains to be determin ed.