Abstract
We have characterised a protein of approximately 80kD previously observed to co-immunoprecipitate with the α3β1 integrin in lysates of surface labelled human epiderrnalkerati-nocytes. The 80kD protein only appeared when keratinocytes were harvested with trypsin/EDTA prior to lysis and a protein of similar molecular mass could be immunoprecipitated from human dermal fibroblasts following treatment of the cells with trypsin/EDTA. N terminal sequencing established that the 80kD protein had homology with the as integrin subunit. Peptide-mass fingerprinting was used to confirm that the protein comprised the amino terminus of α3 and established that the site of cleavage was after amino acid 629. The 80kD fragment could be coimmunoprecipitated with α3β1 using an antibody to the cytoplasmic domain of the α3 subunit, showing that the fragment remained complexed with intact α3β1. When antibodies to the cytoplasmic and extracellular domains of α3 were used to label human epidermis by immunofluorescence, the staining patterns were indistinguishable and there is therefore no evidence that proteolysis of α3 plays a role in keratinocyte detachment from the basement membrane during terminal differentiation. Whether the 80kD fragment has any effects, positive or negative, on α3β1-mediated adhesion remains to be determined.