PROTEOLYTIC CLEAVAGE OF THE INTEGRIN BETA(4), SUBUNIT

The integrin beta(4) subunit often undergoes proteolytic cleavage with in its long cytoplasmic tail to yield a characteristic protein pattern of 205, 165, and 125 kDa. The results in this study suggest that beta (4) cleavage often occurs during or after cell lysis, where it was rea dily inhibitable by calcium chelators (EDTA, EGTA) and inhibitors of c ysteine proteases (E64c, leupeptin). The cleavage of beta(4) is cataly zed by a calpain-like enzyme because (i) it requires calcium, (ii) it is mimicked by purified milli-calpain, and (iii) it is inhibited by se veral calpain inhibitors including the calpain-specific inhibitor calp astatin. Within intact cells, cleavage of beta(4) was cell type-specif ic and observed only when the cells were made permeable to calcium. Su bstantial cell viability was retained during beta(4) cleavage induced by ionomycin plus calcium, indicating that cleavage within intact cell s was not necessarily a consequence of cell death. However, manipulati ons of cells including suspension, synchronization, and stimulation wi th serum, phorbol esters, or other agents all failed to induce cleavag e, suggesting that if cleavage is physiologically relevant, it is not easily duplicated in vitro. Analysis of multiple cell types showed a w ide variation in beta(4) sensitivity to proteolytic cleavage, suggesti ng that this process might be differentially regulated depending on th e cellular environment. (C) 1994 Academic Press, Inc.