Abstract
In the present report the biosynthesis of the integrin alpha-chains endowed with constitutive endoproteolytic cleavage was evaluated in LoVo cells where furin, a subtilisin-like convertase involved in post-translational endoproteolytic processing, is not functional. It was found that cell-surface alpha 3, alpha 6 and alpha v subunits were not processed endoproteolytically into heavy and light chains as they were in HT29-D4 cells, a furin-competent cell line. Complete removal of N-linked oligosaccharides and pulse-chase experiments confirmed that the cleavage of the alpha 6 integrin subunit occurring 45 min after translation in HT29 cells did not take place in LoVo cells. Apart from cleavage deficiency, alpha 6 subunit glycosylation, association with beta 4 subunits and targeting to the plasma membrane seemed comparable in LoVo and HT29 cells. The pro-alpha 6 and the pro-alpha 3 subunits immunopurified from LoVo cells were highly sensitive to endoproteolysis by recombinant furin. Furin cleavage was calcium dependent and resulted in the conversion of the 140 kDa pro-alpha 6 into a 120 kDa heavy chain. These results suggest strongly that furin is involved in the endoproteolytic processing of cleavable integrin alpha subunits.