LACK OF INTEGRIN ALPHA-CHAIN ENDOPROTEOLYTIC CLEAVAGE IN FURIN-DEFICIENT HUMAN COLON ADENOCARCINOMA CELLS LOVO

In the present report the biosynthesis of the integrin alpha-chains en dowed with constitutive endoproteolytic cleavage was evaluated in LoVo cells where furin, a subtilisin-like convertase involved in post-tran slational endoproteolytic processing, is not functional. It was found that cell-surface alpha 3, alpha 6 and alpha v subunits were not proce ssed endoproteolytically into heavy and light chains as they were in H T29-D4 cells, a furin-competent cell line. Complete removal of N-linke d oligosaccharides and pulse-chase experiments confirmed that the clea vage of the alpha 6 integrin subunit occurring 45 min after translatio n in HT29 cells did not take place in LoVo cells. Apart from cleavage deficiency, alpha 6 subunit glycosylation, association with beta 4 sub units and targeting to the plasma membrane seemed comparable in LoVo a nd HT29 cells. The pro-alpha 6 and the pro-alpha 3 subunits immunopuri fied from LoVo cells were highly sensitive to endoproteolysis by recom binant furin. Furin cleavage was calcium dependent and resulted in the conversion of the 140 kDa pro-alpha 6 into a 120 kDa heavy chain. The se results suggest strongly that furin is involved in the endoproteoly tic processing of cleavable integrin alpha subunits.