A 2ND MUTANT ALLELE OF FURIN IN THE PROCESSING-INCOMPETENT CELL-LINE,LOVO - EVIDENCE FOR INVOLVEMENT OF THE HOMO-B DOMAIN IN AUTOCATALYTICACTIVATION

Furin is a Gels membrane-associated endoprotease that is involved in c leavage of various precursor proteins predominantly at Arg-X-Lys/Arg-A rg sites. Furin itself is synthesized as an inactive precursor, which is activated through intramolecular autocatalytic cleavage at an Arg-X -Lys-Arg site. We previously found that human colon carcinoma LoVo cel ls have a frameshift mutation within the home B domain of furin and th ereby lack processing activity toward Arg-X-Lys/Arg-Arg sites. In this study, however, we identified a second furin mutation in this cell li ne. The mutation, a replacement of a conserved Trp residue within the home B domain with Arg, results in lack of processing activity of the mutant furin. The combination of both mutations can account for the re cessive nature of the processing incompetence of LoVo cells. Immunoflu orescence analysis with three distinct anti-furin monoclonal antibodie s revealed that neither furin mutant underwent the autocatalytic activ ation or left the endoplasmic reticulum for the Golgi. These data indi cate that the home B domain as well as the catalytic domain is require d for autocatalytic activation of furin.