THE CHLAMYDOMONAS-REINHARDTII MOCO CARRIER PROTEIN IS MULTIMERIC AND STABILIZES MOLYBDOPTERIN COFACTOR IN A MOLYBDATE CHARGED FORM

In Chlamydomonas reinhardtii,, molybdopterin cofactor (MoCo) able to r econstitute active nitrate reductase (NR) with apoenzyme from the Neur ospora crassa mutant nit-1 was found mostly bound to a carrier protein (CP), This protein is scarce in the algal free extracts and has been purified 520-fold. MoCoCP is a protein of 64 kDa with subunits of 16.5 kDa and an isoelectric point of 4,5. In contrast to free MoCo, MoCo b ound to CP was remarkably protected against inactivation under both ae robic conditions and basic pH. MocoCP transferred active MoCo to apoNR in vitro without addition of molybdate, though reconstituted activity was 20% higher in the presence of molybdate, Incubation with tungstat e specifically inhibited MoCoCP activity but had no effect on the acti vity of free MoCo released from milk xanthine oxidase, MoCoCP did not charge molybdate unless in the presence of N, crassa extracts. Our dat a support that MoCoCP stabilizes MoCo in an active form charged with m olybdate to provide MoCo to apomolybdoenzymes. (C) 1998 Federation of European Biochemical Societies.