The FAC protein encoded by the Fanconi anemia (FA) complementation group C gene is thought to function in the cytoplasm at a step before DNA repair. Because FA cells are susceptible to mitomycin C, we considered the possibility that FAC might interact with enzymes involved in the bioreductive activation of this drug. Here we report that FAC binds to NADPH cytochrome-P450 reductase (RED), a microsomal membrane protein involved in electron transfer, in both transfected COS-1 and normal murine liver cells. FAC-RED interaction requires the amino-terminal region of FAC and the cytosolic, membrane-proximal domain of the reductase. The latter contains a known binding site for flavin mononucleotide (FMN). Addition of FMN to cytosolic lysates disrupts FAC-reductase complexes, while flavin dinucleotide, which binds to a distinct carboxy-terminal domain, fails to alter FAC-RED complexes at concentrations similar to FMN. FAC is also functionally coupled to this enzyme as its expression in COS-1 cells suppresses the ability of RED to reduce cytochrome c in the presence of NADPH. We propose that FAC plays a fundamental role in vivo by attenuating the activity of RED, thereby regulating a major detoxification pathway in mammalian cells.
Copyright 1998 by The American Society of Hematology