NAD kinase phosphorylates NAD(+) to form NADP(+). Conversely, NADP phosphatase, which has not yet been identified, dephosphorylates NADP(+) to produce NAD(+). Among the NAD kinase homologs, the primary structure of MJ0917 of hyperthermophilic archaeal Methanococcus jannaschii is unique. MJ0917 possesses an NAD kinase homologous region in its C-terminal half and an inositol-1-phosphatase homologous region in its N-terminal half. In this study, MJ0917 was biochemically shown to possess both NAD kinase and phosphatase activities toward NADP(+), NADPH, and fructose 1,6-bisphosphate, but not toward inositol 1-phosphate. With regard to the phosphatase activity, kinetic values indicated that NADP(+) is the preferred substrate and that MJ0917 would function as a novel NADP phosphatase/NAD kinase showing conflicting dual activities, viz. synthesis and degradation of an essential NADP(+). Furthermore, in vitro analysis of MJ0917 showed that, although MJ0917 could supply NADP(+), it prevented excess accumulation of NADP(+); thus, it has the ability to maintain a high NAD(+)/NADP(+) ratio, whereas 5'-AMP would decrease this ratio. The evolutionary process during which MJ0917 arose is also discussed.