The human nm23-H4 gene product is a mitochondrial nucleoside diphosphate kinase

We demonstrate here the catalytic activity and subcellular localization of the Nm23-H4 protein, product of nm23-H4, a new member of the human nm23/nuc leoside diphosphate (NDP) kinase gene family (Milon, L., Rousseau-Merck, M. , Munier, A., Erent, M., Lascu, I., Capeau, J., and Lacombe, M. L. (1997) H um. Genet. 99, 550-557). Nm3-H4 was synthesized in escherichia coli as the full-length protein and as a truncated form missing the N-terminal extensio n characteristic of mitochondrial targeting. The truncated form possesses N DP kinase activity, whereas the full-length protein is inactive, suggesting that the extension prevents enzyme folding and/or activity. X-ray crystall ographic analysis was performed on active truncated Nm23-H4. Like other euk aryotic NDP kinases, it is a hexamer. Nm23-H4 naturally possesses a serine residue at position 129, equivalent to the li-pn mutation of the Drosophila NDP kinase. The x-ray structure shows that the presence of Ser(129) has lo cal structural effects that weaken subunit interactions. Site-directed muta genesis shows that the serine is responsible for the lability of Nm23-H4 to heat and urea treatment, because the S129P mutant is greatly stabilized. E xamination of human embryonic kidney 293 cells transfected with green fluor escent protein fusions by confocal microscopy shows a specific mitochondria l localization of Nm23-H4 that was also demonstrated by Western blot analys is of subcellular fractions of these cells. Import into mitochondria is acc ompanied by cleavage of the N-terminal extension that results in NDP kinase activity. Submitochondrial fractionation indicates that Nm23-H4 is associa ted with mitochondrial membranes, possibly to the contact sites between the outer and inner membranes.