A new human nm23 homologue (nm23-H5) specifically expressed in testis germinal cells

We have identified a cDNA encoding a 212 amino acid protein (Nm23-H5) with 27-31% identity to the human members of the nm23/nucleoside diphosphate (NDP) kinase gene family. The nm23-H5 gene is located on chromosome 5q23-31 and is transcribed as one main transcript of 1.1 kb which is highly and specifically expressed in testis, in the spermatogonia and early spermatocytes. Nm23-H5 possesses most of the residues conserved among NDP kinases plus an additional COOH-terminus end of 55 amino acids unique to this protein. However, under usual assay conditions, Nm23-H5 expressed in Escherichia coli did not exhibit NDP kinase activity. These results suggest that Nm23-H5 is specifically involved in early stages of spermatogenesis.