Molecular cloning and functional expression of bovine spleen ecto-NAD+ glycohydrolase:: structural identity with human CD38

Bovine spleen ecto-NAD(+) glycohydrolase, an archetypal member of the mammalian membrane-associated NAD(P)(+) glycohydrolase enzyme family (EC 3.2.2.6), displays catalytic features similar to those of CD38, i.e. a protein originally described as a lymphocyte differentiation marker involved in the metabolism of cyclic ADP-ribose and signal transduction. Using amino acid sequence information obtained from NAD(+) glycohydrolase and from a truncated and hydrosoluble form of the enzyme (hNADase) purified to homogeneity, a full-length cDNA clone was obtained. The deduced sequence indicates a protein of 278 residues with a molecular mass of 31.5 kDa. It predicts that bovine ecto-NAD(+) glycohydrolase is a type II transmembrane protein, with a very short intracellular tail. The bulk of the enzyme, which is extracellular and contains two potential N-glycosylation sites, yields the fully catalytically active hNADase which is truncated by 71 residues. Transfection of HeLa cells with the full-length cDNA resulted in the expression of the expected NAD(+) glycohydrolase, ADP-ribosyl cyclase and GDP-ribosyl cyclase activities at the surface of the cells. The bovine enzyme, which is the first 'classical' NAD(P)(+) glycohydrolase whose structure has been established, presents a particularly high sequence identity with CD38, including the presence of 10 strictly conserved cysteine residues in the ectodomain and putative catalytic residues. However, it lacks two otherwise conserved cysteine residues near its C-terminus. Thus hNADase, the truncated protein of 207 amino acids, represents the smallest functional domain endowed with all the catalytic activities of CD38/NAD(+) glycohydrolases so far identified. Altogether, our data strongly suggest that the cloned bovine spleen ecto-NAD(+) glycohydrolase is the bovine equivalent of CD38.