Requirement of Cys399 for processing of the human ecto-ATPase (NTPDase2) and its implications for determination of the activities of splice variants of the enzyme

Ecto-ATPase (CD39L1) corresponds to the type 2 enzyme of the ecto-nucleoside triphosphate diphosphohydrolase family (E-NTPDase). We have isolated from human ECV304 cells three cDNAs with high homology with members of the E-NTPDase family that encode predicted proteins of 495, 472, and 450 amino acids. Sequencing of a genomic DNA clone confirmed that these three sequences correspond to splice variants of the human ecto-ATPase (NTPDase2alpha, -2beta, and -2gamma). Although all three enzyme forms were expressed heterologously to similar levels in Chinese hamster ovary cells clone K-1 (CHO-K1) cells, only the 495-amino acid protein (NTPDase2alpha exhibited ecto-ATPase activity. Immunolocalization studies demonstrated that NTPDase2alpha is fully processed and trafficked to the plasma membrane, whereas the NTPDase2beta and -2gamma splice variants were retained in not fully glycosylated forms in the endoplasmic reticulum. The potential roles of two highly conserved residues, Cys(399) and Asn(443), in the activity and cellular trafficking of the ecto-ATPase were examined. Mutation of Cys(399), which is absent in NTPDase2beta and -2gamma, produced a protein completely devoid of nucleotidase activity, while mutation of Asn(443) to Asp resulted in substantial loss of activity. Neither the Cys(399) nor Asn(443) mutants were fully glycosylated, and both were retained in the endoplasmic reticulum. These results indicate that the lack of ecto-nucleotidase activity exhibited by NTPDase2beta and -2gamma and the (CS)-S-399 mutant, as well as the large reduction of activity in the N443D mutant are due to alterations in the folding/maturation of these proteins.