Expression, purification and crystallization of the ecto-enzymatic domain of rat E-NTPDase1 CD39

Crystallographic evidence for a domain motion in rat nucleoside triphosphate diphosphohydrolase (NTPDase) 1

Nucleoside triphosphate diphosphohydrolases (NTPDases) are a physiologically important class of membrane-bound ectonucleotidases responsible for the regulation of extracellular levels of nucleotides. CD39 or NTPDase1 is the dominant NTPDase of the vasculature. By hydrolyzing proinflammatory ATP and platelet-activating ADP to AMP, it blocks platelet aggregation and supports blood flow. Thus, great interest exists in understanding the structure and dynamics of this prototype member of the eukaryotic NTPDase family. Here, we report the crystal structure of a variant of soluble NTPDase1 lacking a putative membrane interaction loop identified between the two lobes of the catalytic domain. ATPase and ADPase activities of this variant are determined via a newly established kinetic isothermal titration calorimetry assay and compared to that of the soluble NTPDase1 variant characterized previously. Complex structures with decavanadate and heptamolybdate show that both polyoxometallates bind electrostatically to a loop that is involved in binding of the nucleobase. In addition, a comparison of the domain orientations of the four independent proteins in the crystal asymmetric unit provides the first direct experimental evidence for a domain motion of NTPDases. An interdomain rotation angle of up to 7.4° affects the active site cleft between the two lobes of the protein. Comparison with a previously solved bacterial NTPDase structure indicates that the domains may undergo relative rotational movements of more than 20°. Our data support the idea that the influence of transmembrane helix dynamics on activity is achieved by coupling to a domain motion.

The stability of chicken nucleoside triphosphate diphosphohydrolase 8 requires both of its transmembrane domains

Chicken nucleoside triphosphate diphosphohydrolase 8 (NTPDase8) is a cell surface ectonucleotidase with a large extracellular domain (ECD) containing the active site and is anchored to the membrane by two transmembrane domains (TMDs) at the N- and C-termini. Unlike other cell surface NTPDases that have been characterized, the chicken NTPDase8 is not susceptible to substrate inactivation or agents that cause membrane perturbation. To determine if the stability of the enzyme is inherent in its ECD, the cDNA construct of the soluble chicken NTPDase8 was expressed and the protein purified. The ATPase activity of the purified soluble chicken NTPDase8 was less than 15% of that of the purified full-length enzyme. Strikingly, in contrast to the membrane-bound enzyme, the activity of the soluble chicken NTPDase8 decreased with time in a temperature-dependent manner as a result of inactivation by ATP, ADP, and P(i). Truncated mutants in which the ECD is anchored by a single TMD at either the N- or the C-terminus by the native chicken NTPDase TMDs or a TMD from a different NTPDase, human NTPDase2, also displayed a nonlinear time course of ATP hydrolysis. While removal of the N- or C-terminal TMD affected protein expression differently, the truncated mutants were generally similar to the soluble chicken NTPDase8 with respect to ATP, ADP, and P(i) inactivation. Other biochemical characteristics, e.g., ATPase/ADPase ratios, inhibition by azide, and affinity for ATP, were also altered when one or both of the TMDs were removed from the chicken NTPDase8. These results indicate that (1) both TMDs of the chicken NTPDase8 are required to maintain stability of the enzyme and maximal catalytic activity and (2) the conformations of the ectodomain in the soluble enzyme and the truncated mutants differ from that of the full-length chicken NTPDase8.