Either the carboxyl- or the amino-terminal region of the human ecto-ATPase (E-NTPDase 2) confers detergent and temperature sensitivity to the chicken ecto-ATP-diphosphohydrolase (E-NTPDase 8)

Human ecto-ATPase (E-NTPDase 2) and chicken ecto-ATP-diphosphohydrolase (E-NTPDase 8) are cell surface nucleotidases with two transmembranous domains, one each at the N- and C-termini. Hydrolysis of substrates occurs in active sites residing in their extracellular domains. Human ecto-ATPase activity is decreased by NP-40 and at temperatures higher than 37 T. Reduction of activity is abolished by prior cross-linking of the ecto-ATPase by lectin and chemical cross-linking agents [Knowles, A. F., and Chiang, W.-C. (2003) Arch. Biochem. Biophys. 418, 217-227]. In contrast, the chicken ecto-ATP-diphosphohydrolase is not inhibited by NP-40, and activity is approximately 2-fold higher at 55 degrees C. To determine if the transmembranous domains of the two E-NTPDases mediate their respective responses to detergents and high temperature, we first constructed a chimera (ck-hu ACR5) in which the C-terminus of the chicken ecto-ATP-diphosphohydrolase is substituted by the corresponding region of the human ecto-ATPase. While this chimera displays many similar enzymatic characteristics as the parental chicken ecto-ATP-diphosphohydrolase, its inhibition by NP-40, high temperature, and substrate resemble that of the human ecto-ATPase, which donates the C-terminus including the C-terminal transmembranous domain. Additionally, comparison of the effects of ConA, disuccinimidyl suberate, and glutaraldehyde on the parental enzymes and the chimera indicated that catalysis which occurs in the extracellular domains of the two E-NTPDases responds differently to conformational constraints. Enzyme activity of a second chimera (ck-hu ACR1) in which the N-terminus of the chicken ecto-ATP-diphosphohydrolase is substituted by the corresponding region of the human ecto-ATPase is also inhibited by NP-40 and is less active at 55 degrees C; however, its temperature dependence differs from that of ck-hu ACR5. These results indicate that (1) the C- and N-termini of the two E-NTPDases encompassing the two transmembranous domains are important elements in determining the sensitivity of the human ecto-ATPase to NP-40 and high temperatures; (2) incorporation of either the C- or N-terminus of the human ecto-ATPase alone in the chicken ecto-ATP-diphosphohydrolase is sufficient to impart negative regulation on ATP hydrolysis due to membrane perturbation; and (3) interactions of the two sets of heterologous transmembranous domains are not equivalent, which are most likely related to their different amino acid sequences.