CLONING, SEQUENCING, AND EXPRESSION OF A HUMAN BRAIN ECTO-APYRASE RELATED TO BOTH THE ECTO-ATPASES AND CD39 ECTO-APYRASES

An extracellular ATPase (E-type ATPase) clone was isolated from a huma n brain cDNA library and sequenced. The transcript shows similarity to the previously published chicken smooth muscle and rat brain ecto-ATP ase cDNAs, human CD39L1 cDNA (putative human ecto-ATPase), and mammali an CD39 (lymphoid cell activation antigen, ecto-apyrase, ATPDase, ATP- diphosphohydrolase) cDNAs. The full-length human brain cDNA encodes a 529 amino acid glycoprotein with a putative membrane spanning region n ear each terminus, with the majority of the protein found extracellula rly. Expression of this clone in mammalian COS-1 cells yielded NaN3-se nsitive ATPase and ADPase activity detectable both on intact cells and cell membrane preparations. The nucleotide hydrolysis ratio of the ex pressed protein is approx. 2.75:1 (ATPase:ADPase activity), classifyin g it as an ecto-apyrase. However, this hydrolysis ratio is intermediat e between that observed for the ecto-ATPases and the CD39 ecto-apyrase s (L.. Plesner, Int. Rev. Cytol. 158 (1995) 141-214). Quantitative ana lyses of amino acid identities and similarities between this ecto-apyr ase and other vertebrate E-type ATPases suggest that this human brain enzyme is nearly equally related to the ecto-ATPases and the CD39s, an d phylogenetic analysis suggests that it could be an ancestral enzyme from which both ecto-ATPases and CD39 ecto-apyrases are derived. (C) 1 998 Elsevier Science B.V. All rights reserved.