Structure and function of ectoapyrase (CD39)

Abstract

Although the presence of nucleotidase activities on the extracellular surface of cells has been known for many years, the enzymes responsible for these activities in animal cells have only recently been identified. Here, we describe how we cloned the gene for potato apyrase and, thus, found the way to identify CD39 as the animal cell ectoapyrase. CD39 has two transmembrane domains, one at each end of the molecule, small cytoplasmic NH2- and COOH-terminal domains, and a large extracellular domain with the enzymatic activity. One of the characteristic features of this enzyme is the loss of activity caused by solubilization with detergents. Exploration of this phenomenon revealed that ectoapyrase (CD39) is a tetramer in the membrane; solubilization with Triton X-100 causes dissociation of the tetramer to monomers with concomitant loss of activity. On the other hand, the enzyme can be solubilized with digitonin without loss of activity and retention of the tetrameric state. The conclusion is that the transmembrane segments of the protein are involved in association of the monomers to tetramers. Drug Dev. Res. 45:245–252, 1998. © 1998 Wiley-Liss, Inc.