BIOCHEMICAL-CHARACTERIZATION OF HUMAN PC-1, AN ENZYME POSSESSING ALKALINE PHOSPHODIESTERASE-I AND NUCLEOTIDE PYROPHOSPHATASE ACTIVITIES

PC-1 is an ecto-enzyme possessing alkaline phosphodiesterase I and nuc leotide pyrophosphatase activities. In this paper, we demonstrate the expression, biochemical characterization and biosynthesis of human PC- 1. Previously, there has been uncertainty concerning which of two meth ionine residues is the initiator. It is now shown that expression of P C-1 is much greater if the first methionine residue is present, and th at the sequence between the two methionine residues is translated in b oth human and mouse, in both transfected cells and cells naturally exp ressing PC-1. The first methionine residue is therefore the initiator. Human PC-1 is capable of autophosphorylation, and conditions are desc ribed in which PC-1 is the only labelled phosphoprotein on the plasma membranes of intact cells, allowing the demonstration that the mature membrane form of human PC-1 is approximately 10 kDa larger than that o f the mouse form. Pulse-chase biosynthetic studies and treatment with two different endoglycosidases show that most of this difference is du e to N-linked oligosaccharides. The polypeptide backbone of human PC-1 is 20 amino acids longer than that of the mouse PC-1, with most of th e difference in polypeptide chain length being in the cytoplasmic doma in. The revised cytoplasmic domain of human PC-1 has 76 amino acids, w hile the mouse cytoplasmic domain has 58 amino acids. Optimal alignmen t of mouse and human cytoplasmic domains reveals areas of sequence con servation in which the third bases vary. It is suggested that these re gions of conservation may point to functionally important sequences in the cytoplasmic domain.