Structural basis of allotypes of ecto-nucleotide pyrophosphatase/phosphodiesterase (plasma cell membrane glycoprotein PC-1) in the mouse and rat, and analysis of allele-specific xenogeneic antibodies

Ecto-nucleotide pyrophosphatase/phosphodiesterases (E-NPPs) have been implicated in bone calcification, type II diabetes, control of purinergic signalling and tumour invasion. The gene for the plasma cell membrane glycoprotein PC-1 in the mouse (Enpp1) has been known since 1970 to exist in two allelic forms, but their structural basis was heretofore unknown. We show that the Enpp1(a) and Enpp1(b) alleles differ by only two amino acids, at positions 650 and 679 in the C-terminal nuclease-like domain. Histidine 650 but not arginine 679 forms an essential part of the Enpp1(a) epitope recognized by monoclonal antibody IR-518. Sequences of LEW and LOU rats and the rat glioma cell line C6 differ from that of the mouse by about 60 amino acids. The LOU and C6 cell line sequences differ by only three amino acids, but differ from the LEW sequence by 10 amino acids. All three rat strains possess the mouse Enpp1(b) allele at positions 650 and 679. Despite numerous other differences from the mouse, rats immunized with Enpp1(a) mouse cells have generated monoclonal antibodies specific for the Enpp1(a) allele, suggesting that amino acids 650 and 679 may be particularly immunogenic. The cytoplasmic tails of the mouse and rat are highly conserved, but are significantly different from human cytoplasmic tails.