The orthologue of the acatalytic mammalian ART4 in chicken is an arginine-specific mono-ADP-ribosyltransferase

Background: Human ART4, carrier of the GPI-(glycosyl-phosphatidylinositol) anchored Dombrock blood group antigens, is an apparently inactive member of the mammalian mono-ADP-ribosyltransferase (ART) family named after the enzymatic transfer of a single ADP- ribose moiety from NAD+ to arginine residues of extracellular target proteins. All known mammalian ART4 orthologues are predicted to lack ART activity because of one or more changes in essential active site residues that make up the R-S-EXE motif. So far, no other function has been detected. Results: Here we report the identification and characterisation of ART4 in chicken, which to our knowledge is the first true non-mammalian orthologue of a mammalian ART family member. The chicken ART4 gene has the same physical structure as its mammalian counterparts ( three coding exons separated by two introns in phase 0 and phase 1, respectively) and maps to a region of conserved linkage synteny on chromosome 1. Its mRNA encodes a 289 amino acid protein with predicted N-terminal signal peptide and C-terminal GPI- anchor sequences and 47% sequence identity to human ART4. However, in striking contrast to its mammalian orthologues, the chicken protein contains an intact R-S-EXE motif. Upon ectopic expression in C-33A cells, recombinant chicken ART4 localized at the cell surface as a GPI- anchored, highly glycosylated protein, which displayed arginine-specific ART activity (apparent Km of the recombinant protein for etheno-NAD+1.0 +/- 0.18 mu M). Conclusion: The avian orthologue of the acatalytic mammalian ART4 is a mono-ADP-ribosyltransferase with enzymatic activity comparable to that of other, catalytically active and GPI-anchored members of the mammalian ART family.