Chicken guanylate-binding protein. Conservation of GTPase activity and induction by cytokines

To gain further insights into the cytokine network of birds, we used polymerase chain reaction technology to clone a cDNA that codes for a chicken homolog of the interferon-induced guanylate-binding proteins (GBPs). In its N-terminal moiety, the 64-kDa chicken GBP contains two sequence blocks of 100 and 19 amino acids, respectively, that are about 70% identical to mammalian GBPs. The first region includes two motifs of the canonical GTP-binding consensus element. The other parts of chicken GBP are poorly conserved, except for a CAAX motif at the extreme C terminus which might signal isoprenylation. Like mammalian GBPs, recombinant chicken GBP specifically bound to agarose-immobilized guanine nucleotides and hydrolyzed GTP to both GDP and GMP. Regulation by interferons was also conserved: chicken GBP RNA was barely detectable in uninduced chicken cells. Low GBP RNA levels were found in cells treated with type I interferon, whereas very high levels were observed in cells treated with supernatant of a chicken T cell line that secretes a gamma-interferon-like activity. Together with recent phylogenetic studies of interferon genes, these results suggest that in spite of low sequence conservation, the various components of the avian interferon system are functionally well conserved.