The three-dimensional structures of two isoforms of nucleoside diphosphatekinase from bovine retina

The crystal structures of two isoforms of nucleoside diphosphate kinase fro m bovine retina overexpressed in Escherischia coli have been determined to 2.4 Angstrom resolution. Both the isoforms, NBR-A and NBR-B, are hexameric and the fold of the monomer is in agreement with NDP-kinase structures from other biological sources. Although the polypeptide chains of the two isofo rms differ by only two residues, they crystallize in different space groups . NBR-A crystallizes in space group P2(1)2(1)2(1) with an entire hexamer in the asymmetric unit, while NBR-B crystallizes in space group P4(3)2(1)2 wi th a trimer in the asymmetric unit. The highly conserved nucleotide-binding site observed in other nucleoside diphosphate kinase structures is also ob served here, Both NBR-A and NBR-B were crystallized in the presence of cGMP . The nucleotide is bound with the base in the anti conformation. The NBR-A active site contained both cGMP and GDP each bound at half occupancy. Pres umably, NBR-A had retained GDP (or GTP) from the purification process. The NBR-B active site contained only cGMP.