Cloning and sequence analysis of lily and tobacco guanylate kinases

Guanylate kinase is an essential enzyme in the nucleotide biosynthetic pathway, catalyzing the reversible transfer of the terminal phosphoryl group of ATP to GMP or dGMP. This enzyme has been well studied from several organisms and many structural and functional details have been characterized. Animal GMP kinases have also been implicated in signal transduction pathways. However, the corresponding role by plant derived GMP kinases remains to be elucidated. Full-length cDNA clones encoding enzymatically active guanylate kinases were isolated from cDNA libraries of lily and tobacco. Lily cDNA is predicted to encode a 392-amino acid protein with a molecular mass of 43.1 kDa and carries amino- and carboxy- terminal extensions of the guanylate kinase (GK)-like domain. But tobacco cDNA is predicted to encode a smaller protein of 297-amino acids with a molecular mass of 32.7 kDa. The amino acid residues known to participate in the catalytic activity of functionally characterized GMP kinases, are also conserved in GK domains of LGK-1 and NGK-1. The GK domains of NGK-1, LGK-1 and previously characterized AGK-1 from Arabidopsis exhibit 74-84% identity, whereas their N- and C-terminal domains are more divergent with amino acid conservation in the order of 48-55%. Phylogenetic analysis on the deduced amino acid sequences reveals that NGK-1 and LGK-1 form one distinct subgroup along with AGK-1 and AGK-2 homologues from Arabidopsis. Isolation of GMP kinases from diverse plant species like lily and tobacco adds a new dimension in understanding their role in cell signaling pathways that are associated with plant growth and development.