Characterization of a eukaryotic type serine/threonine kinase in Spodoptera litura nucleopolyhedrovirus-I (SpltNPV-I)

An open reading frame (ORF) of 819 nt coding for a predicted protein of 272 amino acids was identified in the genome of Spodoptera litura nucleopolyhedrovirus (SpltNPV-I). Sequence derived amino acid sequence analysis of this ORF suggested it to be a eukaryotic type protein kinase having conserved I–XI subdomains of Hanks kinase. In addition to kinase catalytic domains, this hypothetical protein had two bromodomains which could play regulatory roles in transcription. The ORF was expressed as 31 kDa apoprotein in E. coli and 33 kDa glycoprotein in Sf9 cells, and was called SpltNPV-I pk1 or pk1. The protein was localized in the nuclei of infected cells of the SpltNPV-I permissive cell line, NIV-HA-197. The recombinant protein had autophosphorylation and substrate phosphorylation activities in presence of Mn2+ or Mg2+, and these activities were inhibited by staurosporine. Mutation of Lys-50 to Met but not Lys-44 to Gln abolished pk1 kinase activity. Kinetics of pk1 showed that the rate of phosphorylation of SpltNPV-I pk1 > MBP > histone H1, and both MBP and histone H1 had the Kms of 3 μM. Analysis of phosphorylated protein showed the phosphorylation of serine and threonine residues, but not tyrosine. All these results suggested that identified SpltNPV-I ORF codes for a serine/threonine kinase.