Protein kinase activity of Kaposi's sarcoma associated herpesvirus (KSHV) ORF 36

Abstract

The Kaposi``s Sarcoma associated Herpesvirus(KSHV/HHV-8) is a γ herpesvirus that is implicated in the pathogenesis of Kaposi``s sarcoma. The nucleotide sequence of the KSHV open reading frame 36 predicts a polypeptide with significant sequence homology to the catalytic domain of known protein kinases. To determine whether the KSHV ORF36 polypeptide is a protein kinase, we have expressed it in COS-1 cell as a fusion with glutathione S-transferase. Affinity precipitated preparations of fusion protein were found to be autophosphorylated. Mutation of lysine 108 to glutamine within the conserved kinase catalytic domain II decreased protein kinase activity dramatically, supporting the notion that the protein kinase activity is inherent to the ORF36 polypeptide. The divalent cation preference of KSHV ORF36 was determined by measuring kinase activity in the presence of various concentrations of $Mn^{2+} or Mg^{2+}$, and KSHV ORF36 was shown to prefer $Mn^{2+} to Mg^{2+} for maximal kinase activity. Phosphoamino acid analysis showed that KSHV ORF36 fusion protein was phosphorylated on a serine residue, implicating that KSHV ORF36 encoded a serine/threonine kinase. To define the subcellular localization of KSHV ORF36, we expressed Green Fluorescence Protein(GFP) KSHV ORF36 fusion protein in COS-1 cell, and subsequent microscopy experiment showed that KSHV ORF36 localized in the nucleus.