Phosphorylation of the nucleocapsid protein of Hantaan virus by casein kinase II

Hantaanvirus (HTNV) is the prototype of the genus Hantavirus, which belongs to the family Bunyaviridae. Hantaviruses are carried and transmitted by rodents and are known to cause two serious disease syndromes in humans i.e., hemorrhagic fever with renal syndrome (HFRS) and the hantavirus pulmonary syndrome (HPS). HTNV is an enveloped virus that contains a tripartite genome consisting of three negative-sense RNA segments (L, M, S), and the S and M segment of HTNV, respectively, encode the viral nucleocapsid protein (NP) and envelope glycoproteins. Possible phosphorylation motifs of casein kinase II (CKII) and protein kinase C (PKC) were identified in HTNV NP through bioinformatics searches. Sucrose gradient SDS-PAGE analysis indicated that dephosphorylated HTNV NP migrated faster than non-dephosphorylated NP, suggesting that HTNV NP is phosphorylated in infected Vero E6 cells. Immunoblot anaylsis of HTNV particles with anti-phosphoserine antibody and anti-phosphothreonine antibody after immunoprecipitation showed that viral particles are readily phosphorylated at threonine residues. In vitro kinase assay further showed that HTNV NP is phosphorylated by CK II, but not by PKC. Full length or truncated HTNV NPs expressed in E. coli were phosphorylated in vitro by CKII suggesting that phosphorylation may occur in vivo at multiple sites. Site specific mutagenesis studies suggest that HTNV NP phosphorylation might occur at unknown sites excluding the site-directly mutagenized locations. Taken together, HTNV NP can be phosphorylated mainly at threonine residues in vivo by CK II treatment.