RNA binding of recombinant nucleocapsid proteins of hantaviruses

Genes encoding the nucleocapsid (N) proteins of two hantaviruses, Hantaan virus strain 76-118 (HTN) and Puumala virus strain CG 18-20 (PUU), were expressed in Escherichia coli as histidine-tagged proteins. They were purified by metal-chelate affinity chromatography under native or denaturing conditions to near homogeneity. The soluble form of HTN N protein was associated with RNA of E. coli. Renatured N proteins were shown to bind in vitro transcribed RNA representing the hantaviral small genomic (S) RNA segment. RNA binding was shown by affinity to filter-immobilized N proteins and by gel mobility shift assays. Competition experiments using tRNA, poly(U) and poly(A)+ U indicated that binding of RNA by the N protein is nonspecific. However, direct binding of ds-RNA resulted in efficient formation of large complexes suggesting that double-stranded nucleic acids are bound preferentially. Carboxyterminal fragments of HTN and PUU N proteins containing about 100 amino acids of the carboxy termini retained full binding capacity indicating that RNA binding occurs via a carboxyterminal domain.