Influenza virus RNA encapsidation: discrimination for vRNA versus cRNA molecules

Abstract

The packaging signal of influenza viral RNA is shown to reside within the 5′ bulged promoter structure, caused by the central unpaired residue A10 in its 5′ branch. Upon insertion of two uridine residues in the 3′ branch opposite A10, the minus-strand vRNA promoter is converted into a 3′ bulged structure, while the plus-strand cRNA promoter adopts the 5′ bulged conformation. The cRNA is packaged exclusively in the progeny virions of this promoter variant. Upon insertion of a single uridine nucleotide opposite A10, the two de-bulged structures of the vRNA and cRNA promoters are rendered identical, and both vRNA and cRNA molecules are packaged indiscriminately, in a 1:1 ratio. We propose that interaction of viral polymerase with the two differently bulged vRNA and cRNA promoter structures results in either of two conformations of the protein, which in turn are recognized in a two-step procedure by matrix protein M1. While both, viral nucleoprotein particles (vRNPs) and cRNPs, are exported from the nucleus in association with M1, attachment to only the 5′ bulged vRNPs is maintained during encapsidation at the plasma membrane.