SPECIFIC ENCAPSIDATION OF NODAVIRUS RNAS IS MEDIATED THROUGH THE C-TERMINUS OF CAPSID PRECURSOR PROTEIN-ALPHA

Flock house virus (FHV) is a small icosahedral insect virus with a bip artite, messenger-sense RNA genome. Its T=3 icosahedral capsid is init ially assembled from 180 subunits of a single type of coat protein, ca psid precursor protein alpha (407 amino acids). Following assembly, th e precursor particles undergo a maturation step in which the alpha sub units autocatalytically cleave between Asn363 and Ala364. This cleavag e generates mature coat proteins beta (363 residues) and gamma (44 res idues) and is required for acquisition of virion infectivity. The X-ra y structure of mature FIN shows that gamma peptides located at the fiv efold axes of the virion form a pentameric helical bundle, and it has been suggested that this bundle plays a role in release of viral RNA d uring FIN uncoating. To provide experimental support for this hypothes is, we generated mutant coat proteins that carried deletions in the ga mma region of precursor protein alpha. Surprisingly, we found that the se mutations interfered with specific recognition and packaging of vir al RNA during assembly. The resulting particles contained large amount s of cellular RNAs and varying amounts of the viral RNAs, Single-site amino acid substitution mutants showed that three phenylalanines locat ed at positions 402, 405, and 407 of coat precursor protein alpha were critically important for specific recognition of the FIN genome, Thus , in addition to its hypothesized role in uncoating and RNA delivery, the C-terminal region of coat protein alpha plays a significant role i n recognition of FIN RNA during assembly. A possible link between thes e two functions is discussed.