The nucleolus and specific
nucleolar proteins are involved in the life cycles of some plant and animal viruses, but the functions of these
proteins and of nucleolar trafficking in
virus infections are largely unknown. The ORF3
protein of the plant virus, groundnut rosette virus (an umbravirus), has been shown to cycle through the nucleus, passing through Cajal bodies to the nucleolus and then exiting back into the cytoplasm. This journey is absolutely required for the formation of viral
ribonucleoprotein particles (RNPs) that, themselves, are essential for the spread of the virus to noninoculated leaves of the shoot tip. Here, we show that these processes rely on the interaction of the ORF3
protein with
fibrillarin, a major
nucleolar protein. Silencing of the
fibrillarin gene prevents long-distance movement of groundnut rosette virus but does not affect viral replication or cell-to-cell movement. Repressing
fibrillarin production also localizes the ORF3
protein to multiple Cajal body-like aggregates that fail to fuse with the nucleolus. Umbraviral ORF3
protein and
fibrillarin interact in vitro and, when mixed with umbravirus
RNA, form an RNP complex. This complex has a filamentous structure with some regular helical features, resembling the RNP complex formed in vivo during umbravirus
infection. The filaments formed in vitro are infectious when inoculated to plants, and their infectivity is resistant to
RNase. These results demonstrate previously undescribed functions for
fibrillarin as an essential component of translocatable viral RNPs and may have implications for other plant and animal viruses that interact with the nucleolus.