Umbraviruses have been a long-term topic of research at SCRI. These viruses differ from other plant viruses in that they do not have a coat protein and do not produce conventional particles. The normal requirement of a coat protein for efficient spread through the plant is compensated for by the ORF3 protein that forms filamentous ribonucleoprotein particles (RNPs) with the viral RNA. Formation of these RNPs required for long-distance movement is dependent on the interaction of the ORF3 protein with fibrillarin, a major host nucleolar protein.

TEM analysis of complexes formed in vitro shows that ORF3 protein and viral RNA alone merely formed non-uniform structures comprising interwoven filaments. In contrast, complexes of viral RNA, ORF3 protein and fibrillarin formed regular filamentous structures similar to viral RNPs formed in vivo, which were infectious and RNase resistant. In TEM the particles had a diameter of about 20nm, and helical repeat structures were observed on some filaments, in agreement with observations for in vivo RNPs.