Structure of a pentameric virion-associated fiber with a potential role in Orsay virus entry to host cells

Despite the wide use of caenorhabditis elegans as a model organism,the first virus naturally infecting this organism was not discovered until six years ago. the orsay virus and its related nematode viruses have a positive-sense rna genome,encoding three proteins: cp,rdrp,and a novel δ protein that shares no homology with any other proteins. δ can be expressed either as a free δ or a cp-δ fusion protein by ribosomal frameshift,but the structure and function of both δ and cp-δ remain unknown. using a combination of electron microscopy,x-ray crystallography,computational and biophysical analyses,here we show that the orsay δ protein forms a ~420-å long,pentameric fiber with an n-terminal α-helical bundle,a β-stranded filament in the middle,and a c-terminal head domain. the pentameric nature of the δ fiber has been independently confirmed by both mass spectrometry and analytical ultracentrifugation. recombinant orsay capsid containing cp-δ shows protruding long fibers with globular heads at the distal end. mutant viruses with disrupted cp-δ fibers were generated by organism-based reverse genetics. these viruses were found to be either non-viable or with poor infectivity according to phenotypic and qrt-pcr analyses. furthermore,addition of purified δ proteins to worm culture greatly reduced orsay infectivity in a sequence-specific manner. based on the structure resemblance between the orsay cp-δ fiber and the fibers from reovirus and adenovirus,we propose that cp-δ functions as a cell attachment protein to mediate orsay entry into worm intestine cells. © 2017 fan et al.