Abstract
The vesivirus feline calicivirus (FCV) is a positive strand RNA virus encapsidated by an icosahedral T=3 shell formed by the viral VP1 protein. Upon its expression in the insect cell - baculovirus system in the context of vaccine development, two types of virus-like particles (VLPs) were formed, a majority built of 60 subunits (T=1) and a minority probably built of 180 subunits (T=3). The structure of the small particles was determined by x-ray crystallography at 0.8 nm resolution helped by cryo-electron microscopy in order to understand their formation. Cubic crystals belonged to space group P213. Their self-rotation function showed the presence of an octahedral pseudo-symmetry similar to the one described previously by Agerbandje and co-workers for human parvovirus VLPs. The crystal structure could be solved starting from the published VP1 structure in the context of the T=3 viral capsid. In contrast to viral capsids, where the capsomers are interlocked by the exchange of the N-terminal arm (NTA) domain, this domain is disordered in the T=1 capsid of the VLPs. Furthermore it is prone to proteolytic cleavage. The relative orientation of P (protrusion) and S (shell) domains is alerted so as to fit VP1 to the smaller T=1 particle whereas the intermolecular contacts around 2-fold, 3-fold and 5-fold axes are conserved. By consequence the surface of the VLP is very similar compared to the viral capsid and suggests a similar antigenicity. The knowledge of the structure of the VLPs will help to improve their stability, in respect to a use for vaccination.
Highlights
The Vesivirus feline calicivirus (FCV) belongs to the family of Caliciviridae which carry a positive strand RNA genome which is enclosed in a capsid composed of 180 copies of the majorPLOS ONE | DOI:10.1371/journal.pone.0119289 March 20, 2015FCV virus-like particles (VLPs) Structure and the GIS-IBISA
High-resolution crystallographic or cryoelectron microscopy structures of several caliciviruses elucidated the capsid structure for the Vesiviruses San Miguel sea lion virus (SMSV) [1], FCV [2], the Noroviruses Norwalk virus (NV) [3] and murine norovirus (MNV) [4] and the Lagovirus rabbit hemorrhagic disease virus (RHDV) [5]
As the viral particles of FCV have a diameter of 40 nm [2], we assumed that the big form corresponded to T = 3 icosahedral VLPs composed of 180 molecules of VP1 whereas the small form corresponded to T = 1 VLPs composed of 60 subunits
Summary
The Vesivirus feline calicivirus (FCV) belongs to the family of Caliciviridae which carry a positive strand RNA genome which is enclosed in a capsid composed of 180 copies of the major. High-resolution crystallographic or cryoelectron microscopy structures of several caliciviruses elucidated the capsid structure for the Vesiviruses San Miguel sea lion virus (SMSV) [1], FCV [2], the Noroviruses Norwalk virus (NV) [3] and murine norovirus (MNV) [4] and the Lagovirus rabbit hemorrhagic disease virus (RHDV) [5] These structures showed that the major capsid protein (VP1 in vesiviruses) is composed of 3 domains: a short disordered peptide downstream of the proteolytic cleavage site We observed that the majority of the FCV VLPs obtained from the expression of VP1 in the baculovirus system are small sub-viral particles made of 60 subunits with T = 1 symmetry different from viral capsids We determined their structure by x-ray crystallography helped by an initial reconstruction from cryo-electron microscopy. As an octahedral self-organization of the recombinant protein in the context of the crystal could not be excluded in beforehand, the pseudo-symmetry was misleading
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