Novel Inter-Subunit Contacts in Barley Stripe Mosaic Virus Revealed by Cryo-Electron Microscopy

Abstract

SummaryBarley stripe mosaic virus (BSMV, genus Hordeivirus) is a rod-shaped single-stranded RNA virus similar to viruses of the structurally characterized and well-studied genus Tobamovirus. Here we report the first high-resolution structure of BSMV at 4.1 Å obtained by cryo-electron microscopy. We discovered that BSMV forms two types of virion that differ in the number of coat protein (CP) subunits per turn and interactions between the CP subunits. While BSMV and tobacco mosaic virus CP subunits have a similar fold and interact with RNA using conserved residues, the axial contacts between the CP of these two viral groups are considerably different. BSMV CP subunits lack substantial axial contacts and are held together by a previously unobserved lateral contact formed at the virion surface via an interacting loop, which protrudes from the CP hydrophobic core to the adjacent CP subunit. These data provide an insight into diversity in structural organization of helical viruses.

Helical capsids of plant viruses: architecture with structural lability

“…1b). This contact involves both charged and hydrophobic residues (Clare et al, 2015). The functional importance of this contact is confirmed by site-directed mutagenesis: mutation of Ile86 and Tyr91 in the interacting loop to Gly residues results in the inability of BSMV CP to form stable virions.…”

“…In 2015, the structure of two other potexviruses was resolved at high resolution by cryoelectron microscopy, which shed new light on the details of intersubunit interactions in filamentous virions (Agirrezabala et al, 2015;DiMaio et al, 2015). Additionally, the structure of rod-shaped Barley stripe mosaic virus (BSMV, genus Hordeivirus) has recently been obtained at near-atomic resolution, showing striking differences from TMV in intersubunit contacts (Clare et al, 2015).…”

“…However, structural studies of the BSMV CP and virions employing analyses of circular dichroism spectra, differential scanning calorimetry and tryptophan spectra have demonstrated that the BSMV virions are considerably more labile than the TMV virions and, in general, the physicochemical characteristics of BSMV virions resemble those of flexuous filamentous viruses rather than TMV (Makarov et al, 2013). The recently reported high-resolution structure of BSMV at 4.1 Å obtained by cryoelectron microscopy (Protein Data Bank accession numbers 5a79 and 5a7a) provides an explanation for this contradiction (Clare et al, 2015).…”

“…Wide virions have 111 subunits per helix period, while narrow ones have 106 subunits, corresponding to a difference of one CP subunit per helix turn (Clare et al, 2015). Accordingly, both the conformation of CP molecules and their contacts differ in the two virion forms, providing another example of quasi-equivalent interactions in plant virus helical capsids.…”

“…Both forms co-exist in populations of BSMV virions, but it is not known whether the two virion forms are assembled simultaneously and independently from each other in one cell or tissue, or if only one virion form is assembled under the given conditions and later undergoes a structural transition resulting in the observed mixed virion populations. Structural data show that the narrow BSMV virions are more stable than wide virions due to stronger CP contacts with RNA and additional contacts between CP subunits (Clare et al, 2015). Hypothetically, the stable narrow virion form could be considered as a bona fide storage container for the viral genome, while the wide form could be an intermediate in virion uncoating.…”