Structure of native and expanded sobemoviruses by electron cryo-microscopyand image reconstruction

Rice yellow mottle virus (RYMV) and southern bean mosaic virus, cowpea stra in (SCPMV) are members of the Sobemovirus genus of RNA-containing viruses. We used electron cryo-microscopy (cryo-EM) and icosahedral image analysis t o examine the native structures of these two viruses at 25 A resolution. Bo th viruses have a single tightly packed capsid layer with 180 subunits asse mbled on a T = 3 icosahedral lattice. Distinctive crown-like pentamers eman ate from the 12 5-fold axes of symmetry. The exterior face of SCPMV display s deep valleys along the 2-fold axes and protrusions at the quasi-3-fold ax es. While having a similar topography, the surface of RYMV is comparatively smooth. Two concentric shells of density reside beneath the capsid layer o f RYMV and SCPMV, which we interpret as ordered regions of genomic RNA. Ln the presence of divalent cations, SCPMV particles swell and fracture, where as the expanded form of RYMV is stable. We previously proposed that the cel l-to-cell movement of RYMV in xylem involves chelation of Ca2+ from pit mem branes of infected cells, thereby stabilizing the capsid shells and allowin g a pathway for spread of RYMV through destabilized membranes. In the conte xt of this model, we propose that the expanded form of RYMV is an intermedi ate in the in vivo assembly of virions. (C) 2000 Academic Press.