RNA and Nucleocapsid Are Dispensable for Mature XXXX Capsid Assembly.

@entity2858 @entity645 @entity5024 @entity368
@entity2858 ) is released from infected cells in an immature, noninfectious form in which the structural polyprotein @entity5024 is arranged in a hexameric lattice, forming an incomplete spherical shell. Maturation to the infectious form is mediated by the viral protease, which cleaves @entity5024 at five sites, releasing the CA (capsid) protein, which forms a conical capsid encasing the condensed RNA genome. The pathway of this structural rearrangement is currently not understood, and it is unclear how cone assembly is initiated. RNA represents an integral structural component of retroviruses, and the viral nucleoprotein core has previously been proposed to nucleate mature capsid assembly. We addressed this hypothesis by replacing the RNA-binding NC (nucleocapsid) domain of @entity2858 @entity5024 and the adjacent spacer peptide 2 (SP2) by a @entity368 zipper (LZ) protein-protein interaction domain [ @entity5024 (LZ)] in the viral context. We found that @entity5024 (LZ)-carrying virus [ @entity645 (LZ)] was efficiently released and viral polyproteins were proteolytically processed, though with reduced efficiency. Cryo-electron tomography revealed that the particles lacked a condensed nucleoprotein and contained an increased proportion of aberrant core morphologies caused either by the absence of RNA or by altered @entity5024 processing. Nevertheless, a significant proportion of @entity645 (LZ) particles contained mature capsids with the wild-type morphology. These results clearly demonstrate that the nucleoprotein complex is dispensable as a nucleator for mature @entity2858 capsid assembly in the viral context. IMPORTANCE: Formation of a closed conical capsid encasing the viral RNA genome is essential for @entity2858 infectivity. It is currently unclear what viral components initiate and regulate the formation of the capsid during virus morphogenesis, but it has been proposed that the ribonucleoprotein complex plays a role. To test this, we prepared virus-like particles lacking the viral nucleocapsid protein and RNA and analyzed their three-dimensional structure by cryo-electron tomography. While most virions displayed an abnormal morphology under these conditions, some particles showed a normal mature morphology with closed conical capsids. These data demonstrate that the presence of RNA and the nucleocapsid protein is not required for the formation of a mature, cone-shaped @entity2858 capsid.
