EARLY STEPS IN REOVIRUS INFECTION ARE ASSOCIATED WITH DRAMATIC CHANGES IN SUPRAMOLECULAR STRUCTURE AND PROTEIN CONFORMATION - ANALYSIS OF VIRIONS AND SUBVIRAL PARTICLES BY CRYOELECTRON MICROSCOPY AND IMAGE-RECONSTRUCTION

Three structural forms of type 1 Lang reovirus (virions, intermediate subviral particles [ISVPs], and cores) have been examined by cryoelectron microscopy (cryoEM) and image reconstruction at 27 to 32-angstrom resolution. Analysis of the three-dimensional maps and known biochemical composition allows determination of capsid protein location, globular shape, stoichiometry, quaternary organization, and interactions with adjacent capsid proteins. Comparisons of the virion, ISVP and core structures and examination of difference maps reveal dramatic changes in supramolecular structure and protein conformation that are related to the early steps of reovirus infection. The intact virion (approximately 850-angstrom diam) is designed for environmental stability in which the dsRNA genome is protected not only by tight sigma3-mu1, lambda2-sigma3, and lambda2-mul interactions in the outer capsid but also by a densely packed core shell formed primarily by lambda1 and sigma2. The segmented genome appears to be packed in a liquid crystalline fashion at radii < 240 angstrom. Depending on viral growth conditions, virions undergo cleavage by enteric or endosomal/lysosomal proteases, to generate the activated ISVP (approximately 800-angstrom diam). This transition involves the release of an outer capsid layer spanning radii from 360 to 427 angstrom that is formed by 60 tetrameric and 60 hexameric clusters of ellipsoidal subunits of sigma3. The vertex-associated cell attachment protein, sigma1, also undergoes a striking change from a poorly visualized, more compact form, to an extended, flexible fiber. This conformational change may maximize interactions of sigma1 with cell surface receptors. Transcription of viral mRNAs is mediated by the core particle (approximately 600-angstrom diam), generated from the ISVP after penetration and uncoating. The transition from ISVP to core involves release of the 12 sigma1 fibers and the remaining outer capsid layer formed by 200 trimers of rod-shaped mu1 subunits that span radii from 306 to 395 angstrom. In the virion and ISVP, flower-shaped pentamers of the lambda2 protein are centered at the vertices. In the ISVP-to-core transition, domains of the lambda2 subunits rotate and swing upward and outward to form a turretlike structure extending from radii 305 to 400 angstrom, with a diameter of 184 angstrom, and a central channel 84 angstrom wide. This novel conformational change allows the potential diffusion of substrates for transcription and exit of newly synthesized mRNA segments. The essence of these orchestrated events is that reovirus is superbly designed to undergo stages of controlled disassembly in which the release of oligomeric protein layers in the outer capsid is coordinated with dramatic protein conformational changes at the icosahedral vertices.