INTERMEDIATES IN THE ASSEMBLY PATHWAY OF THE DOUBLE-STRANDED-RNA VIRUS PHI-6

The double-stranded RNA bacteriophage phi 6 contains a nucleocapsid en closed by a lipid envelope. The nucleocapsid has an outer layer of pro tein P8 and a core consisting of the four proteins P1, P2, P4 and P7. These four proteins form the polyhedral structure which acts as the RN A packaging and polymerase complex, Simultaneous expression of these f our proteins in Escherichia coli gives rise to procapsids that can car ry out the entire RNA replication cycle. Icosahedral image reconstruct ion from cryo electron micrographs was used to determine the three-dim ensional structures of the virion-isolated nucleocapsid and core, and of several procapsid-related particles expressed and assembled in E. c oli. The nucleocapsid has a T = 13 surface lattice, composed primarily of P8. The core is a rounded structure with turrets projecting from t he 5-fold vertices, while the procapsid is smaller than the core and m ore dodecahedral. The differences between the core and the procapsid s uggest that maturation involves extensive structural rearrangements pr oducing expansion. These rearrangements are co-ordinated with the pack aging and RNA polymerization reactions that result in virus assembly, This structural characterization of the phi 6 assembly intermediates r eveals the ordered progression of obligate stages leading to virion as sembly along with striking similarities to the corresponding Reovirida e structures.