FUNGAL VIRUS CAPSIDS, CYTOPLASMIC COMPARTMENTS FOR THE REPLICATION OFDOUBLE-STRANDED-RNA, FORMED AS ICOSAHEDRAL SHELLS OF ASYMMETRIC GAG DIMERS

The primary functions of most virus capsids are to protect the viral g enome in the extra-cellular milieu and deliver it to the host. In cont rast, the capsids of fungal viruses, like the cores of all other known double stranded RNA viruses, are not involved in host recognition but do shield their genomes, and they also carry out transcription and re plication. Nascent (+) strands are extruded from transcribing virions. The capsids of the yeast virus L-A are composed of Gag (capsid protei n; 76 kDa), with a few molecules of Gag-Pol (170 kDa). Analysis of the se 420 Angstrom diameter shells and those of the fungal P4 virus by cr yo-electron microscopy and image reconstruction shows that they share the same novel icosahedral structure. Both capsids consist of 60 equiv alent Gag dimers, whose two subunits occupy non-equivalent bonding env ironments. Stoichiometry data on other double-stranded RNA viruses ind icate that the 120-subunit structure is widespread, implying that this molecular architecture has features that are particularly favorable t o the design of a capsid that is also a biosynthetic compartment.