ASSEMBLY AND ENTRY MECHANISMS OF SEMLIKI FOREST VIRUS

The alphavirus Semliki Forest (SFV) is an enveloped virus with a posit ive single-stranded RNA genome. The genome is complexed with 240 copie s of a capsid protein into a nucleocapsid structure. In the membrane t he virus carries an equal number of copies of a membrane protein heter odimer. The latter oligomers are grouped into clusters of three. These structures form the spikes of the virus and carry its entry functions , that is receptor binding and membrane fusion activity. The membrane protein heterodimer is synthesized as a p62E1 precursor protein which upon transport to the cell surface is cleaved into the mature E2E1 for m. Recent studies have given much new information on the assembly and entry mechanism of this simple RNA virus. Much of this work has been p ossible through the construction of a complete cDNA clone of the SFV g enome which can be used for in vitro transcription of infectious RNA. One important finding has been to show that a spike deletion variant a nd a capsid protein deletion variant are budding-negative when express ed separately but can easily complement each other when transfected in to the same cell. This shows clearly that enveloped viruses use differ ent budding strategies: one which depends on a nucleocapsid-spike inte raction as exemplified by SFV and another one which is based on a dire ct core-lipid bilayer interaction as shown before to be the case with retroviruses. Another important finding concerns the activation proces s of the presumed fusion protein of SFV, the E1 subunit. In the origin al p62E1 heterodimer E1 is completely inactive. Activation proceeds in several steps. First p62 cleavage activates the potential for low pH inducible fusion. Next the low pH which surrounds incoming virus in en dosomes induces dissociation of the heterodimeric structure. This is f ollowed by a rearrangement of E1 subunits into homotrimers which are f usion active.