PREFORMED CYTOPLASMIC NUCLEOCAPSIDS ARE NOT NECESSARY FOR ALPHAVIRUS BUDDING

According to the present model for assembly of alphaviruses, e.g. Seml iki Forest virus (SFV), the viral genome is first encapsidated into a nucleocapsid (NC) in cytoplasm and this is then used for budding at pl asma membrane (PM), The preformed NC is thought to act as a template o n which the viral envelope can be organized, In the present work we ha ve characterized two SFV deletion mutants which did not assemble NCs i n the cytoplasm but which instead appeared to form NCs at the PM simul taneously with virus budding, The deletions were introduced in a conse rved 14 residue long linker peptide that joins the amino-terminal RNA- binding domain with the carboxy-terminal serine-protease domain of the capsid protein, Despite the deletions and the change in morphogenesis , wild-type (wt)-like particles were produced with almost wt efficienc y, It is suggested that the NC assembly defect of the mutants is rescu ed through spike-capsid interactions at PM, The results show that the preassembly of NCs in the cytoplasm is not a prerequisite for alphavir us budding, The apparent similarities of the morphogenesis pathways of wt and mutant SFV with those of type D and type C retroviruses are di scussed.