The core protein of the alphavirus sindbis virus assembles into core-like nucleoproteins with the viral genome RNA and with other single-stranded nucleic acids in vitro

Abstract A system has been developed that allows the reconstruction of a core-like (CL) ribonucleoprotein (RNP) from Sindbis virus-specific core protein and genome RNA in vitro . The RNP particles were analyzed by equilibrium density gradient centrifugation and electron microscopy. The CL RNP is similar in size, shape, and texture to authentic viral core. The assembly of the homologous CL RNP in vitro depends on the relative concentrations of protein and RNA in the reaction: At low concentrations of protein incomplete particles of rather high density are made; increase of the protein concentration leads to an optimum concentration at which the protein is quantitatively incorporated into complete CL particles; further increase in protein concentration leads to the formation of a precipitate which has not been analyzed in detail. No identifiable structures were generated in vitro in the absence of nucleic acid, but all single-stranded deoxyribonucleic and ribonucleic acids analyzed were incorporated into particles similar to those formed in the presence of viral genome RNA. These complexes are called heterologous CL nucleoproteins. Since nucleic acids differing in size between about 100 and 6000 nucleotides (e.g., tRNA and fd DNA), which vary widely in secondary structure are efficiently incorporated into heterologous CL particles, probably all single-stranded nucleic acids in this size range can be efficiently incorporated into such particles in vitro . Some implications of a possible interaction between viral core protein and single-stranded nucleic acids other than the viral genome in vivo , e.g., during the synthesis of defective interfering particles or during inhibition of host cell DNA synthesis, are discussed.