Properties of Semliki Forest Virus Nucleocapsid

The nucleocapsid of the Semliki Forest virus is composed of 34% RNA and 66% protein, or one RNA and about 240 capsid protein molecules. The particle is spherical, with a diameter of 38--39 nm. If the nucleocapsid is exposed to slightly acid pH (6.4--5.6) it undergoes a structural change and is contracted to a 32 nm state. A similar contraction can be effected by RNase treatment, in this case, however, in connection with a loss of RNA. Treatment of the nucleocapsid with 0.2 mM SDS results in dissociation of capsid protein from RNA, an effect which suggests strong RNA-protein interaction. At 0.05 mM SDS the protein remains associated with the RNA, but the S-value is reduced from 150 S to 100 S. Electron micrographs of the 100 S ribonucleoprotein showed irregular and strand-like structures.