Purified Semliki Forest virus was analysed by analytical sedimentation, rate zonal sedimentation, isopycnic sedimentation, polyacrylamide gel electrophoresis and electron microscopy. It was found that the spherical virus particles had a sedimentation coefficient of 241 S (249 S determined by rate zonal sedimentation), and a buoyant density in sucrose of 1.190 g/ml; one protein and one glycoprotein were present in the virus; after fixation and negative staining with phosphotungstic acid the diameter of the particles was measured to be 50 to 56 nm. Treatment of Semliki Forest virus with 0.1% Tween 80 and 1% tri(-butyl)phosphate (TNBP) for 1 h at 4 °C destroyed infectivity and resulted in the appearance of the following haemagglutinating and immunizing virus components. (1) A spherical 211 S (217 S) component, 69 to 75 nm in diameter, consisting of an envelope and a core; one protein and one glycoprotein were present; the [H]-uridine:[C)-protein ratio was 2.8-fold higher than that of double-labelled untreated virus. (2) A spherical component, 70 to 73 nm in diameter, consisting of the virus envelope lacking intact cores; = 140 S (153 S); one glycoprotein was detected; the [H]-uridine: [C]-protein ratio was about 1/3 of untreated virus. (3) A 135 S component. (4) Structures assumed to represent envelope fragments; much of them had a sedimentation coefficient of 49 S. It was concluded from experiments with [H]-uridine-[C]-amino acids double-labelled and [C]-stearic acid-labelled virus that Tween 80-TNBP dissociates lipids and some protein components from the virus.


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