Temperature-dependent compositional changes in the envelope of phi 6.

Bacteriophage φ6 contains lipid in an envelope that covers an icosahedral core. The envelope has six proteins, of which two constitute the attachment apparatus. The latter are designated P3, the host attachment protein, and P6, its anchor in the phage envelope. The number of molecules of P3 and P6 seem to be equivalent in the particle, but the relative amounts of the two proteins compared to other phage proteins vary with temperature. In phage grown above 24° there is about one-fourth as much P3 and P6 in the particles compared to phage grown below 24°. A result of this compositional change is a reduction in the rate of sedimentation of the virus formed at high temperature. A nonsense mutant defective in the synthesis of protein P6, which makes particles that contain lipid and other membrane proteins but lack P3 and P6 in the envelope, sediments at the same rate as wild-type virus grown at high temperatures. The sedimentation rate of such P3, P6-deficient particles is independent of the temperature at which they are formed. The variation in the amount of P3 and P6 inserted into the phage membrane reflects an aspect of assembly other than synthesis, since the relative amount of the two proteins synthesized with respect to other phage proteins does not change much with temperature.