Molecular organization and selective solubilization of lipids and proteins in the envelope of mycoplasmal virus L2.

The composition and molecular organization of mycoplasmal virus L2 (MVL2) were studied and compared with those of the cell membrane of the host, Acholeplasma laidlawii strain JA1. The virus contained 0.2-0.25 mumol of polar lipids per mg of viral protein. The lipid species of the MVL2 were the same as those of the host cells. Nevertheless, the proportions between the various polar lipids were different, with a much lower content of phosphatidylglycerol in the viral lipids. Despite these quantitative differences, the fatty acid composition of MVL2 was similar to that found in the host cells, a similarly allowing alteration of the fatty acid composition and study of its effect on viral absorption, penetration, and release. Pulse-chase experiments revealed that the lipids incorporated into the virus were synthesized before and after infection. Electron paramagnetic resonance spectrometry suggested that the viral lipid domain had the properties of a lipid bilayer. Nevertheless, the hydrocarbon chains in the MVL2 envelope were less mobile than those in membranes of the host cells, a difference apparently due to the different content, composition, and disposition of proteins in the MVL2 envelope. The electrophoretic pattern of MVL2 polypeptides was dominated by four major and five minor bands distinct from the polypeptides present in A. laidlawii membranes. None of the polypeptides gave a positive periodic acid-Schiff reaction, a result suggesting the absence of glycoproteins. Selective solubilization experiments excluded the possibility that one or more of the major polypeptides was associated with a capsid structure. Lactoperoxidase-mediated iodination of intact viral particles revealed that at least two of the major polypeptides were localized on the external surface of the viral envelope. The susceptibility of these polypeptides to brief proteolytic treatment and the finding that the infectivity of the virus was dramatically affected by such treatment suggested that these polypeptides were playing a role in recognition and/or attachment of the virus to the host cells.