1. Intact F glycoprotein is required to induce permeability changes in Lettrée cells or in erythrocytes. Some HN glycoproteins may also be required. Permeability changes thus offer a simple, accurate and rapid means of assaying the integrity of F glycoprotein in certain viral preparations. 2. The ‘1-day’ virus (which contains intact F glycoprotein but which differs morphologically from ‘3 day’ virus) does not cause permeability changes; it can be rendered active by various physical treatments. It is concluded that the environment in which F glycoprotein is embedded is a determining factor for permeability changes. 3. The entry of fluorescently labelled peptides into cells made permeable by virus has been measured. Peptides having a molecular weight in excess of 1000 enter poorly, suggesting a ‘pore’ size of approx. 1 nm in diameter. 4. Two novel assay methods concerned with virus—cell fusion are described. The first measures the fluorescence enhancement that occurs when anthroylstearate is transferred from anthroylstearate-labelled virus to cells. The second measures the giant-cell formation that occurs when partially fused erythrocytes are exposed to hypo-osmotic treatment. The ‘1-day’ virus is active in these assays. In contrast with permeability changes, virus—cell fusion is insensitive to changes in external Ca2+-concentration. 5. The results are compatible with a model [Knutton & Pasternak (1979) Trends Biochem. Sci. 4, 220—223; Impraim, Foster, Micklem & Pasternak (1980) Biochem. J. 186, 847—860] in which virus—cell fusion is a prerequisite for permeability changes, and in which permeability changes are the cause of haemolysis and giant-cell (polykaryon) formation.
1. The phospholipid composition and cholesterol/phospholipid ratio of plasma membrane is the same in normal as in transformed BHK (baby-hamster kidney) cells; no significant difference in length or degree of unsaturation of the contributing acyl chains is apparent. 2. The turnover of acetate-labelled phosphatidylcholine species in the plasma membrane of normal and transformed BHK cells is the same. 3. Intramembranous particles of normal and transformed 3T3-cell plasma membrane are randomly distributed, whether at 4degreesC or at 37degreesC, in sparse or in dense cultures. There is no correlation between distribution of particles and the movement of concanavalin A receptor sites. 4. It is concluded that transformation of fibroblastic cells by oncogenic viruses does not lead to major changes in the lipid fluidity of the plasma membrane. 5. Details of the phospholipid composition of nuclei, mitochondria and endoplasmic reticulum in normal and transformed BHK cells have been deposited as Supplementary Publication SUP 50061 (5 pages) at the British Library Lending Division, Boston, Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies may be obtained on the terms given in Biochem. J. (1976) 153, 5.