Protein-facilitated intermembrane transfer of squalene. Demonstration by density gradient centrifugation

Squalene-enriched, trypsinized microsomes display no squalene epoxidase activity either as such or when combined with normal microsomes. On addition of microgram quantities of supernatant protein factor to the combined system, squalene epoxidation commences at once and continues at a rapid rate (Friedlander, E. J., Caras, I. W., Lin, L. F., and Bloch, K. (1980) J. Biol. Chem. 255, 8042-8045). When mixtures of trypsin-treated, [3H]squalene-containing microsomes and normal microsomes are subjected to isopycnic density gradient centrifugation, the two microsomal populations separate readily. Essentially all of the radioactive squalene remains associated with the lighter (trypsinized) fraction of microsomes. However, if the mixture of microsomes is initially incubated with supernatant protein factor and then centrifuged, a large fraction of labeled squalene sediments with the denser, normal microsomes. Thus, supernatant protein factor mediates the transfer of squalene from one microsome population to another. This conclusion had previously been reached on the basis of less direct experiments (Friedlander, E. J., Caras, I. W., Lin, L. F., and Bloch, K. (1980) J. Biol. Chem. 255, 8042-8045). Evidence is presented that the process of supernatant protein factor-mediated squalene transfer does not involve membrane fusion and proceeds also in the reverse direction.