Phospholipid-dependence of plant UDP-glucose-sterol-β-D-glucosyltransferase. I. Detergent-mediated delipidation by selective solubilization

Abstract

The plasma membrane (PM)-bound uridine-diphosphate-glucose: sterol β-D-glucosyltransferase (UDPG-SGTase) of etiolated maize celeoptiles is activated by Triton X-100 and other non-ionic detergents, either at low (Triton X-100/protein = 0.2, w/w) or at high concentration (Triton/protein = 2) E. Quantin et al., Plant Sci. Lett. 17 (1980) 193. At intermediate concentrations (Triton/protein = 0.4–0.8) this stimulation is abolished. If plasma membrane is pretreated at these intermediate Triton concentrations and centrifuged at 100 000 × g, both supernatant and resuspended pellet are poorly active when tested for UDPG-SGTase activity in the presence of optimal (high) Triton concentration. But the supernatant activity is strongly stimulated (2–3 times) by a lipidic extract of the corresponding pellet. Phospholipids purified from this extract produce the same stimulation. Our results clearly show that in this concentration range of Triton X-100, most of the enzyme activity is solubilized whereas an essential phospholipidic factor is still membrane-bound. This original delipidation process, together with the associated inhibition and the subsequent reactivation by phospholipids, strongly suggest the phospholipid-dependence of plant UDPG-SGTase. This is confirmed by the accompanying paper, P. Ullmann et al., Plant Sci. Lett., 36 (1984) 29.