Native isocitrate lyase from castor bean and a C-terminally truncated variant were expressed in Saccharomyces cerevisiae under the control of a galactose-inducible promoter. Both forms of isocitrate lyase were targeted to the yeast peroxisomes. They co-fractionated with catalase on sucrose-density-gradient centrifugation of a post-nuclear supernatant prepared from cells grown on oleic acid plus galactose, but were found in the cytosolic fractions when the cells were grown under conditions that repress peroxisome formation. The endogenous S. cerevisiae isocitrate lyase was found solely in the cytoplasmic fractions, even under growth conditions that induce peroxisome proliferation. This result shows that the presence of isocitrate lyase in peroxisomes is not essential for a functional glyoxylate cycle. Although the heterologous enzyme was transported to peroxisomes it was not enzymically active. Immunocytochemical studies provide independent evidence that the plant enzyme is imported into the matrix of yeast peroxisomes.
Present address: Department of Pharmacology, College of Medicine, Ohio State University, Columbus, OH 43210-1239, U.S.A.
Present address: Department of Virus Research, John Innes Centre, Colney Lane, Norwich NR4 7UH, U.K.