OLEOSIN OF PLANT SEED OIL BODIES IS CORRECTLY TARGETED TO THE LIPID BODIES IN TRANSFORMED YEAST

Yeast (Saccharomyces cerevisiae) has been used extensively as a hetero logous eukaryotic system to study the intracellular targeting of prote ins to different organelles. The lipid bodies in yeast have not been p reviously subjected to such studies. These organelles are functionally equivalent to the subcellular storage oil bodies in plant seeds. A pl ant oil body has a matrix of oils (triacylglycerols) surrounded by a l ayer of phospholipids embedded with abundant structural proteins calle d oleosins. We tested whether plant oleosin could be correctly targete d to the lipid bodies in transformed yeast. The coding region of a mai ze (Zea mays L.) oleosin gene was incorporated into yeast high copy an d low copy number plasmids in which its expression was under the contr ol of GAL1 promoter. Yeast strains transformed with these plasmids pro duced oleosin when grown in a medium containing galactose but not gluc ose. The oleosin produced in yeast had a molecular mass slightly highe r than that of the native protein in maize. Oleosin accumulated concom itantly with the storage lipids during growth of the transformed yeast , and it was not secreted. Subcellular fractionation of the cell extra cts obtained by two different cell breakage procedures revealed that t he oleosin was largely restricted to the lipid bodies. Oleosin apparen tly did not affect the lipid contents and composition of the trans for med yeast lipid bodies but replaced some of the native proteins associ ated with the organelles. Immunocytochemistry of the transformed yeast cells showed that the oleosin was present mostly on the periphery of the lipid bodies. Oleosin isolated from maize or transformed yeast str ain, alone or in the presence of phospholipids or SDS, did not bind to the yeast lipid bodies in vitro. We conclude that plant oleosin is co rrectly targeted to the lipid bodies in transformed yeast and that yea st may be used as a heterologous system to dissect the intracellular t argeting signals in the oleosin.