Cloning, expression and isoform classification of a minor oleosin in sesame oil bodies

The oil bodies of plant seeds contain a triacylglycerol matrix surrounded by a monolayer of phospholipids embedded with alkaline proteins termed oleosins. Two distinct oleosins are present in the oil bodies of diverse angiosperms, and classified as high and low Mr isoforms according to their relative molecular masses in each species. In sesame oil bodies, besides the two ubiquitous oleosin isoforms (17 and 15 kDa), an additional minor oleosin (15.5 kDa) was revealed on Tricine SDS-PAGE. A full-length cDNA fragment was cloned, sequenced and deduced to be a putative oleosin of 15,446 Da. The gene was constructed in a fusion or non-fusion vector and then over-expressed with different efficiency in Escherichia coli. All three oleosins purified from sesame oil bodies were subjected to immunoassaying using antibodies raised against the over-expressed oleosin. The results confirmed that this gene encodes the sesame 15.5 kDa oleosin. Sequence comparisons with other known oleosins revealed that sesame 15.5 kDa oleosin does not represent a new oleosin isoform class but may have been derived through gene duplication and truncation of sesame 17 kDa oleosin, and possesses the minimal structure of the high Mr oleosin isoform. A conserved amphipathic alpha-helix is predicted in sesame 15.5 kDa oleosin, which may imply a potential biological function associated with this isoform.