Identification of caleosin and two oleosin isoforms in oil bodies of pine megagametophytes

Numerous oil bodies of 0.2-2 μm occupied approximately 80% of intracellular space in mature pine (Pinus massoniana) megagametophytes. They were stably isolated and found to comprise mostly triacylglycerols as examined by thin layer chromatography analysis and confirmed by both Nile red and BODIPY stainings. Fatty acids released from the triacylglycerols of pine oil bodies were mainly unsaturated, including linoleic acid (60%), adrenic acid (12.3%) and vaccenic acid (9.7%). Proteins extracted from pine oil bodies were subjected to immunological cross-recognition, and the results showed that three proteins of 28, 16 and 14 kDa were detected by antibodies against sesame seed caleosin, sesame oleosin-L and lily pollen oleosin-P, respectively. Complete cDNA fragments encoding these three pine oil-body proteins, tentatively named caleosin, oleosin-L and oleosin-G, were obtained by PCR cloning and further confirmed by mass spectrometric analysis. Consistently, phylogenetic tree analyses showed that pine caleosin was closely-related to the caleosin of cycad megagametophyte among known caleosin sequences. While pine oleosin-L was found clustered with seed oleosin isoforms of angiosperm species, oleosin-G was distinctively grouped with the oleosin-P of lily pollen. The oleosin-G identified in pine megagametophytes seems to represent a new class of seed oleosin isoform evolutionarily close to the pollen oleosin-P.