Characterization of a new class of oleosins suggests a male gametophyte-specific lipid storage pathway

The expression and sequences of two related antherspecific cDNAs (I3 and C98) of Brassica napus (oilseed rape) were examined. These cDNAs were found to exhibit significant predicted amino acid sequence similarity with a group of seed-specific proteins, the oleosins, which are involved in oil body membrane structure. Pollen of B. napus also contains oil bodies, and the synthesis and accumulation of these organelles correlates with expression of the I3 and C98 transcripts. The protein content of purified pollen oil bodies was therefore examined in order to determine the presence or absence of possible oleosins. One major protein species of 14 kDa was identified and subjected to N-terminal amino acid sequence determination. The sequence of this pollen oil body associated protein is homologous to the predicted sequence of one of the anther-specific cDNs, I3, implying that this cDNA represents an anther-specific oleosin gene expressed in developing pollen. The expression of other genes active during seed oil body formation was also examined, and these were found to be specific to the sporophyte, inferring the existence of specific components of the pathway of lipid synthesis and storage in the male gametophyte. These possibilities and the relationship between different oleosin genes are discussed.