Biosynthesis, targeting and processing of oleosin-like proteins, which are major pollen coat components in Brassica...

The purpose of this study is to characterise the biosynthesis, targeting and processing of some of the major protein components of the pollen coat, or tryphine, of Brassica napus. The authors have N-terminally sequenced 11 of the most abundant pollen coat polypeptides, and nine of these sequences correspond to proteolytically cleaved products of seven oleosin-like genes, i.e. Oln B;1 to Oln B;6 and Oln B;11. The Oln B;11 gene product is co- or post-translationally targeted in vitro to canine microsomal membranes. This implies that the oleosin-like protein is targeted to the endoplasmic reticulum in tapetal cells in vivo. Affinity-purified antibodies raised against a 20-residue domain of Oln B;3 and B;4 gene products cross-reacted with full-length proteins of 45-48 kDa in early developing (< 2 mm to 5 mm) buds and anthers, but recognised truncated proteins of 32-38 kDa at later (4 mm to 7 mm) stages of development. The 45-48 kDa immunoreactive proteins were associated with a floating lipid body fraction obtained from a tapetal/locular fluid extract from maturing anthers and a major 48 kDa polypeptide from this fraction was confirmed by N-terminal sequencing to be a full length product of the Oln B;3 gene. Quantitative immunocytochemical studies showed that the full length 45-48 kDa oleosin-like proteins were specifically localised in the interior of tapetal cytoplasmic lipid bodies where they were associated with a regular hexagonal-like fibrous reticulum. No significant labelling of elaioplasts was observed. The same antibodies specifically labelled 32-38 kDa oleosin-like proteins on the extracellular pollen coat of maturing pollen grains. These results demonstrate for the first time that many of the major pollen coat proteins are derived from an endoproteolytic cleavage of precursor oleosin-like proteins that originally accumulate within the large cytoplasmic lipid bodies of tapetal cells.