Protein storage vacuoles of Brassica napus zygotic embryos accumulate a BURP domain protein and perturbation of its production distorts the PSV

BNM2 is a prototypical member of the enigmatic BURP domain protein family whose members contain the signature FX(6-7)GX(10-28)PX(25-31)CX(11-12)X(2)SX(45-56)CHX(10)CHX(25-29)CHX(2)TX(15-16)PX(5)CH in the C-terminus. This protein family occurs only in plants, and the cognate genes vary very widely in their expression contexts in vegetative and reproductive tissues. None of the BURP family members has been assigned any biochemical function. BNM2 was originally discovered as a gene expressed in microspore-derived embryos (MDE) of Brassica napus but we found that MDE do not contain the corresponding protein. We show that BNM2 protein production is confined to the seeds and localized to the protein storage vacuoles (PSV) even though the transcript is found in vegetative parts and floral buds as well. In developing seeds, transcript accumulation precedes protein appearance by more than 18 days. RNA accumulation peaks at similar to 20 days post anthesis (DPA) whereas protein accumulation reaches its maximum at similar to 40 DPA. Transgenic expression of BNM2 does not abrogate this regulation to yield ectopic protein production or to alter the temporal aspect of BNM2 accumulation. Overexpression of BNM2 led to spatial distortion of storage protein accumulation within PSV and to some morphological alterations of PSVs. However, the overall storage protein content was not altered.