Generation mechanism of novel, huge protein bodies containing wild type or hypoallergenic derivatives of birch pollen allergen Bet v 1 in rice endosperm.

Tree pollen chimera 7 (TPC7), a hypoallergenic Bet v 1 tolerogen against birch pollen allergy, induces the formation of novel, huge protein bodies (referred to as TPC7 bodies) in rice endosperm, and is accumulated in high level. In the present study, we found that native Bet v 1 and TPC9, analog proteins of TPC7, were also deposited into novel protein bodies in rice endosperm. However, the novel protein bodies in Bet v 1 and TPC9 rice were much smaller and less abundant than those in TPC7 rice, reflected in lower amounts of accumulation of Bet v 1 and TPC9 than that of TPC7. A domain swapping experiment between TPC7 and Bet v 1 revealed that the latter half of TPC7 is important for the formation of the TPC7 body. We found that chaperons and folding enzymes such as BiP and protein disulfide isomerase were localized within the TPC7 body. TPC7 protein was extracted from TPC7 seeds as large aggregates with molecular masses greater than 669 kDa, or approximately 75 kDa under native or semi-native conditions. These TPC7 aggregates are thought to be responsible for the induction of TPC7 body formation. TPC7 accumulated to a maximum level of 550 μg/seed, which amounts to 23% of total seed protein, while Bet v 1 and TPC9 accumulated much lower levels. The TPC7 body represents a promising reservoir, which may serve as a fusion partner for high-level production and sequestering storage of recombinant proteins.