Abstract
In maturing seeds, storage protein precursors are synthesized on rER and are sorted to PSVs, where they are processed into mature forms and accumulated. Arabidopsis mag2 mutants abnormally accumulate the precursors of 12S globulins and 2S albumins. Novel structures with high electron-dense core were found in mag2 seed cells. Some of them are surrounded with ribosomes. Immunogold analysis suggests that the high electron-dense core is an aggregate of pro2S albumins and that pro12S globulins are localized around the core. ER chaperons, BiP and PDI, are abnormally accumulated and colocalized with precursors of 12S globulins around the pro2S core. These results suggest that mag2 mutants have defect(s) in the exit step from ER, resulting in ER stress. MAG2 gene encodes a novel protein exhibiting homology to mammalian RINT-1 and yeast TIP20. MAG2 might function in membrane traffic between Golgi and ER for seed storage proteins.
