MIP proteins Are Novel Factors Responsible for Transport of Seed Storage Proteins in Arabidopsis

Abstract

In maturing seeds, storage proteins are synthesized on rough ER as precursors and then are transported to protein storage vacuoles for further processing into mature forms. We previously reported that maigo2 mutant accumulated a large number of MAG2 bodies which are composed of storage protein precursors and ER chaperones within the ER lumen. MAG2 protein might be involved in the exit of storage protein precursors from the ER. In order to clarify mechanism underlying the MAG2-dependent transport, we studied on interacting proteins with MAG2 and found three MAG2-interacting proteins (designated as MIPs). MIP proteins were associated with the ER membrane together with MAG2. mip mutants abnormally accumulated storage protein precursors in their dry seeds. MAG2 body-like structures were also observed in mip seed cells. Our results suggest that MIP proteins cooperate with MAG2, playing a significant role in the transport of storage proteins in maturing seeds.