Ca2+‐Calmodulin regulates nuclear translocation of the rice seed‐specific MADS‐box transcription factor OsMADS29

Abstract

OsMADS29 (M29) is a crucial regulator of seed development in rice. The expression of M29 is strictly regulated at transcriptional as well as post‐transcriptional levels. The MADS‐box proteins are known to bind to DNA as dimers. However, in the case of M29, the dimerization also plays a vital role in its localization into the nucleus. The factor(s) that affect oligomerization and nuclear transport of MADS proteins have not yet been characterized. By using BiFC in transgenic BY‐2 cell lines and Yeast‐2‐hybrid assay (Y2H), we show that calmodulin (CaM) interacts with M29 in a Ca2+‐dependent manner. This interaction specifically takes place in the cytoplasm, probably in association with the endoplasmic reticulum. By generating domain‐specific deletions, we show that both sites in M29 are involved in this interaction. Further, by using BiFC‐FRET‐FLIM, we demonstrate that CaM may also help in the dimerization of two M29 monomers. Since most MADS proteins have CaM binding domains, the interaction between these proteins could be a general regulatory mechanism for oligomerization and nuclear transport.