Degradation of SERRATE via ubiquitin-independent 20S proteasome to survey RNA metabolism.

SERRATE (SE) is an RNA processing factor that regulates plant growth and development starting from the embryo stage. Here, interacting partners of SE that are also the key components of the ubiquitin-independent 20S proteasome are identified and characterized to play important roles in regulating SE degradation and RNA metabolism inArabidopsis. SERRATE (SE) is a key factor in RNA metabolism. Here, we report that SE binds 20S core proteasome alpha subunit G1 (PAG1) among other components and is accumulated in their mutants. Purified PAG1-containing 20S proteasome degrades recombinant SE via an ATP- and ubiquitin-independent manner in vitro. Nevertheless,PAG1is a positive regulator forSEin vivo, aspag1shows comparable molecular and/or developmental defects relative tose. Furthermore, SE is poorly assembled into macromolecular complexes, exemplified by the microprocessor inpag1compared with Col-0.SEoverexpression triggered the destruction of both transgenic and endogenous protein, leading to similar phenotypes ofseandSEoverexpression lines. We therefore propose that PAG1 degrades the intrinsically disordered portion of SE to secure the functionality of folded SE that is assembled and protected in macromolecular complexes. This study provides insight into how the 20S proteasome regulates RNA metabolism through controlling its key factor in eukaryotes.