Nuclear basket localized proteasomes maintain circadian period through nuclear TOC1 proteolysis

Post-translational control of circadian period can involve changes in protein intracellular localization to affect clock function. Many clock proteins rely on a nuclear presence for their activity. As the primary gateway regulating the movement of molecules between the cytosol and the nucleus, the nuclear pore may assist in circadian system maintenance. We describe roles for the nuclear basket proteins, NUA and NUP136, in the maintenance of Arabidopsis circadian period, through effects on the stability of the core clock protein, TOC1. The circadian period of nua and nup136 mutants is significantly longer than that of wildtype plants. We show that NUP136 interacts with NUA, proteasome components and TOC1 in vivo, recruiting them to the inner nuclear rim. TOC1 interaction with the NUP136-NUA complex leads to a proteasome-dependent degradation of TOC1. Loss of NUP136 or NUA disrupts this regulatory environment, leading to aberrant nuclear TOC1 accumulation and consequent lengthening of circadian period. Our work thus identifies nuclear basket-localized proteasomes as key to the maintenance of circadian period.