Nucleocytoplasmic Trafficking of the Arabidopsis WD40 Repeat Protein XIW1 Regulates ABI5 Stability and Abscisic Acid Responses

Abstract WD40 repeat-containing proteins (WD40 proteins) serve as versatile scaffolds for protein-protein interactions, modulating a variety of cellular processes such as plant stress and hormone responses. Here, we describe a WD40 protein, XIW1 (for XPO1-Interacting WD40 protein 1), that positively regulates the Abscisic acid (ABA) response in Arabidopsis . XIW1 is located in the cytoplasm and nucleus. It interacts with the nuclear transport receptor XPO1 and is exported by XPO1 from the nucleus. Mutation of XIW1 reduces the induction of ABA-responsive genes and the accumulation of ABA Insensitive 5 (ABI5), leading to ABA-insensitive phenotypes during seed germination and seedling growth, and decreased drought stress resistance. ABA treatment upregulates the expression of XIW1 , and both ABA and abiotic stresses promote the nuclear accumulation of the XIW1 protein. In the nucleus, XIW1 interacts with ABI5, and loss of XIW1 results in rapid proteasomal degradation of ABI5. Taken together, these data suggest that XIW1 is a nucleocytoplasmic shuttling protein that has a positive role in ABA responses by interacting with and maintaining the stability of ABI5 in the nucleus.