The nucleocytoplasmic exchange is of fundamental importance to eukaryotic life and is mediated by karyopherins, a superfamily of nuclear transport receptors. However, the function and cargo spectrum of plant karyopherins are largely obscure. Here, we report proximity-labeling-based proteomic profiling of in vivo substrates of KA120, a karyopherin-β required for suppressing autoimmune induction in Arabidopsis. We identify multiple components of the MOS4-associated complex (MAC), a conserved splicing regulatory protein complex. Surprisingly, we find that KA120 does not affect the nucleocytoplasmic distribution of MAC proteins but rather prevents their protein condensation in the nucleus. Furthermore, we demonstrate that MAC condensation is robustly induced by pathogen infection, which is sufficient to activate defense gene expression, possibly by sequestrating negative immune regulators via phase transition. Our study reveals a noncanonical chaperoning activity of a plant karyopherin, which modulates the nuclear condensation of an evolutionarily conserved splicing regulatory complex to coordinate plant immune activation.

核质交换对于真核生物至关重要，由核转运蛋白（核转运受体超家族）介导。然而，植物核转运蛋白的功能和货物谱在很大程度上还不清楚。在这里，我们报告了基于邻近标记的 KA120体内底物的蛋白质组学分析，KA120 是抑制拟南芥自身免疫诱导所需的核转运蛋白-β 。我们鉴定了 MOS4 相关复合物 (MAC) 的多个成分，这是一种保守的剪接调节蛋白复合物。令人惊讶的是，我们发现KA120并不影响MAC蛋白的核质分布，而是阻止其蛋白在细胞核中凝结。此外，我们证明 MAC 凝结是由病原体感染强烈诱导的，这足以激活防御基因表达，可能是通过相变隔离负免疫调节剂。我们的研究揭示了植物核传递蛋白的非典型陪伴活性，它调节进化上保守的剪接调节复合物的核凝结以协调植物免疫激活。