Department of Plant Biotechnology and Bioinformatics, Ghent University, Ghent, Belgium.
Center for Plant Systems Biology, VIB, Ghent, Belgium.
Nature. 2018 Nov;563(7732):574-578. doi: 10.1038/s41586-018-0714-x. Epub 2018 Nov 14.
Stomatal cell lineage is an archetypal example of asymmetric cell division (ACD), which is necessary for plant survival. In Arabidopsis thaliana, the GLYCOGEN SYNTHASE KINASE3 (GSK3)/SHAGGY-like kinase BRASSINOSTEROID INSENSITIVE 2 (BIN2) phosphorylates both the mitogen-activated protein kinase (MAPK) signalling module and its downstream target, the transcription factor SPEECHLESS (SPCH), to promote and restrict ACDs, respectively, in the same stomatal lineage cell. However, the mechanisms that balance these mutually exclusive activities remain unclear. Here we identify the plant-specific protein POLAR as a stomatal lineage scaffold for a subset of GSK3-like kinases that confines them to the cytosol and subsequently transiently polarizes them within the cell, together with BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE (BASL), before ACD. As a result, MAPK signalling is attenuated, enabling SPCH to drive ACD in the nucleus. Moreover, POLAR turnover requires phosphorylation on specific residues, mediated by GSK3. Our study reveals a mechanism by which the scaffolding protein POLAR ensures GSK3 substrate specificity, and could serve as a paradigm for understanding regulation of GSK3 in plants.
气孔细胞谱系是不对称细胞分裂（ACD）的典型范例，这对于植物的生存是必要的。在拟南芥中，糖原合酶激酶 3（GSK3）/类 SHAGGY 激酶 BRASSINOSTEROID INSENSITIVE 2（BIN2）分别磷酸化丝裂原活化蛋白激酶（MAPK）信号模块及其下游靶标转录因子 SPEECHLESS（SPCH），以分别促进和限制同一气孔谱系细胞中的 ACD。然而，平衡这些相互排斥的活性的机制仍不清楚。在这里，我们鉴定出植物特异性蛋白 POLAR 作为一组 GSK3 样激酶的气孔谱系支架，将它们限制在细胞质中，并随后在 ACD 之前将它们与 BREAKING OF ASYMMETRY IN THE STOMATAL LINEAGE（BASL）一起短暂地在细胞内极化。结果，MAPK 信号被减弱，从而使 SPCH 能够在核内驱动 ACD。此外，POLAR 的周转需要特定残基上的磷酸化，由 GSK3 介导。我们的研究揭示了支架蛋白 POLAR 确保 GSK3 底物特异性的机制，并且可以作为理解植物中 GSK3 调节的范例。
