Modulation of an RNA-binding protein by abscisic-acid-activated protein kinase

Abstract

Protein kinases are involved in stress signalling in both plant and animal systems. The hormone abscisic acid mediates the responses of plants to stresses such as drought, salinity and cold. Abscisic-acid-activated protein kinase (AAPK)—found in guard cells, which control stomatal pores—has been shown to regulate plasma membrane ion channels1. Here we show that AAPK-interacting protein 1 (AKIP1), with sequence homology to heterogeneous nuclear RNA-binding protein A/B, is a substrate of AAPK. AAPK-dependent phosphorylation is required for the interaction of AKIP1 with messenger RNA that encodes dehydrin, a protein implicated in cell protection under stress conditions. AAPK and AKIP1 are present in the guard-cell nucleus, and in vivo treatment of such cells with abscisic acid enhances the partitioning of AKIP1 into subnuclear foci which are reminiscent of nuclear speckles. These results show that phosphorylation-regulated RNA target discrimination by heterogeneous nuclear RNA-binding proteins2 may be a general phenomenon in eukaryotes, and implicate a plant hormone in the regulation of protein dynamics during rapid subnuclear reorganization.

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Figure 1: AKIP1 encodes a protein that binds single-stranded RNA.
Figure 2: AKIP1 specifically interacts with AAPK in the yeast two-hybrid system.
Figure 3: AKIP1 and AAPK localization in guard cells.
Figure 4: Phosphorylation of AKIP1 by ABA-activated AAPK regulates AKIP1 binding to dehydrin mRNA.
Figure 5: Interaction of AAPK-phosphorylated AKIP1 with dehydrin transcript.

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Acknowledgements

We thank S. Gilroy and E. Kunze for the use of their confocal and fluorescence microscopes; M. Guiltinan for use of the gene gun; H. Ma for providing the SNF1 and SNF4 constructs; P. James for providing the PJ69-4A strain; L. Ding for assistance with AKIP1 constructs; and P. Minnich for technical support. This research was supported by NSF (S.M.A.) and partly supported by a Grant-in-Aid for Scientific Research on Priority Areas from Ministry of Education, Sports and Culture of Japan (K.S. and T.K.).

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Correspondence to Jiaxu Li or Sarah M. Assmann.

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