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Reversible acetylation in brassinosteroid signaling

Deacetylation of the kinase BIN2 by HDA6 promotes brassinosteroid signaling in plants. Plants have homologs of the mammalian kinase glycogen synthase kinase 3 (GSK-3) and one of these, BIN2, limits brassinosteroid (BR) signaling by phosphorylating and inhibiting the BR-responsive transcription factors BES1 and BZR1. Hao et al. identified the deacetylase HDA6 in a mass spectrometry screen for proteins that coimmunoprecipitated with BIN2, showed that the two proteins interacted in biomolecular fluorescence complementation assays and affinity pull-down assays, and coimmunoprecipitated tagged overexpressed forms of BIN2 and HDA6 from transgenic Arabidopsis thaliana. In vitro kinase assays indicated that BIN2 did not have kinase activity toward HDA6. Acetylation of tagged BIN2 was detected when the protein was expressed in Escherichia coli or in plants, and coexpression of HDA6 reduced the amount of the acetylated BIN2. Pharmacological inhibition of deacetylase activity resulted in a short hypocotyl phenotype, consistent with BR deficiency or enhanced BIN2 activity, which was rescued by adding sugar to the growth medium. Analysis of transgenic plants overexpressing HDA6 or plants in which HDA6 was knocked out indicated that HDA6 promoted BR signaling; however, the BR-associated phenotypes were only observed when the plants were grown in sugar-limiting medium or in BR-deficient plants. Genetic interactions with the gain-of-function mutant bin1-2 were consistent with HDA6 inhibiting BIN2. Mass spectrometry analysis identified Lys189 as the site in BIN2 that was acetylated in the protein expressed in E. coli. BIN2 with this residue mutated to Arg (BIN2K189R) exhibited reduced kinase activity in vitro. Transgenic plants expressing BIN2K189R or bin2-1K189R exhibited phenotypes and biochemical and cellular events (phosphorylation status of the BR-regulated transcription factors and expression of BR-dependent target genes) that were consistent with this mutant enzyme having less activity. The authors propose that HDA6 may enhance BR signaling by deacetylating BIN2 to integrate nutrient status with the BR pathway. Y. Hao, H. Wang, S. Qiao, L. Leng, X. Wang, Histone deacetylase HDA6 enhances brassinosteroid signaling by inhibiting the BIN2 kinase. Proc. Natl. Acad. Sci. U.S.A. 113, 10418–10423 (2016). [PubMed]