Modulation of BIN2 kinase activity by HY5 controls hypocotyl elongation in the light

Abstract

ELONGATED HYPOCOTYL 5 (HY5), a basic domain/leucine zipper (bZIP) transcription factor, acts as a master regulator of transcription to promote photomorphogenesis. At present, it's unclear whether HY5 uses additional mechanisms to inhibit hypocotyl elongation. Here, we demonstrate that HY5 enhances the activity of GSK3-like kinase BRASSINOSTEROID-INSENSITIVE 2 (BIN2), a key repressor of brassinosteroid signaling, to repress hypocotyl elongation. We show that HY5 physically interacts with and genetically acts through BIN2 to inhibit hypocotyl elongation. The interaction of HY5 with BIN2 enhances its kinase activity possibly by the promotion of BIN2 Tyr 200 autophosphorylation, and subsequently represses the accumulation of the transcription factor BRASSINAZOLE-RESISTANT 1 (BZR1). Leu 137 of HY5 is found to be important for the HY5-BIN2 interaction and HY5-mediated regulation of BIN2 activity, without affecting the transcriptional activity of HY5. HY5 levels increase with light intensity, which gradually enhances BIN2 activity. Thus, our work reveals an additional way in which HY5 promotes photomorphogenesis, and provides an insight into the regulation of GSK3 activity.

Regulation of Three Key Kinases of Brassinosteroid Signaling Pathway

“…Given the earlier discovery of a direct BIN2-BES1/BZR1 binding [ 165 ] that was recently confirmed by a single-molecular analysis [ 191 ], it is no surprise that BIN2 is also regulated by its competitive binding with other cellular proteins in plant cells. Recent studies showed that BIN2 interacted in vivo with several well-studied light signaling components, including CRY1 (Cryptochrome1, a blue-light photoreceptor) [ 192 ], the COP1 (Constitutive Photomorphogenesis1)/SPA (Suppressor of phyA-105 ) complex (a light regulated E3 ubiquitin ligase) [ 193 ], and HY5 (long hypocotyl5, a bZIP-type transcriptional factor) [ 194 ]. CRY1 exhibits a blue light-dependent binding with both BIN2 and BZR1 to enhance the BIN2-catalyzed BZR1 phosphorylation [ 192 ], thus inhibiting its nuclear translocation (likely by increased interaction with 14-3-3 proteins [ 195 ]) and its DNA binding activity through CRY1’s competitive binding to the BZR1’s DNA binding domain.…”

Molecular pathways regulating elongation of aerial plant organs: a focus on light, the circadian clock, and temperature

“…Retention of BZR1 and BES1 in the cytoplasm depends on the interaction of their phosphorylated forms with 14‐3‐3 proteins (Bai et al ., 2007; Gampala et al ., 2007; Ryu et al ., 2007; Ryu et al ., 2010; Tang et al ., 2011). Glycogen synthase kinase 3‐mediated phosphorylation of BZR1 further reduces activity of this transcription factor by promoting its degradation (He et al ., 2002; Li et al ., 2020a). BZR1 , BES1 , and at least one other gene in the six‐member BZR family, BES1 / BZR1 HOMOLOG 4 ( BEH4 ), are involved in hypocotyl responses to BRs (He et al ., 2005; Kang et al ., 2015; Saito et al ., 2018; Galstyan and Nemhauser, 2019).…”

“…Indeed, besides phosphorylating BZRs, BIN2 exhibits kinase activity towards PIF3 and PIF4, and additionally, it has been demonstrated that Ser/Thr to Ala mutations in PIF3 or PIF4 (PIF41A mutant protein) that reduce BIN2‐mediated phosphorylation of these PIFs also stabilize both proteins (Zhao et al ., 2002; Bernardo‐Garcia et al ., 2014; Ling et al ., 2017). This suggests that BIN2 phosphorylation of PIFs promotes their degradation, as it does for BZR1, and furthermore this process appears to be important for repression of hypocotyl growth in light (He et al ., 2002; Bernardo‐Garcia et al ., 2014; Ling et al ., 2017; Li et al ., 2020a). Specifically, ProPIF4::PIF41A‐HA pif4 pif5 seedlings grown under short days (SDs; 8 h light/16 h dark) exhibit significant growth throughout the daytime, which is not observed in the wild type, ProPIF4::PIF4‐HA pif4 pif5 , or even a Pro35S::PIF4‐HA line (Bernardo‐Garcia et al ., 2014).…”

“…Of the transcription factors targeted by COP1/SPAs for degradation, the bZIP transcription factor HY5 is perhaps most important in regard to hypocotyl growth (Figure 3) as it represses elongation in response to R, FR, B, and UV‐B (Koornneef et al ., 1980; Kim et al ., 2002; Oravecz et al ., 2006; Johansson et al ., 2014; Burko et al ., 2020; Bursch et al ., 2020; Li et al ., 2020a). In the dark, HY5 is largely degraded by the 26S proteasome in a COP1/SPA‐dependent manner (Osterlund et al ., 2000; Saijo et al ., 2003; Yang and Wang, 2006; Zhu et al ., 2008).…”

“…Beyond its activity as a transcription factor, HY5 also appears to promote photomorphogenesis by enhancing the activity of BIN2 (Li et al ., 2020a). Specifically, HY5 associates with BIN2 and promotes an activating autophosphorylation event at residue Y200 of this kinase (Kim et al ., 2009; Li et al ., 2020a).…”