Phosphorylation by CK2 Enhances the Rapid Light-induced Degradation of Phytochrome Interacting Factor 1 in Arabidopsis

Maria D. Person,Ling Zhu,Michael D. Dennis,Karen S. Browning,Enamul Huq,Qingyun Bu,Sheen X. Lu,Elaine M. Tobin,Lu Yu

doi:10.1074/jbc.m110.186882

Abstract

The phytochrome family of sensory photoreceptors interacts with phytochrome interacting factors (PIFs), repressors of photomorphogenesis, in response to environmental light signals and induces rapid phosphorylation and degradation of PIFs to promote photomorphogenesis. However, the kinase that phosphorylates PIFs is still unknown. Here we show that CK2 directly phosphorylates PIF1 at multiple sites. α1 and α2 subunits individually phosphorylated PIF1 weakly in vitro. However, each of four β subunits strongly stimulated phosphorylation of PIF1 by α1 or α2. Mapping of the phosphorylation sites identified seven Ser/Thr residues scattered throughout PIF1. Ser/Thr to Ala scanning mutations at all seven sites eliminated CK2-mediated phosphorylation of PIF1 in vitro. Moreover, the rate of degradation of the Ser/Thr to Ala mutant PIF1 was significantly reduced compared with wild-type PIF1 in transgenic plants. In addition, hypocotyl lengths of the mutant PIF1 transgenic plants were much longer than the wild-type PIF1 transgenic plants under light, suggesting that the mutant PIF1 is suppressing photomorphogenesis. Taken together, these data suggest that CK2-mediated phosphorylation enhances the light-induced degradation of PIF1 to promote photomorphogenesis.

Highlights

(which consists of five members, phyA to phyE, in Arabidopsis) senses and responds to a broad spectrum of light signals and pleiotropically regulates plant growth from seed germination to flowering time [3,4,5]
Because CK2mediated phosphorylation stabilizes the positively acting transcription factors involved in light signaling in Arabidopsis, we reasoned that CK2 might phosphorylate the negatively acting transcription factors (e.g. phytochrome interacting factors (PIFs)) and regulate their stability/function in Arabidopsis
This study provides biochemical evidence that PIF1 is a substrate for Arabidopsis CK2, a ubiquitous Ser/Thr kinase present in all organisms

Summary

Introduction

(which consists of five members, phyA to phyE, in Arabidopsis) senses and responds to a broad spectrum (red, far-red, and blue) of light signals and pleiotropically regulates plant growth from seed germination to flowering time [3,4,5]. Because PIF1-6M is still phosphorylated by CK2 and the S466A mutation in the S464 – 465A (two CK2 sites mapped in this study, Fig. 2A) background displayed a strong reduction in the light-induced degradation of PIF1 (Fig. 4), we reasoned that Ser-466 might be a CK2 site, as predicted by the ScanSite 2.0 and KinasePhos software packages (supplemental Table S2).

Results

Conclusion