Role of a white collar-1-white collar-2 complex in blue-light signal transduction

Abstract

Mutations in either white collar-1 (wc-1) or white collar-2 (wc-2) lead to a loss of most blue-light-induced phenomena in Neurospora crassa. Sequence analysis and in vitro experiments show that WC-1 and WC-2 are transcription factors regulating the expression of light-induced genes. The WC proteins form homo-and heterodimers in vitro; this interaction could represent a fundamental step in the control of their activity. We demonstrate in vivo that the WC proteins are assembled in a white collar complex (WCC) and that WC-1 undergoes a change in mobility due to light-induced phosphorylation events. The phosphorylation level increases progressively upon light exposure, producing a hyperphosphorylated form that is degraded and apparently replaced in the complex by a newly synthesized WC-1. WC-2 is unmodified and also does not change quantitatively in the time frame examined. Light-dependent phosphorylation of WC-1 also occurs in a wc-2 mutant, suggesting that a functional WC-2 is dispensable for this light-specific event. These results suggest that light-induced phosphorylation and degradation of WC-1 could play a role in the transient expression of blue-light-regulated genes. Our findings suggest a mechanism by which WC-1 and WC-2 mediate light responses in Neurospora.

Molecular mechanism of light responses in Neurospora: from light-induced transcription to photoadaptation

“…4B, cf. lanes 1 and 5, our WC-1 antibody is more sensitive to the hyperphosphorylated WC-1 species than the hypophosphorylated forms), indicating that light triggered the degradation of a portion of WC-1, consistent with previous results (Talora et al 1999;Lee et al 2000;Franchi et al 2005). In addition, there was an increase of the least phosphorylated WC-1 species after 4 h in DD for the CHX-treated samples (Fig.…”

“…The dark-to-light transition results in transient hyperphosphorylation of WC-1 and WC-2, peaking after 15-30 min in LL (Talora et al 1999;Schwerdtfeger and Linden 2000;Cheng et al 2003a;He et al 2005). In addition, WC-1 becomes less phosphorylated after prolonged exposure in LL, but its phosphorylation level is more extensive than that in DD (Schwerdtfeger and Linden 2001;Cheng et al 2003a).…”

“…The WC proteins heterodimerize via their PAS domains to form a WC complex (WCC) in vivo (Ballario et al 1998;Talora et al 1999;Cheng et al 2002Cheng et al , 2003a, and it is this complex that mediates light responses by activating transcription of light-inducible genes Liu et al 2003). Upon light exposure, the immediate light-inducible genes such as the circadian clock component frequency (frq), genes involved in carotenoid biosynthesis (albino, al-1-al-3; vivid, vvd), are induced to their peak levels in 15-30 min.…”

“…WCs autoregulate their own transcription and their post-translational modifications (Ballario et al 1996;Talora et al 1999;Schwerdtfeger and Linden 2000). wc-1 is an immediate early light-inducible gene.…”

“…The light-induced WC hyperphosphorylation was previously proposed to have two roles. First, it may trigger WC-1 degradation, as WC-1 is less stable in the light than in the dark (Talora et al 1999;Lee et al 2000;Schwerdtfeger and Linden 2000;Franchi et al 2005). Second, it was proposed to be a mechanism that activates WCC, since WC-1 hyperphosphorylation appears to correlate with the activation of transcription.…”