SHORT HYPOCOTYL UNDER BLUE1 Truncations and Mutations Alter Its Association with a Signaling Protein Complex inArabidopsis 

Abstract

Higher plants monitor their ambient light signals through red/far-red absorbing phytochromes and blue/UV-A light absorbing cryptochromes. Subsequent signaling cascades alter gene expression and initiate morphogenic responses. We previously isolated SHORT HYPOCOTYL UNDER BLUE1 (SHB1), a putative transcriptional coactivator in light signaling. SHB1 is homologous to the SYG1 protein family and contains an N-terminal SPX domain and a C-terminal EXS domain. Overaccumulation of the SPX domain caused a long hypocotyl phenotype similar to that of shb1-D under red, far-red, or blue light. By contrast, overaccumulation of the C-terminal EXS domain led to a short hypocotyl phenotype similar to that of shb1 under blue light. The N-terminal SPX domain was associated with a smaller protein complex than the native protein complex associated with endogenous SHB1. By contrast, the EXS domain was associated with a slightly smaller protein complex than the native protein complex, but it largely displaced endogenous SHB1 from its native protein complex. In addition, all six missense mutations that we identified from a suppressor screen were clustered within or close to the SPX domain, and these mutations impaired the assembly of the SHB1-containing protein complex. We propose that both SPX and EXS domains likely anchor SHB1 to a protein complex, and the SPX domain is critical for SHB1 signaling. INTRODUCTIONLight signals trigger deetiolation or photomorphogenic responses, including the inhibition of hypocotyl elongation, the opening of cotyledons and hypocotyl hooks, the expansion of cotyledons, and the development of chloroplasts, in most land plants. The effective wavelengths that regulate photomorphogensis are red and far-red light, which are perceived by phytochromes, and UV-A/blue light, which is perceived by cryptochromes. Among the five phytochrome members in Arabidopsis thaliana, phytochrome A (phyA) mainly regulates far-red light-mediated deetiolation responses, and phyB together with phyC, phyD, and phyE regulate red light-mediated deetiolation responses (Neff et al., 2000;Huq and Quail, 2005). Cryptochrome1 (cry1) and cry2 specifically regulate photomorphogensis under blue light (Ahmad and Cashmore, 1993;Lin et al., 1998). Blue light triggers the phosphorylation of cry1 and cry2 and activates a series of signaling events or activities downstream of blue light perception (Shalitin et al., 2002(Shalitin et al., , 2003. Recently, a basic helix-loop-helix transcription factor has been shown to interact with photoactivated cry2, suggesting the function of this transcription factor in early photoreceptor signaling in response to blue light .Other components downstream of crys include SHORT UN-DER BLUE1 (SUB1) and phosphatase 7 (PP7). SUB1 is a cytosolic calcium binding protein and acts in both blue and far-red light pathways, since sub1 showed a short hypocotyl phenotype under both blue and far-red light (Guo et al., 2001). PP7 is a Ser/ Thr phosphatase, and downregulation of PP7 caused a long hypocotyl phenotype under blue li...

Eukaryotic Phosphate Homeostasis: The Inositol Pyrophosphate Perspective

“…Overexpression of SHB1, or just its SPX domain, extends its signalling activity towards a broader spectrum of light wavelengths, suggesting that these signalling activities might be directly modulated by SPX, perhaps by interaction with other domains [31].…”

“…However, the SPX domain is required for binding to PHO2, an ubiquitin-conjugating E2 enzyme that helps in the degradation of PHO1 for regulation of phosphate homeostasis [29]. Another SPX-EXS subfamily member is SHB1 (Short Hypocotyl under BLUE1, also known as PHO1;H4) [31].…”

“…The ability of PP-IPs to control S. cerevisiae polyP synthesis was originally reported by two studies. One used 31 P-NMR to determine yeast polyP levels [37] and linked the synthesis of polyP to a specific PP-IPs, PP-IP4, which is generated by the pyrophosphorylation of inositol pentakisphosphate (IP5) not directly function as an activator/inhibitor but might work as a molecular glue regulating protein-protein interactions. This mode of action has been observed for several proteins: IPs bind in a pocket between histone deacetylases (HDAC) and co-repressor proteins [38] regulating their interaction; similarly IPs bind to TIR1 and COI1-JAZ, the receptor for the plant hormone auxin [39] and jasmonate, respectively [40].…”

Phosphate homeostasis in the yeast Saccharomyces cerevisiae, the key role of the SPX domain‐containing proteins

“…The SPX domain of the yeast Syg1 was for long the only evidence of the involvement of the SPX domain in protein interaction, interacting with the b subunit of the G-protein heterotrimer, ultimately suppressing the lethality of the G-protein a-subunit deficiency (Table 1) [11]. Since then, two additional studies, in yeast and Arabidopsis thaliana, have shown a role for the SPX domain in protein interaction [12,13]. In addition, a strong link between proteins harbouring the SPX domain and Pi homeostasis has been observed in yeast and plants.…”

“…Moreover, recent work in both yeast and plants report that the SPX domain itself could be involved in fine-tuning of Pi transport and signaling, through mechanisms such as physical interactions with other proteins [12,13,21].…”