The VQ Motif-Containing Protein Family of Plant-Specific Transcriptional Regulators

Abstract

The VQ motif-containing proteins (designated as VQ proteins) are a class of plant-specific proteins with a conserved and single short FxxhVQxhTG amino acid sequence motif. VQ proteins regulate diverse developmental processes, including responses to biotic and abiotic stresses, seed development, and photomorphogenesis. In this Update, we summarize and discuss recent advances in our understanding of the regulation and function of VQ proteins and the role of the VQ motif in mediating transcriptional regulation and protein-protein interactions in signaling pathways. Based on the accumulated evidence, we propose a general mechanism of action for the VQ protein family, which likely defines a novel class of transcriptional regulators specific to plants.

Figure 1.

The consensus sequence of the VQ motif. The sequences of the 31 amino acids of the VQ motif (pfam05678; Marchler-Bauer et al., 2015) were extracted from 34 Arabidopsis, 39 rice, and 25 moss VQ proteins, and the graph was generated by WebLogo (http://weblogo.berkeley.edu/; Crooks et al., 2004). The overall height of the stack indicates the sequence conservation at that position, while the height of the symbols within the stack indicates the relative frequency of each amino acid at that position.

Figure 2.

Summary of 34 Arabidopsis VQ proteins. Protein length^a, Based on data from The Arabidopsis Information Resource (www.arabidopsis.org). Localization^b, Predicted (adopted from Cheng et al. [2012]; the Subcellular Proteomic Database [http://suba.plantenergy.uwa.edu.au]) or supported by experiments. Transcriptional activity^c, Based on the transient expression assay of Li et al. (2014). Group^d, On the basis of the whole sequence similarities, the VQ proteins are classified into 10 groups (Pecher et al., 2014). Blue boxes denote VQ motifs. AGI, Arabidopsis Genome Initiative; M, mitochondria; N, nucleus; P, plastid.

Figure 3.

Transcriptional regulatory mode of VQ proteins. A, VQ23 interacts with WRKY33 and stimulates its binding to the promoter region of defense-responsive genes. B, The interaction between VQ9 and WRKY8 inhibits WRKY8 from binding to the promoter regions of its target genes during salt stress tolerance. C, VQ14 and MINI3 interact and coregulate the abundance of mRNAs encoding proteins involved in seed development. D, The interaction between VQ4 and WRKYs inhibits WRKYs from binding to their target genes in the plant defense response. MPK3/6 phosphorylate VQ4 and induce its degradation. E, After pathogen infection, MPK4 is activated and phosphorylates VQ21, triggering the release of VQ21 and WRKY33, which binds to the promoter of defense-responsive genes, such as PAD3, and activates their expression. VQ21 may optimize the transcriptional activation of WRKY33. In D and E, p denotes phosphorylated residues at multiple sites. F, VQ29 and PIF1 interact directly to regulate XTR7 expression and cell elongation.