Two-component signal transduction pathways in Arabidopsis

Abstract

The two-component system, consisting of a histidine (His) protein kinase that senses a signal input and a response regulator that mediates the output, is an ancient and evolutionarily conserved signaling mechanism in prokaryotes and eukaryotes. The identification of 54 His protein kinases, His-containing phosphotransfer proteins, response regulators, and related proteins in Arabidopsis suggests an important role of two-component phosphorelay in plant signal transduction. Recent studies indicate that two-component elements are involved in plant hormone, stress, and light signaling. In this review, we present a genome analysis of the Arabidopsis two-component elements and summarize the major advances in our understanding of Arabidopsis two-component signaling.

Figure 1

Schematic representation of the two-component and the multistep phosphorelay signaling systems. A, The prototypical two-component pathway uses a single phosphoryl transfer event between a His protein kinase and its cognate response regulator. B, The multistep His-to-Asp phosphorelay system in which a His-containing phosphotransfer protein serves as a phosphoryl acceptor and donor between the hybrid protein kinase and the response regulator. In yeast (Saccharomyces cerevisiae) osmosensing, the phosphorelay connects to an MAPK cascade (Wurgler-Murphy and Saito, 1997). In Arabidopsis cytokinin signaling, a response regulator directly regulates its target gene expression (Hwang and Sheen, 2001). The vertical bars represent transmembrane domains. H, His; D, Asp; P, phosphoryl group.

Figure 2

Primary domain structure of representative two-component elements in Arabidopsis. A, CRE1/AHK4/WOL and CKI1 are similar in domain structure but quite diverged in amino acid sequence. ETR1 and ERS1 are representatives of the ethylene receptor family with/without a receiver domain. Phytochromes (PHY) are soluble proteins with similar overall structure and consisting of the light-sensing domain (the chromophore-binding domain), the PAS repeats, and a domain with His protein kinase homology. TM, Transmembrane domain; ED, extracellular putative input domain; KD, kinase domain; RD, receiver domain; RLD, receiver-like domain; H, His; D, Asp. B, His-containing phosphotransfer protein. C, Response regulators and response regulator-like proteins: ARR2, a B-type response regulator; ARR3, an A-type response regulator; and APRR1 and APRR2 (pseudo-type response regulators), response regulator-like proteins. ARR2 has a receiver domain followed by a DNA-binding domain (B motif) and a Pro-/Gln-rich transactivation domain, whereas ARR3 only carries a receiver domain. APRR1 and APRR2 have an atypical receiver domain similar to ARR2, but with N, E, and K motifs. In addition to a receiver-like domain, APRR1 and APRR2 have C and B motifs, respectively. Diagrams are not to scale. RD, Receiver domain; BD, DNA-binding B motif; AD, transactivation domain; D, Asp; N, Asn; E, Glu; K, Lys.

Figure 3

Unrooted relationship tree of His protein kinases and related proteins in Arabidopsis. The tree is branched into three groups: the ethylene receptors, the photoreceptor phytochromes, and the AHK members. The entire amino acid sequences of His protein kinases were aligned by the Clustal X program (Thompson et al., 1997) and the relationship tree was produced by the TreeView program (Page, 1996). The AHK members, the ethylene receptors, and the phytochromes are in light-gray, gray, and dark-gray shade, respectively.

Figure 4

Amino acid sequence alignment of His protein kinase transmitter (A) and receiver (B) domains of His protein kinases and related proteins in Arabidopsis. Sequences were aligned by the Clustal X program. Conserved amino acids are highlighted. Black and gray backgrounds indicate percentage of amino acid similarity: black, at least 75%; darker gray, 50%; and lighter gray, 25%. Amino acid similarity groups are: D, N; E, Q; S, T; K, R; F, Y, and W; and L, I, V, and M. Conserved motifs are indicated above the alignment. The numbers indicate the amino acid gaps between the motifs.

Figure 5

His-containing phosphotransfer proteins in Arabidopsis. A, Unrooted relationship tree of His-containing phosphotransfer proteins (AHPs). Programs used were Clustal X for alignment and TreeView for graphical output. The entire amino acid sequences were aligned. B, Alignment of deduced amino acid sequences of His phosphotransfer proteins in Arabidopsis. AHP1-5 contains the highly conserved XHQXKGSSXS motif, which includes the His phosphorylation site. The putative His phosphorylation residue of AHP6 is replaced by an Asn. H, Conserved His phosphorylation site; the asterisk marks a potential His phosphorylation residue of AHP6.

Figure 6

Unrooted relationship tree of response regulators and response regulator-like proteins in Arabidopsis. The amino acid sequences of receiver domains of response regulators and response regulator-like proteins in Arabidopsis were aligned by the Clustal X program and the relationship tree was produced by the TreeView program. Response regulators and response regulator-like proteins in Arabidopsis are divided into three major groups: A type (11 genes), B type (12 genes), and pseudoresponse regulator (nine genes). We categorized proteins as response regulator-like (pseudoresponse regulators) if they did not have the conserved phosphate-accepting Asp within the receiver domain. A-, B-, and pseudo-type response regulators are in dark-gray, light-gray, and gray shade, respectively.

Figure 7

Alignment of deduced amino acids sequences of response regulators and response regulator-like proteins in Arabidopsis. A, The amino acid sequences of receiver domains of response regulators and response regulator-like proteins were aligned. The highly conserved amino acids are highlighted. The three conserved motifs are indicated above the alignments. The numbers indicate the amino acid gaps between the motifs. B, Alignment of putative DNA-binding B motifs of B-type response regulators and related proteins. The predicted amino acid sequences of the B motifs were aligned with the conserved Myb DNA-binding motif.

Figure 8

Locations of putative two-component regulators on the Arabidopsis chromosomes. Ovals on the chromosomes represent the centromeres. The arrows show the direction of transcription. The numbers in parentheses indicate the position of the first exon of each two-component gene from one end of the chromosome in megabase (Mb). The bars represent AHKs and ethylene receptors (red), phytochromes (yellow), ARRs (green), and AHPs (blue).

Figure 9

Model of the two-component signal transduction pathways in Arabidopsis. The cytokinin signal is perceived by multiple His protein kinases at the plasma membrane. Upon perception of the cytokinin signal, His protein kinases initiate a signaling cascade via the phosphorelay that results in the nuclear translocation of AHPs (Hwang and Sheen, 2001). Activated AHPs may interact with sequestered ARRs or ARR complexes, transfer the phosphate to the receiver domain of its cognate B-type ARR, releasing these activation-type ARRs from putative repressors in the nucleus. The dephosphorylated AHP shuttles back to the cytosol, where it can be rephosphorylated. The liberated ARRs bind to multiple cis elements in the promoter of target genes. The activation of the repressor-type ARRs as primary cytokinin response genes provides a negative feedback mechanism. In addition to the CTR1 signaling pathway, additional ethylene signaling pathways could be mediated by two-component components (Lohrmann and Harter, 2002). Red light and cytokinin signaling is converged at ARR4. ARR4 stabilizes the active form of PHYB by inhibiting dark reversion (Sweere et al., 2001). Stress and Glc may also modulate two-component signaling (Urao et al., 1998; F. Rolland and J. Sheen, unpublished data). RD, Response domain; BD, DNA binding domain; AD, transactivation domain; PM, plasma membrane; N, nucleus; R, putative repressor; FR, far-red light.