A Conserved Transcription Motif Suggesting Functional Parallels between Caenorhabditis elegans SKN-1 and Cap'n'Collar-related Basic Leucine Zipper Proteins

Abstract

In Caenorhabditis elegans, the predicted transcription factor SKN-1 is required for embryonic endodermal and mesodermal specification and for maintaining differentiated intestinal cells post-embryonically. The SKN-1 DNA-binding region is related to the Cap'n'Collar (CNC) family of basic leucine zipper proteins, but uniquely, SKN-1 binds DNA as a monomer. CNC proteins are absent in C. elegans, however; and their involvement in the endoderm and mesoderm suggests some functional parallels to SKN-1. Using a cell culture assay, we show that SKN-1 induces transcription and contains three potent activation domains. The functional core of one domain is a short motif, the DIDLID element, which is highly conserved in a subgroup of vertebrate CNC proteins. The DIDLID element is important for SKN-1-driven transcription, suggesting a likely significance in other CNC proteins. SKN-1 binds to and activates transcription through the p300/cAMP-responsive elementbinding protein-binding protein (CBP) coactivator, supporting the genetic prediction that SKN-1 recruits the C. elegans p300/CBP ortholog, CBP-1. The DIDLID element appears to act independently of p300/CBP, however, suggesting a distinct conserved target. The evolutionarily preservation of the DIDLID transcriptional element supports the model that SKN-1 and some CNC proteins interact with analogous cofactors and may have preserved some similar functions despite having divergent DNA-binding domains.During development, establishment of cell fates frequently involves conserved regulatory pathways. In the early Caenorhabditis elegans embryo, endodermal (intestinal) fates are specified by GATA family transcription factors (END-1, END-3, ELT-2) that are related to those that mediate endoderm development in Drosophila (Serpent) and vertebrates (GATA-4/5/6) (1-3). This program is triggered by maternally expressed SKN-1, a predicted transcription factor (4) that also specifies mesodermal lineages (pharynx and some body wall muscle) (5). The presence of consensus SKN-1-binding sites adjacent to the end-1 gene along with the timing of its expression suggests that SKN-1 may activate end-1 directly (2). SKN-1 is also required in the endoderm post-embryonically, to prevent differentiated intestinal cells from undergoing severe atrophy (5).Apparent SKN-1 orthologs have not been identified outside of nematodes, but in its DNA-binding region, SKN-1 is related to a subgroup of basic leucine zipper transcription factors (4, 5), the CNC 1 proteins. A basic DNA-binding region at the SKN-1 COOH terminus is particularly similar to those of CNC proteins, but SKN-1 lacks a zipper dimerization domain (Fig. 1, A and C) and, uniquely, binds DNA as a monomer (4). Its DNA binding requires the adjacent ␣-helical "CNC region" (Fig. 1, A and C) (4, 6 -8), which is otherwise found only in CNC proteins. An adjacent SKN-1 element that is lacking in CNC proteins, the NH 2 -terminal arm (Fig. 1, A and C), contributes additional binding affinity and specificity (6, 9). These similarities and differences pose t...

A Caenorhabditis elegans model of insulin resistance: Altered macronutrient storage and dauer formation in an OGT-1 knockout

“…To examine the effect of altered O-GlcNAc synthesis on nuclear pore function, we examined nuclear transport of several well characterized C. elegans transcription factors, including SKN-1, HLH-1, HLH-2, ELT-2, and LIN-26. In C. elegans, SKN-1 is involved in both early blastomere development and the stress response (28)(29)(30)(31). The stress response in larvae and adults causes cytoplasmic SKN-1 in intestinal cells to rapidly translocate to the nucleus (28).…”

SKN-1 linksC. elegansmesendodermal specification to a conserved oxidative stress response

“…SKN-1 is also similar to two vertebrate CNC-group proteins (NF-E2-related factors Nrf1 and Nrf2) within a 14-amino-acid transactivator element (DIDLID; Fig. 1B; Walker et al 2000). These limited similarities suggest that although SKN-1-like proteins have not been identified outside of nematodes, SKN-1 and these particular bZIP proteins might share a common precursor.…”

“…SKN-1 and Nrf proteins are most similar within the 14-amino-acid DIDLID transactivation element ( Fig. 1B; Walker et al 2000), which appears to be present only in SKN-1 and Nrf protein orthologs, suggesting that in metazoans, preservation of this ele- Figure 6. skn-1 mutants are sensitive to oxidative stress and have reduced lifespans.…”

Redox-regulated Turnover of Nrf2 Is Determined by at Least Two Separate Protein Domains, the Redox-sensitive Neh2 Degron and the Redox-insensitive Neh6 Degron

“…Cell Culture, Transfections, Chemical Challenge, and Reporter Assays-ts20TG R and H38.5 cells (26) [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] and the ETGE motif (amino acids 79 -82) of the Neh2 domain are highlighted, as are two highly conserved regions of the Neh6 domain (amino acids 329 -339 and 363-379). A complete sequence alignment of orthologous Nrf2 proteins from human to fish and a description of the known functions of the Neh domains can be found in the supplemental Fig.…”

“…Comparison of the amino acid sequence of SKN-1, a transcription factor in Caenorhabditis elegans that responds to oxidative stress, with that of Nrf2 has revealed the presence of a conserved peptide called the "DIDLID element" in the nematode protein (22). This element is located between residues 99 and 112 of SKN-1, whereas in mouse, rat, and human Nrf2 it resides in the Neh2 domain between amino acids 17 and 32 ( Fig.…”