The N-Terminal Domain of the Arabidopsis thaliana TOPLESS Corepressor

Molly Hunt '27 and Frances Szaraz '25

Contents:

I. Introduction

The corepressor TOPLESS is fundamental for transcriptional repression in the plant Arabidopsis thaliana. Corepressors indirectly repress gene transcription by linking transcription factors to chromatin remodeling complexes, thus making DNA inaccessible for mRNA transcription. TOPLESS (TPL) is widely conserved throughout the green plants, and interacts with a wide variety of transcriptional repressors. The protein is critical for auxin signaling and embryonic development, and appears fundamental to a myriad of pathways.

The N-terminal domain of the Arabidopsis TPL protein (AtTPL) contains a number of motifs critical for its function and tetramerization, which are interdependent. The LIS1 homology (LisH) and C-terminal to LisH (CTLH) domains are critical for dimerization, and for interactions with transcription factors via a hydrophobic peptide region. 

II. General Structure of the TPL Monomer

The AtTPL N-terminal domain (NTD) is a tetramer, comprised of four monomers consisting of . The monomers that eventually tetramerize to form the complete region consist of nine connected alpha-helices, and comprise 3 distinctive regions. α1 and α2 form the domain, α3-α5 form the domain, and α6-α9 form the domain.  

III. Dimerization and Tetramerization

To connect the two monomers into a dimer, the first monomer’s makes contact with the of the adjacent monomer. To tetramerize (form dimers of dimers), the α6 and α7 helices of the adjacent dimers connect via multiple amino acid interactions. Mutagenesis approaches indicate that K102 and T116 are particularly critical for the tetramerization . K102 forms a hydrogen bond with a threonine (T120) of the adjacent monomer, and T116 forms a similar hydrogen bond with a glutamine residue (Q117).

IV. Interactions with Ear Motifs

TPL can interact with transcriptional regulators through hydrophobic peptides, specifically through the binding of ethylene-responsive element binding factors associated with amphiphilic repression (EAR) motifs domain. Three hydrophobic grooves exist in the TPL, and all three are possible candidates for binding with proteins that contain this EAR motif. is formed by C- terminal alpha 9 helix of the CRA domain and the LisH domain. is smaller, and is located between the LisH and the CTLH domains. And finally is formed by a three-helix bundle, including helix alpha 5 of the CTLH domain and then helices alpha 7 and alpha 9 of the CRA domains. Through mutations, G2 and G3 are the most likely candidate to be responsible for . The mechanics of such interactions can be examined by looking at van der waals interactions between the ear motifs of auxin responsive protein IAA27 and individual hydrophobic grooves.


IV. Evolutionary Significance

Despite their repressive effect on transcription, TPL and other transcriptional corepressors are critical for standard growth and development in land plants, and their loss is detrimental to the continuation of stem cells, shoot growth, and organ formation throughout a plant’s lifetime. The name TOPLESS arises from several striking loss-of-function mutant phenotypes, the most severe of which lack an embryonic shoot and instead form a second root (Long et al., 2006). Loss of TPL results in a pleiotropic cascade, implicating TPL in a number of upstream roles outside of transcriptional repression, including embryogenesis, meristem maintenance, organ growth, circadian clock maintenance, and reproduction (Plant et al., 2021).

TPL has long been a source of interest to plant evolutionary biologists due to its high level of conservation across diverse plant lineages (Causier, 2023). The protein’s function in the initiation of 3D growth is both instrumental to and likely predates terrestrialization (Causier, 2023). It has been suggested that TPL proteins were recruited from pre-existing gene regulatory networks to new pathways through transcription factors containing a broad range of repressive domain (RD) binding sequences, such as the G motifs featured here. Researchers have postulated that the charophyte algal ancestors were preadapted to land before the emergence of embryophyte plants and that TPL may have already been recruited by this point (Plant et al., 2021). Throughout several hundred million years of evolution, TPL proteins have become critical for facilitating phytoterrestrialization and the ongoing dominance of land plants.


VI. References

Causier, B.,McKay, M., Hopes, T., Lloyd, J., Wang, D., Harrison, C.J. and Davies, B. 2023. The TOPLESS corepressor regulates developmental switches in the bryophyte Physcomitrium patens that were critical for plant terrestrialisation. Plant J, 115: 1331-1344.

Long, J. A., Ohno, C., Smith, Z. R., & Meyerowitz, E. M. 2006. TOPLESS regulates apical embryonic fate in Arabidopsis. Science 312(5779), 1520-1523.

Martin-Arevalillo, R., Nanao, M.H., Larrieu, A., Vinos-Poyo, T., Mast, D., Galvan-Ampudia, C., Brunoud, G., Vernoux, T., Dumas, R., Parcy, F. 2017. Structure of the Arabidopsis TOPLESS corepressor provides insight into the evolution of transcriptional repression Proc. Natl Acad. Sci. U.S.A. 114 (30) 8107-8112

Plant, A.R., Larrieu, A. and Causier, B. 2021. Repressor for hire! The vital roles of TOPLESS-mediated transcriptional repression in plants. New Phytologist 231: 963-973.

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