Understanding the Role of GH1 Domain Proteins in Chromatin Organization and Gene Regulation

My current research focuses on three families of GH1 domain proteins: Linker histone H1, High Mobility Group A (GH1-HMGA) and Telomere Repeat Binding (GH1-TRB):

Nucleosomes interact with the globular domain (GH1) of linker histone H1, which plays a crucial role in chromatin compaction. In addition to H1, plants have two unique families of proteins containing the GH1 domain: GH1-HMGA (high mobility group A) proteins and TRB (telomere repeat binding) proteins. GH1-HMGA proteins are characterised by their AT-hook domains that recognise the AT-rich region, whereas TRB proteins have a Myb DNA-binding domain that allows interaction with telomeric sequences. We are studying how these GH1 domain-containing proteins either cooperate or compete for chromatin binding sites influencing processes such as chromatin structure maintenance and gene expression modulation.

Key Findings

Our work participates in the understanding on the function of the five TRB proteins (TRB1-5), which recognize the telobox DNA-binding motif present at telomeres, but also at thousands of promoters. At some of these promoters, TRB1, TRB2, and TRB3 proteins recruit the Polycomb group complex 2 (PRC2) to deposit H3K27me3 and JMJ14 to remove H3K4me3, thus repressing transcription at these loci. We contributed to show that H1 histones prevent TRB1-3 binding to telobox-rich interstitial telomere repeats (ITRs) (Teano, 2023).

We are specifically interesting to elucidate the function of TRB4 and TRB5, two related paralogues forming a distinct TRB clade. In contrast to TRB1, 2, and 3, the absence of these two proteins does not affect the levels of H3K4me3 or H3K27me3 suggesting a different mode of action. Our results indicate an antagonism between the two clades, as combining clade I mutants with a prc2 mutant exacerbate prc2 phenotypes, while clade II mutants compensate prc2 defects. Transcriptomic studies revealed that gene upregulation in the prc2 mutant requires TRB4 and TRB5, particularly at MADS-Box genes like SOC1 and SEPs, which are essential for correct plant development and require fine-tuned transcriptional regulation, revealing the unexpected function of TRB4 and TRB5 as transcriptional activators (Amiard, 2024).

Research Directions: Role of GH1 domain proteins in genome regulation and coordinating Polycomb activity

Our aim is to dissect the role of Telomere Repeat Binding (TRB) proteins, GH1-High mobility group proteins (GH1-HMGA) and the linker histone H1 in regulating Polycomb activity. To explain the surprising observation that TRB4 and 5, in contrast to TRB1, 2 and 3, act as transcriptional activators of PRC2-controlled genes, we have screened an EMS-mutagenized population of trb45 mutants for enhancer or suppressor phenotypes. This screen resulted in several promising suppressor mutants, that will help us to dissect mechanistically how TRB4 and TRB5 regulate gene expression.

To study GH1-HMGA proteins, we have developed specific antibodies against GH1-HMGA1/2 to map their genome-wide distribution in wild type and mutant background for histone H1 and TRB proteins, probing competitive or synergistic binding among GH1-domain proteins. Our studies on GH1-HMGAs have revealed their specific binding to transposable elements (TEs) and their altered chromatin enrichment profiles in the absence of histone H1. These findings suggest a potential role in regulating TEs and chromatin dynamics.

Finally, in collaboration with the company LUMICKS (Netherlands) and the team of G. Farge (UCA), we are developing single molecule approaches to investigate the competitive interactions and DNA binding dynamics of GH1 proteins at the single molecule level and exploring their biochemical properties using advanced single-molecule techniques.

Methodologies

To unravel the mechanisms of action for these proteins, we employ a multidisciplinary approach: genetics, proteomics, epigenomics and biophysics.

Funding

ANR EpiLinks, I-site Emergence GH1-domain.

People

Emmanuel Vanrobays (MCU-UCA)

Sylvie Tutois (MCU-UCA)

Léa Feit (PhD student – CNRS)

Hafsa El Idrissi Mboutassim (PhD student – UCA)

Collaborators

1. Barnèche (IBPS, Paris)
2. Quadrana (IPS2, Paris)
3. Farge (UCA), Clermont-Ferrand)
4. Lorković (GMI, Vienna)
5. Bischoff (UZH, Zurich)
6. Carles (CEA, Grenoble)