Biology subjects

Heidarsson, P. O.

Publications and source records attributed to Heidarsson, P. O..

4 recordsLinked to original sources

bioRxiv2026

PolyA/polyQ-mediated conformational rewiring regulates DNA engagement and drives aggregation in the neuronal transcription factor Ascl1

Ascl1 is a pioneer transcription factor that drives neuronal fate decisions, yet the structural basis of its activity remains elusive. Besides the basic helix-loop-helix (bHLH) domain which dimerizes with other transcription factors and binds DNA, Ascl1 contains long low-complexity intrinsically disordered regions (IDRs), including a polyA/polyQ tract of unknown function. Here we use single-molecule FRET to map the conformational ensemble of full-length Ascl1 across monomeric, heterodimeric, and DNA-bound states. Monomeric Ascl1 is largely disordered but displays multivalent coupling between the N-terminal IDR and the bHLH domain, with the polyA/polyQ tract opposing bHLH compaction. E12 binding folds the bHLH domain and remodels the N-IDR, increasing dynamics across the N-IDR while suppressing them locally within the polyA/polyQ tract. Deleting the tract weakens nonspecific DNA binding by the Ascl1/E12 heterodimer without disrupting heterodimerization or high-affinity E-box binding, abolishes aggregation in vitro, and increases Ascl1 abundance in HEK293T cells. Thus, the polyA/polyQ tract acts as a regulatory module that promotes nonspecific DNA engagement while imposing a cost in solubility and cellular abundance.

biophysics↗

bioRxiv2023

Novel mechanisms of MITF regulation and melanoma predisposition identified in a mouse suppressor screen

MITF, a basic-Helix-Loop-Helix Zipper (bHLHZip) transcription factor, plays vital roles in melanocyte development and functions as an oncogene. To explore MITF regulation and its role in melanoma, we conducted a genetic screen for suppressors of the Mitf-associated pigmentation phenotype. An intragenic Mitf mutation was identified, leading to termination of MITF at the K316 SUMOylation site and loss of the C-end intrinsically disordered region (IDR). The resulting protein is more nuclear but less stable than wild-type MITF and retains DNA-binding ability. Interestingly, as a dimer, it can translocate wild-type and mutant MITF partners into the nucleus, improving its own stability and ensuring an active nuclear MITF supply. Interactions between K316 SUMOylation and S409 phosphorylation sites across monomers largely explain the observed effects. Notably, the recurrent melanoma-associated E318K mutation in MITF, which affects K316 SUMOylation, also alters protein regulation in concert with S409, unraveling a novel regulatory mechanism with unexpected disease insights.

molecular biology↗

bioRxiv2023

DNA binding redistributes activation domain ensemble and accessibility in pioneer factor Sox2

More than 1600 human transcription factors orchestrate the transcriptional machinery to control gene expression and cell fate. Their function is conveyed through intrinsically disordered regions (IDRs) containing activation or repression domains but lacking quantitative structural ensemble models prevents their mechanistic decoding. Here we integrate single-molecule FRET and NMR spectroscopy with molecular simulations showing that DNA binding can lead to complex changes in the IDR ensemble and accessibility. The C-terminal IDR of pioneer factor Sox2 is highly disordered but its conformational dynamics are guided by weak and dynamic charge interactions with the folded DNA binding domain. Both DNA and nucleosome binding induce major rearrangements in the IDR ensemble without affecting DNA binding affinity. Remarkably, interdomain interactions are redistributed in complex with DNA leading to variable exposure of two activation domains critical for transcription. Charged intramolecular interactions allowing for dynamic redistributions may be common in transcription factors and necessary for sensitive tuning of structural ensembles.

biophysics↗

bioRxiv2020

Disordered Proteins Enable Histone Chaperoning on the Nucleosome

Proteins with highly charged disordered regions are abundant in the nucleus, where many of them interact with nucleic acids and control key processes such as transcription. The functional advantages conferred by protein disorder, however, have largely remained unclear. Here we show that disorder can facilitate a remarkable regulatory mechanism involving molecular competition. Single-molecule experiments demonstrate that the human linker histone H1 binds to the nucleosome with ultra-high affinity. However, the large-amplitude dynamics of the positively charged disordered regions of H1 persist on the nucleosome and facilitate the interaction with the highly negatively charged and disordered histone chaperone prothymosin . Consequently, prothymosin can efficiently invade the H1-nucleosome complex and displace H1 via competitive substitution. By integrating experiments and simulations, we establish a molecular model that rationalizes this process structurally and kinetically. Given the abundance of charged disordered regions in the nuclear proteome, this mechanism may be widespread in cellular regulation.

biophysics↗