Mechanisms Governing Target Search and Binding Dynamics of Hypoxia-Inducible Factors

Transcription factors (TFs) are classically attributed a modular construction, containing well-structured sequence specific DNA-binding domains (DBDs) paired with disordered activation domains (ADs) responsible for protein-protein interactions targeting cofactors or the core transcription initiation machinery. However, this simple division of labor model struggles to explain why TFs with identical DNA binding sequence specificity determined in vitro exhibit distinct non-overlapping binding profiles in vivo. The family of Hypoxia-Inducible Factors (HIFs) offer a stark example: aberrantly expressed in several cancer types, HIF-1α and HIF-2α subunit isoforms recognize the same DNA motif in vitro — the hypoxia response element (HRE) — but only share a subset of their target genes in vivo, while eliciting contrasting effects on cancer development and progression under certain circumstances. To probe the mechanisms mediating isoform-specific gene regulation, we used live cell single particle tracking (SPT) to investigate HIF nuclear dynamics and how they change upon genetic perturbation or drug treatment. We found that HIF-α subunits and their dimerization partner HIF-1β exhibit distinct diffusion and binding characteristics that are exquisitely sensitive to concentration and subunit stoichiometry. Using domain-swap variants, mutations, and a HIF-2α specific inhibitor, we found that although the DBD and dimerization domains are important, a major determinant of chromatin binding and diffusion behavior is dictated by the AD-containing intrinsically disordered regions. These findings reveal a previously unappreciated role of IDRs in regulating the TF search process that may play a role in selective functional target site binding on chromatin.