Biology subjects

Graham, T. G. W.

Publications and source records attributed to Graham, T. G. W..

7 recordsLinked to original sources

bioRxiv2025

Collective unstructured interactions drive chromatin binding of transcription factors

Eukaryotic transcription factors (TFs) contain both structured DNA-binding domains (DBDs) and intrinsically disordered regions (IDRs). While the structures and sequence preferences of DBDs have been extensively characterized, the role of IDR-mediated interactions in chromatin binding and nuclear organization remains poorly understood, in part because these interactions have been difficult to measure in living cells. Here, we use a recently developed single-molecule technique, proximity-assisted photoactivation (PAPA), to investigate how IDRs influence TF associations with each other and with chromatin, focusing on the factors Sp1 and Klf1. We find that the number and patterning of aromatic and basic residues within IDRs govern both TF self-association and chromatin binding. Unexpectedly, the isolated DBD of Sp1 binds chromatin very weakly and non-specifically. The isolated IDR, by contrast, interacts poorly with chromatin-bound wild-type Sp1, yet this interaction is enhanced when even minimal DNA-binding capacity is restored. Strikingly, replacing Sp1s native DBD with those of heterologous TFs recovers both IDR-mediated interactions and chromatin association, despite divergent sequence preferences. PAPA measurements also reveal extensive heterotypic interactions between wild-type Sp1 and other TFs. Together, these results establish PAPA as a powerful method for studying unstructured interactions in their native context and suggest that IDRs participate in widespread cooperative associations scaffolded by transient DBD-DNA contacts, which concentrate disordered regions along chromatin. In contrast to classical models, we propose that TF specificity in vivo emerges not solely from DBD sequence preferences, but from a constellation of weak, dynamic, and diverse interactions mediated by IDRs.

biochemistry↗

bioRxiv2024

High-speed 3D Imaging with 25-Camera Multifocus Microscope

We here report an aberration-corrected 25-plane camera array Multifocus microscope (M25) for high-speed, high-resolution wide-field optical microscopy in three spatial dimensions (3D). We demonstrate live imaging of 25-plane 3D volumes of up to 180x180x50um at >100 volumes per second. 3D data is recorded simultaneously by an array of 25 small, sensitive, synchronized machine-vision cameras. M25 employs aberration-corrected Multifocus microscopy--an optical method where diffractive Fourier optics are used for multiplexing and refocusing light-- with a simplified design for chromatic dispersion correction where a corrective diffractive gratings is placed on each camera in the array. This elegant architecture for chromatic correction will be applicable in a broad range of diffractive imaging applications. M25 is a powerful optical tool for high-speed 3D microscopy in that it allows both non-invasive, label-free bright-field and highly sensitive fluorescence microscopy. We showcase M25 capabilities in 3D particle tracking, bright-field, and fluorescence imaging in D. melanogaster, and locomotion and neural activity studies in C. elegans.

neuroscience↗

bioRxiv2024

Spatial microenvironments tune immune response dynamics in the Drosophila larval fat body

Immune responses in tissues display complex spatial patterns of gene expression that are linked to disease outcomes. However, the mechanisms that generate these patterns--including the relative roles of noisy gene expression dynamics, microbial transport, and tissue anatomy-- are poorly understood. As a tractable model of spatial immune responses, we investigated heterogeneous expression of antimicrobial peptides in the larval fly fat body, an organ functionally analogous to the liver. Using live light sheet fluorescence microscopy, we discovered that individual fat body cells express antimicrobial peptides at approximately constant rates following infection, but that the average rate varies along the anterior-posterior axis of the fat body, with rapid expression in the anterior and posterior lobes. Overexpression of immune signaling components and analysis of spatial transcriptomes revealed that these tissue microenvironments are predefined independently of infection, with the rate-limiting step of antimicrobial peptide induction downstream of peptidoglycan sensing. The locations of these microevironments correlate with heartbeat-dependent fluid flow in a manner resembling the strategic positioning of immune cells in the liver, gut, and lymph nodes of mammals. We speculate that this spatial compartmentalization helps the fat body efficiently perform its diverse metabolic, enzymatic, and immunological functions. Author SummaryRecent sequencing and imaging technologies have revealed that immune responses in our organs are not spatially uniform, but occur in complex patterns in which clusters of nearby cells are strongly active. There is increasing evidence that these spatial interactions are important for controlling disease outcomes. However, little is known about the dynamics of how these spatial patterns form: are they created through randomness, are they shaped by external signals, such as pathogen localization, or are they predetermined, representing a fine-grained tissue anatomy? While it is practically infeasible to directly observe these types of cellular dynamics in humans or even mice, small, transparent organisms like fruit fly larvae offer a literal window into the inner workings of immune responses. We used a combined imaging and genetics approach to study heterogeneous spatial patterns of antimicrobial peptide production in the fruit fly equivalent of the liver. We discovered that these spatial patterns were in fact predetermined and represent previously unknown immune microenvironments within this important tissue that correlate with areas of fast blood flow. Since innate immune signaling in highly conserved, this spatial logic may be a general feature of immunological tissues that is relevant to other animals, including humans.

immunology↗

bioRxiv2024

Single-molecule live imaging of subunit interactions and exchange within cellular regulatory complexes

Cells are built from vast networks of competing molecular interactions, most of which have been impossible to monitor in vivo. We recently devised a new strategy, proximity-assisted photoactivation (PAPA), to detect these interactions at single-molecule resolution in live cells. Here we apply PAPA to visualize the network of interactions that regulate the central transcription elongation factor P-TEFb. PAPA between multiple pairs of endogenous proteins, combined with fast single-molecule tracking (fSMT), revealed that inactive P-TEFb within the 7SK ribonucleoprotein complex is largely unbound to chromatin, that this complex dissociates within minutes of treatment with a P-TEFb kinase inhibitor, and that heterogeneous ribonucleoproteins (hnRNPs) bind 7SK concomitant with P-TEFb release. Unlike 7SK-bound P-TEFb, P-TEFb associated with the coactivator BRD4 exhibited increased binding to chromatin. Our results address longstanding questions about a key transcriptional regulator and demonstrate that PAPA-fSMT can probe subunit interactions and exchange within endogenous regulatory complexes in live cells.

molecular biology↗

bioRxiv2023

Surprising Features of Nuclear Receptor Interaction Networks Revealed by Live Cell Single Molecule Imaging

Type 2 Nuclear Receptors (T2NRs) require heterodimerization with a common partner, the Retinoid X Receptor (RXR), to bind cognate DNA recognition sites in chromatin. Based on previous biochemical and over-expression studies, binding of T2NRs to chromatin is proposed to be regulated by competition for a limiting pool of the core RXR subunit. However, this mechanism has not yet been tested for endogenous proteins in live cells. Using single molecule tracking (SMT) and proximity-assisted photoactivation (PAPA), we monitored interactions between endogenously tagged retinoid X receptor (RXR) and retinoic acid receptor (RAR) in live cells. Unexpectedly, we find that higher expression of RAR, but not RXR increases heterodimerization and chromatin binding in U2OS cells. This surprising finding indicates the limiting factor is not RXR but likely its cadre of obligate dimer binding partners. SMT and PAPA thus provide a direct way to probe which components are functionally limiting within a complex TF interaction network providing new insights into mechanisms of gene regulation in vivo with implications for drug development targeting nuclear receptors.

biophysics↗

bioRxiv2021

Proximity-assisted photoactivation (PAPA): Detecting molecular interactions in live-cell single-molecule imaging

Single-molecule imaging provides a powerful way to study biochemical processes in live cells, yet it remains challenging to track single molecules while simultaneously detecting their interactions. Here we describe a novel property of rhodamine dyes, proximity-assisted photoactivation (PAPA), in which one fluorophore (the "sender") can reactivate a second fluorophore (the "receiver") from a dark state. PAPA requires proximity between the two fluorophores, yet it operates at a longer average intermolecular distance than Forster resonance energy transfer (FRET). We show that PAPA can be used in live cells both to detect protein-protein interactions and to highlight a sub-population of labeled protein complexes in which two different labels are in proximity. In proof-of-concept experiments, PAPA detected the expected correlation between androgen receptor self-association and chromatin binding at the single-cell level. These results establish a new way in which a photophysical property of fluorophores can be harnessed to study molecular interactions in single-molecule imaging of live cells.

biophysics↗

bioRxiv2021

Tuning levels of low-complexity domain interactions to modulate endogenous oncogenic transcription

Gene activation by mammalian transcription factors (TFs) requires dynamic, multivalent, and selective interactions of their intrinsically disordered low-complexity domains (LCDs), but how such interactions mediate transcription remains unclear. It has been proposed that extensive LCD-LCD interactions culminating in liquid-liquid phase separation (LLPS) of TFs is the dominant mechanism underlying transactivation. Here, we investigated how tuning the amount and localization of LCD-LCD interactions in vivo affects transcription of endogenous human genes. Quantitative single-cell and single-molecule imaging reveals that the oncogenic TF EWS/FLI1 requires a finely tuned range of LCD-LCD interactions to efficiently activate target genes. Modest or more dramatic increases in LCD-LCD interactions toward putative LLPS repress EWS/FLI1-driven transcription in patient cells. Likewise, ectopically creating LCD-LCD interactions to sequester EWS/FLI1 into a bona fide LLPS compartment, the nucleolus, inhibits EWS/FLI1-driven transcription and oncogenic transformation. Our findings reveal fundamental principles underlying LCD-mediated transcription and suggest mislocalizing specific LCD-LCD interactions as a novel therapeutic strategy for targeting disease-causing TFs.

molecular biology↗