Biology subjects

Struhl, G.

Publications and source records attributed to Struhl, G..

3 recordsLinked to original sources

bioRxiv2024

An in vivo screen for proteolytic switches that can mediate Notch activation

Notch proteins are single pass transmembrane receptors activated by sequential extracellular and intramembrane cleavages that release their cytosolic domains to function as transcription factors in the nucleus. Upon binding, Delta/Serrate/LAG-2 (DSL) ligands activate Notch by exerting a "pulling" force across the intercellular ligand/receptor bridge. This pulling force is generated by Epsin-mediated endocytosis of ligand into the signal-sending cell, and results in the extracellular cleavage of the force-sensing Negative Regulatory Region (NRR) of the receptor by an ADAM10 protease [Kuzbanian (Kuz) in Drosophila]. Here, we have used chimeric Notch and DSL proteins to screen for other domains that can function as ligand-dependent proteolytic switches in place of the NRR in the developing Drosophila wing. While most of the tested domains are either refractory to cleavage or constitutively cleaved, we identify several that mediate Notch activation in response to ligand. These NRR analogues derive from widely divergent source proteins and have strikingly different predicted structures. Yet, almost all depend on force exerted by Epsin-mediated ligand endocytosis and cleavage catalyzed by Kuz. We posit that the sequence space of protein domains that can serve as force-sensing proteolytic switches in Notch activation is unexpectedly large, a conclusion that has implications for the mechanism of target recognition by Kuz/ADAM10 proteases and is consistent with a more general role for force dependent ADAM10 proteolysis in other cell contact-dependent signaling mechanisms. Our results also validate the screen for increasing the repertoire of proteolytic switches available for synthetic Notch (synNotch) therapies and tissue engineering. One sentence summaryA scalable in vivo screen for Epsin and ADAM10 dependent proteolytic switches that can mediate Notch activation in response to force.

cell biology↗

bioRxiv2022

Context-dependent transcriptional regulation by Drosophila Polycomb Response Elements

Polycomb Response Elements (PREs) are cis-acting DNA sequences that confer heritable states of Drosophila HOX gene expression by anchoring Polycomb and Trithorax Group (PcG and TrxG) chromatin modifiers. PREs are also associated with hundreds of other Drosophila genes, most of which are regulated dynamically in response to developmental and physiological context, rather than heritably like HOX genes. Here, we assess the role(s) PREs play at these other loci by analyzing how genomic inserts of a transgenic form of the HOX gene Ultrabithorax (Ubx) can both control and respond to neighboring genes depending on the presence of a single, excisable PRE. Our results support the view that PREs and their associated PcG and TrxG modifiers act primarily to confer quantitative, rather than qualitative, influences on gene expression with the response of any given gene depending on how it integrates this information with other regulatory elements in the local genomic milieu. They also show that PREs can act on neighboring genes selectively and at remarkably long range, but that any given gene can be susceptible or impervious to PRE/PcG/TrxG input depending on context. Finally, we find that transcription and PRE/PcG-dependent silencing are not mutually exclusive: a Ubx transgene inside the intron of a continuously transcribed "host" gene is nevertheless silenced by its resident PRE. We posit that the widely accepted roles of PcG and TrxG complexes in maintaining heritable states of gene expression apply only to a limited coterie of target genes such as HOX genes that are evolutionarily selected to exclude regulatory elements that can over-ride this control.

developmental biology↗

bioRxiv2021

The C. elegans Notch proteins LIN-12 and GLP-1 are tuned to lower force thresholds for activation than Drosophila Notch

The conserved transmembrane receptor Notch mediates cell fate decisions in all animals. In the absence of ligand, a Negative Regulatory Region (NRR) in the Notch ectodomain adopts an autoinhibited confirmation, masking an ADAM protease cleavage site [1, 2]; ligand binding makes the cleavage site accessible, leading to shedding of the Notch ectodomain as the first step of signal transduction [3, 4]. In Drosophila and vertebrates, the ligands are all single-pass transmembrane Delta/Serrate/LAG-2 (DSL) proteins; the endocytic adaptor Epsin binds to the ubiquitinated intracellular domain, and the resulting Clathrin-mediated endocytosis exerts a "pulling force" that exposes the cleavage site in the NRR [4-6]. However, in C. elegans, the presence of natural secreted DSL proteins [7] and other observations suggested that Epsin-mediated endocytosis may not be required to activate the Notch proteins LIN-12 and GLP-1. Here, we confirm that neither Epsin nor the cytosolic domains of DSL proteins are required for Notch signaling in C. elegans. Furthermore, we provide evidence that the NRRs of LIN-12 and GLP-1 are tuned to a lower force level than the NRR of Drosophila Notch. Finally, we show that adding a Leucine "plug" that occludes the cleavage site in vertebrate and Drosophila Notch proteins but is absent in the C. elegans Notch proteins [1, 2] renders the LIN-12 and GLP-1 NRRs dependent on Epsin-mediated ligand endocytosis, indicating that greater force is now required to expose the cleavage site. Thus, the NRRs of LIN-12 and GLP-1 appear to be tuned to a lower force threshold, accounting for the different requirements for signaling in C. elegans.

developmental biology↗