Biology subjects

Morgan, R. M.

Publications and source records attributed to Morgan, R. M..

4 recordsLinked to original sources

bioRxiv2026

A potent macrocyclic peptide binder enables effector-free crystallization of the GTPase RAB27A

RAB27A is a member of a highly conserved family of small GTPases that regulate intracellular vesicle trafficking. Multiple studies have linked RAB27A cellular activity to exosome-mediated modulation of the tumour microenvironment and inflammatory signalling, highlighting it as an attractive target for drug discovery. However, identifying structurally validated and selective inhibitors of the RAB27A-effector interaction has proven challenging, partly because wild-type RAB27A crystallizes only in complex with effector proteins such as Slp2a. Using mRNA display, we identified macrocyclic peptide ligands that target the RAB27A effector-binding interface. Optimisation of initial hits led to IMP2660, a nanomolar-affinity RAB27A binder that effectively disrupts RAB27A-effector interactions. IMP2660 enabled determination of a 2.1[A] structure of active-state RAB27A bound to a noncovalent macrocyclic peptide in the absence of an effector, revealing structural features of the effector-binding surface and providing a platform for future structure-guided ligand development.

biochemistry↗

bioRxiv2026

Discovery of stereoselective targeted covalent inhibitors of the RAB27-effector protein-protein interaction

RAB27A and RAB27B are homologous small GTPases that regulate intracellular vesicle trafficking, orchestrating endocytic and exocytic processes that affect cellular communication, immune responses, and dynamics of the cellular microenvironment. Through their interactions with effector proteins, RAB27A/B play roles in tumor metastasis and chronic inflammation. However, pharmacological modulation of their activity faces challenges typical of small GTPases, including a lack of well-defined pockets outside the conserved GTP binding site, and large RAB27-effector protein-protein interaction (PPI) surfaces. Here, we present the discovery and development of the first cell-active, rationally designed covalent inhibitors of the RAB27-effector interaction, targeting a non-conserved cysteine residue flanking the PPI interface. An electrophile-first biochemical screen led to a novel class of acrylamide covalent inhibitors, and X-ray crystallography structure-guided design led to optimized inhibitors and probes that enantioselectively target RAB27A/B-Cys123 in cells. Potency and selectivity were confirmed through biochemical and cellular assays, including chemical proteomics and phenotype recapitulation in melanocytes alongside a matched inactive enantioprobe control. In contrast, a previously reported compound, Nexinhib-20, was found to be toxic and to exert its activity through non-selective reactivity. This work provides the first toolbox of cell-active chemical probes for RAB27 which can be used in future studies to shed light on the function of this protein and its potential as a therapeutic target.

biochemistry↗

bioRxiv2022

A DNA damage-induced phosphorylation circuit enhances Mec1ATR-Ddc2ATRIP recruitment to Replication Protein A.

The cell cycle checkpoint kinase Mec1ATR and its integral partner Ddc2ATRIP are vital for the DNA damage and replication stress response. Mec1-Ddc2 senses single-stranded DNA (ssDNA) by being recruited to the ssDNA binding Replication Protein A (RPA) via Ddc2. In this study, we show that a DNA-damage induced phosphorylation circuit modulates checkpoint recruitment and function. We demonstrate that Ddc2-RPA interactions modulate the association between RPA and ssDNA and that Rfa1-phosphorylation aids in the further recruitment of Mec1-Ddc2. We also uncover an underappreciated role for Ddc2 phosphorylation that enhances its recruitment to RPA-ssDNA that is important for the DNA damage checkpoint in yeast. The crystal structure of a phosphorylated Ddc2 peptide in complex with its RPA interaction domain provides molecular details of how checkpoint recruitment is enhanced, which involves Zn2+. Using electron microscopy and structural modelling approaches, we propose that Mec1-Ddc2 complexes can form higher order assemblies with RPA when Ddc2 is phosphorylated. Together, our results provide insight into Mec1 recruitment and suggest that formation of supramolecular complexes of RPA and Mec1-Ddc2, modulated by phosphorylation, would allow for rapid clustering of damage foci to promote checkpoint signalling. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=71 SRC="FIGDIR/small/521831v1_ufig1.gif" ALT="Figure 1"> View larger version (20K): org.highwire.dtl.DTLVardef@139e23corg.highwire.dtl.DTLVardef@133d6e6org.highwire.dtl.DTLVardef@6198b5org.highwire.dtl.DTLVardef@1709a73_HPS_FORMAT_FIGEXP M_FIG C_FIG HighlightsO_LIRfa1-S178 phosphorylation promotes Ddc2 recruitment and Ddc2-RPA complexes modulate RPA-ssDNA behaviour. C_LIO_LIDdc2 phosphorylation enhances Mec1-Ddc2 recruitment and is important for the DNA damage checkpoint in yeast. C_LIO_LIStructure of a Ddc2:RPA complex shows phosphorylation-dependent higher order assemblies stabilised by Zn2+. C_LIO_LIWe propose a Mec1-Ddc2 recruitment strategy that allows fast accumulation of Mec1-Ddc2 through DNA damage-induced phosphorylation and promotes autophosphorylation. C_LI

biochemistry↗

bioRxiv2022

Light activation of Orange Carotenoid Protein reveals initial C8'-C7' double bond trans/cis photoisomerization

Orange Carotenoid protein (OCP) is the only known photoreceptor which uses carotenoid for its activation. It is found exclusively in cyanobacteria, where it functions to control light-harvesting of the photosynthetic machinery. However, the photochemical reactions and structural dynamics of this unique photosensing process are not yet resolved. We present time-resolved crystal structures at second-to-minute delays under bright illumination, capturing the early photoproduct and structures of the subsequent reaction intermediates. The first stable photoproduct shows concerted isomerization of C9-C8 and C7-C6 single bonds in the bicycle-pedal (BP) manner and structural changes in the N-terminal domain with minute timescale kinetics. These are followed by a thermally-driven recovery of the BP isomer to the dark state carotenoid configuration. Structural changes propagate to the C-terminal domain, resulting, at later time, in the H-bond rupture of the carotenoid keto group with protein residues. The isomerization and its transient nature are confirmed in OCP crystals and solution by FTIR and UV/Vis spectroscopy. This study reveals the single bond isomerization of the carotenoid in the BP manner and subsequent thermal structural reactions as the basis of OCP photoreception. Understanding and potentially controlling the OCP dynamics offers the prospect of novel applications in biomass engineering as well as in optogenetics and bioimaging.

biophysics↗