Biology subjects

Hume, A. N.

Publications and source records attributed to Hume, A. N..

4 recordsLinked to original sources

bioRxiv2026

An ancient polymorphism in myosin I a/b determines the left-right asymmetry of Japanese snails

Left-right (LR) asymmetry is a fundamental feature of animal development, yet progress in understanding its establishment has been limited by the fact that almost all animals exhibit invariant chirality. Snails are the exception because some wild-living species show abundant chiral variation, yet the existing knowledge is from snails in which the chirality mutation is rare and causes pathology. Here, we show that a chiral polymorphism in Japanese Euhadra snails is due to functional variation in an unconventional myosin I a/b, an isoform not previously implicated in LR specification in any animal. Both the sinistral and dextral gene variants are ancient, impart minimal transcriptional differences in the single-cell embryo and are not pathological. Phylogenetic and structural modelling suggests that dominant-acting amino acid substitutions in the myosin actin-binding domain and/or motor-level junction were enabled by relaxed selection. These results broaden the known molecular repertoire underlying LR asymmetry, suggest key mutations and positions that should be explored in other model animals, and highlight snails as a powerful model for understanding the origins of animal LR asymmetry.

evolutionary biology↗

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↗

bioRxiv2020

How to navigate the myosin-V motor through the actin network

Myosin-V (MyoV) is a ubiquitous motor protein that transports an astonishingly diverse set of cargos on the actin network in eukaryotes. Phosphorylation-dependent processes often regulate MyoV-mediated cargo transport, molecular details of which remain largely unknown. We previously showed that phosphorylation regulates MyoVs switching from microtubules onto actin filaments, not its motor activity. Regulation of switching at reconstituted microtubule-actin-crossings in fact sufficed to recapitulate the MyoVa-driven redistribution of pigment-organelles in amphibian melanophores. However, in those cells, MyoVa also encounter many actin-actin crossings. Here, we show that isolated MyoVa motors switch with equal probabilities at reconstituted actin-actin-crossings. Under the control of its adaptor-protein melanophilin (Mlph), however, the motor differentiates between the actin filaments at crossing points in a phosphorylation-regulated manner. Whereas phosphorylation of Mlph forced about [~]2/3 of MyoVa to ignore the intersections, dephosphorylation completely reversed this behavior and forced [~]2/3 to switch. We show that the filament-binding domain (FBD) of Mlph controls this switching behavior. This property evolved in amphibians, but not in the early vertebrate zebrafish. By protein engineering, we demonstrate that changes of a few residues are sufficient to impart actin-binding capability onto the zebrafish Mlph. We thus unmask the molecular beginnings of dual filament binding in Mlph that allow it to control the switching behavior of MyoVa at cytoskeletal crossings. We therefore propose a direct link between intracellular phosphorylation activity and the adaptor-protein, not to regulate MyoVa activity, but to navigate the motor through the entire cytoskeletal maze for correct positioning of cargo. Significance statementIn virtually all eukaryotic cells, numerous myosin motors have to navigate through an elaborate actin network for timely transport of intracellular cargo. Here, we unmask an unintuitive regulation of the myosin-Va motor that is involved in pigment organelle transport. We demonstrate that myosin-Va differentiates between the same actin filaments and displays regulated switching at reconstituted actin-actin crossings, an unexpected behavior that has been predicted from previous theoretical work. We trace this regulation back to the adaptor protein of the myosin-Va motor and show that this regulation was present in amphibian but had not evolved in the early vertebrate zebrafish. Notably, we demonstrate that the evolution of actin-binding capability is achieved by changing a few residues in the adaptor protein.

biochemistry↗

bioRxiv2018

Rab27a co-ordinates actin-dependent transport by controlling organelle-associated motors and track assembly proteins.

Cell biologists generally consider that microtubules and actin play complementary roles in long- and short-distance transport in animal cells. On the contrary, using melanosomes of melanocytes as a model, we recently discovered that motor myosin-Va, works with dynamic actin tracks, to drive long-range organelle dispersion in microtubule depleted cells. This suggests that in animals, as in yeast and plants, myosin/actin can drive long-range transport. Here we show that SPIRE1/2 and formin-1 (FMN1) proteins generate actin tracks required for myosin-Va-dependent transport in melanocytes. Moreover we show that, in addition to melanophilin/myosin-Va, Rab27a can recruit SPIRE1/2 to melanosomes, thereby integrating motor and track assembly activity at the organelle membrane. Based on this we suggest a model in which organelles and force generators (motors and track assemblers) are linked forming a cell-wide network that allows their collective activity to rapidly disperse the population of organelles long-distance throughout the cytoplasm.

cell biology↗