Publication highlights

Researchers at the Crick and Imperial College report a method to characterise and track sugar-coated cell sensors called proteoglycans using click chemistry. Through a 'bump and hole' engineering technique, they modified a hole in an enzyme and a bump in a sugar, to alter an enzyme that glues the two together so it accepts a bumped version of the sugar. This modified sugar contains a chemical tag which means it can be traced using click chemistry, such as attaching a fluorescent molecule to 'see' the molecule by imaging, or a molecule acting like an anchor to isolate and further study it. In the future, these molecules could be tagged and tracked in different contexts, or proteoglycan function could be altered by replacing the sugar chain with a different biological or synthetic molecule.

Chemical reactions are initiated by an energy source that can be provided by heat, electric current or light, and are generally governed by the rules of thermodynamics. Mechanical forces are an alternative means of activating chemical reactions, often steering reaction pathways that result in products different from those obtained under thermodynamic control. In this work, researchers from the Crick demonstrated that mechanical forces activate chemical reactions that are chemically forbidden, such as the reduction of an individual protein disulfide bond by an inorganic sulfur-oxyanion. Occurring within the core of a protein with a physiological mechanical role, the force-unlocked reactivity has a direct impact on protein elasticity.

The Arp2/3 complex initiates the growth of new actin filaments from the side of pre-existing filaments to generate branched actin networks that are essential for many different cellular processes. However, it can also nucleate single linear actin filaments when activated by WISH/DIP/SPIN90 family proteins. Unexpectedly, researchers at the Crick together with collaborators at Birkbeck, found Arp2/3 can nucleate bidirectional linear actin filaments when activated by SPIN90. By determining the structure of SPIN90 bound to actin filaments, they uncovered the mechanism by which this bidirectional nucleation occurs. Their analysis demonstrates that single filament nucleation by Arp2/3 is mechanistically more like branch formation than previously appreciated.

The DNA replication checkpoint is essential for maintaining genome stability. Without it, when DNA copying restarts after a stall, too many replication origins—the starting points for copying—are mistakenly activated, ultimately leading to cell death. Researchers at the Crick showed, in human cells lacking this checkpoint, that excessive DNA synthesis from surplus origins consumes the vital replication proteins PCNA and RFC, preventing normal restart of stalled copying at replication forks. Without the protection of PCNA and RFC, the ends of the forks are attacked by a protein called HLTF, causing irreversible damage. Removing HLTF helps cells survive even in the absence of the checkpoint, which has implications for how resistance to anti-checkpoint cancer therapies may arise.

Researchers at the Crick and the University of Pittsburgh have used x-ray crystallography and cryo-electron microscopy to determine the structures of betaglycan - a co-receptor involved in cell signalling - in complex with the TGF-β protein and its signalling receptors. They found that both domains in betaglycan are involved in ligand binding, demonstrated how this occurs, and revealed that their arrangement also allows for signalling receptor recruitment. The results provide a structural explanation for how betaglycan functions to capture the ligand and hand it over to the receptors in a sequential manner, to selectively enhance TGF-β signalling.

Researchers in the Signalling and Structural Biology Lab have described a near-complete multisite phosphorylation reaction cycle for the aPKC-Par6 kinase and Lgl substrate. This mechanism explains how a trapped Lgl phospho-intermediate antagonises aPKC-Par6 until it encounters Cdc42-GTP, in an assembly required for cell polarity maintenance.

Fignl1 is an essential gene in mice and mutations have been found in various cancers and genetic disorders. FIGNL1 plays critical roles in maintaining genome stability via modulating RAD51 recombinase, a key protein in repairing DNA and maintaining genome integrity. In collaboration with Marin Jasin in Sloan Memorial Kettering Cancer Center, researchers at the Crick reveal that FIGNL1 prevents RAD51 chromatin accumulation under normal conditions as well as under DNA damaging conditions, and this is responsible for its essential activity in cell viability. CryoEM structure and comprehensive in vitro and in vivo studies reveal a unique mechanistic model for RAD51 dissociation where FIGNL1, a molecular motor, encloses and pulls RAD51 into its structure, leading to RAD51 remodeling and dissociation from chromatin. This unique mode suggests that FIGNL1 can dissociate RAD51 from many bound substrates, including DNA and nucleosomes and account for its vitality in cellular activities.