Molecular catch bonds are ubiquitous in biology and essential for processes like leucocyte extravasion¹ and cellular mechanosensing². Unlike normal (slip) bonds, catch bonds strengthen under tension. The current paradigm is that this feature provides ‘strength on demand³’, thus enabling cells to increase rigidity under stress^1,4–6. However, catch bonds are often weaker than slip bonds because they have cryptic binding sites that are usually buried^7,8. Here we show that catch bonds render reconstituted cytoskeletal actin networks stronger than slip bonds, even though the individual bonds are weaker. Simulations show that slip bonds remain trapped in stress-free areas, whereas weak binding allows catch bonds to mitigate crack initiation by moving to high-tension areas. This ‘dissociation on demand’ explains how cells combine mechanical strength with the adaptability required for shape change, and is relevant to diseases where catch bonding is compromised^7,9, including focal segmental glomerulosclerosis¹⁰ caused by the α-actinin-4 mutant studied here. We surmise that catch bonds are the key to create life-like materials.

The collapse of polypeptides is thought important to protein folding, aggregation, intrinsic disorder, and phase separation. However, whether polypeptide collapse is modulated in cells to control protein states is unclear. Here, using integrated protein manipulation and imaging, we show that the chaperonin GroEL-ES can accelerate the folding of proteins by strengthening their collapse. GroEL induces contractile forces in substrate chains, which draws them into the cavity and triggers a general compaction and discrete folding transitions, even for slow-folding proteins. This collapse enhancement is strongest in the nucleotide-bound states of GroEL and is aided by GroES binding to the cavity rim and by the amphiphilic C-terminal tails at the cavity bottom. Collapse modulation is distinct from other proposed GroEL-ES folding acceleration mechanisms, including steric confinement and misfold unfolding. Given the prevalence of collapse throughout the proteome, we conjecture that collapse modulation is more generally relevant within the protein quality control machinery.

Many proteins form dynamic complexes with DNA, RNA, and other proteins, which often involves protein conformational changes that are key to function. Yet, methods to probe these critical dynamics are scarce. Here we combine optical tweezers with fluorescence imaging to simultaneously monitor the conformation of individual proteins and their binding to partner proteins. Central is a protein–DNA coupling strategy, which uses exonuclease digestion and partial re-synthesis to generate DNA overhangs of different lengths, and ligation to oligo-labeled proteins. It provides up to 40 times higher coupling yields than existing protocols and enables new fluorescence-tweezers assays, which require particularly long and strong DNA handles. We demonstrate the approach by detecting the emission of a tethered fluorescent protein and of a molecular chaperone (trigger factor) complexed with its client. We conjecture that our strategy will be an important tool to study conformational dynamics within larger biomolecular complexes.

The ability to reverse protein aggregation is vital to cells^1,2. Hsp100 disaggregases such as ClpB and Hsp104 are proposed to catalyse this reaction by translocating polypeptide loops through their central pore^3,4. This model of disaggregation is appealing, as it could explain how polypeptides entangled within aggregates can be extracted and subsequently refolded with the assistance of Hsp70^4,5. However, the model is also controversial, as the necessary motor activity has not been identified^6–8 and recent findings indicate non-processive mechanisms such as entropic pulling or Brownian ratcheting^9,10. How loop formation would be accomplished is also obscure. Indeed, cryo-electron microscopy studies consistently show single polypeptide strands in the Hsp100 pore^11,12. Here, by following individual ClpB–substrate complexes in real time, we unambiguously demonstrate processive translocation of looped polypeptides. We integrate optical tweezers with fluorescent-particle tracking to show that ClpB translocates both arms of the loop simultaneously and switches to single-arm translocation when encountering obstacles. ClpB is notably powerful and rapid; it exerts forces of more than 50 pN at speeds of more than 500 residues per second in bursts of up to 28 residues. Remarkably, substrates refold while exiting the pore, analogous to co-translational folding. Our findings have implications for protein-processing phenomena including ubiquitin-mediated remodelling by Cdc48 (or its mammalian orthologue p97)¹³ and degradation by the 26S proteasome¹⁴.