An early step in intracellular transport is the selective recognition of
a vesicle by its appropriate target membrane, a process regulated by Rab GTPases
via the recruitment of tethering effectors1–4. Membrane tethering
confers higher selectivity and efficiency to membrane fusion than the pairing of
SNAREs alone5,6,7. Here, we
addressed the mechanism whereby a tethered vesicle comes closer towards its
target membrane for fusion by reconstituting an endosomal asymmetric tethering
machinery consisting of the dimeric coiled-coil protein EEA16,7
recruited to phosphatidylinositol 3-phosphate membranes and binding vesicles
harboring Rab5. Surprisingly, structural analysis revealed that Rab5:GTP induces
an allosteric conformational change in EEA1, from extended to flexible and
collapsed. Through dynamic analysis by optical tweezers we confirmed that EEA1
captures a vesicle at a distance corresponding to its extended conformation, and
directly measured its flexibility and the forces induced during the tethering
reaction. Expression of engineered EEA1 variants defective in the conformational
change induced prominent clusters of tethered vesicles in vivo.
Our results suggest a new mechanism in which Rab5 induces a change in
flexibility of EEA1, generating an entropic collapse force that
pulls the captured vesicle toward the target membrane to initiate docking and
fusion.
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.
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