Regulation of chaperone function by coupled folding and oligomerization

Abstract

The homotrimeric molecular chaperone Skp of Gram-negative bacteria facilitates the transport of outer membrane proteins across the periplasm. It has been unclear how its activity is modulated during its functional cycle. Here, we report an atomic-resolution characterization of the Escherichia coli Skp monomer-trimer transition. We find that the monomeric state of Skp is intrinsically disordered and that formation of the oligomerization interface initiates folding of the α-helical coiled-coil arms via a unique “stapling” mechanism, resulting in the formation of active trimeric Skp. Native client proteins contact all three Skp subunits simultaneously, and accordingly, their binding shifts the Skp population toward the active trimer. This activation mechanism is shown to be essential for Salmonella fitness in a mouse infection model. The coupled mechanism is a unique example of how an ATP-independent chaperone can modulate its activity as a function of the presence of client proteins.

Redefining Molecular Chaperones as Chaotropes

“…The residues comprising the inner surface of the pockets are pivotal to formation of chaotropic pockets and the chaotropicity of these pockets could be modulated by conformational changes. Mechanisms to regulate the activation of chaperones pockets have been shown for ATP-independent chaperones, such as regulation by different transition mechanisms such as oligomer disassembly, order-to-disorder transition or coupled folding/oligomerization ( Reichmann et al, 2012 ; Haslbeck and Vierling, 2015 ; Suss and Reichmann, 2015 ; Mas et al, 2020 ). Such transitions drastically modulate the surface accessibility to the client proteins, providing a potential mechanism for the regulation of chaotropicity in ATP-independent chaperones.…”

Chaperones Skp and SurA dynamically expand unfolded outer membrane protein X and synergistically disassemble oligomeric aggregates

“…In a first set of experiments, we complexed uOmpX with Skp at 37°C, which binds unfolded OMPs as a trimer 56,57 , denoted thereafter as Skp 3 . As reference, we first performed a measurement in the absence of the chaperone, and again observed a broad distribution at intermediate E values, representing the heterogeneous unfolded conformational ensemble of uOmpX aq , and a minor compact population at E values, representing uOmpX compact (Fig.…”

“…We determined CVs of 0.204 ± 0.036 and 0.304 ± 0.022 for Skp 3 -bound and SurA-bound uOmpX, respectively. Compared to SurA–uOmpX, the CV of the Skp 3 –uOmpX complex is reduced and indicates less conformational heterogeneity for the bound substrate, likely due to the binding of uOmpX in the cavity of Skp 3 56,57 . By contrast, the higher CV for the SurA–uOmpX complex marks an increased heterogeneity.…”

Architecture and assembly dynamics of the essential mitochondrial TIM chaperone systems

“…The ability to rapidly adapt the chaperone concentration likely outweighs the energetic cost related to the need of continuously replenishing the chaperone pool. Furthermore, assembly of chaperones from monomeric subunits may allow the chaperone activation as client proteins emerge (28).…”