Conditional disorder in chaperone action

Abstract

Protein disorder remains an intrinsically fuzzy concept. Its role in protein function is difficult to conceptualize and its experimental study is challenging. Although a wide variety of roles for protein disorder have been proposed, establishing that disorder is functionally important, particularly in vivo, is not a trivial task. Several molecular chaperones have now been identified as conditionally disordered proteins; fully folded and chaperone-inactive under non-stress conditions, they adopt a partially disordered conformation upon exposure to distinct stress conditions. This disorder appears to be vital for their ability to bind multiple aggregation-sensitive client proteins and to protect cells against the stressors. The study of these conditionally disordered chaperones should prove useful in understanding the functional role for protein disorder in molecular recognition.

Figure 1

Proteins have wide ranges of flexibility that span a continuous spectrum including those that are globally intrinsically disordered (shown as an ensemble of structures generated by the program flexible-meccano) [77] to proteins that show well-folded, stable structures (shown in orange). Many proteins are conditionally disordered and can transition between order and disorder depending on the presence or absence of binding partners or other conditions (shown by the double headed arrow). Proteins can also show portions that are well folded and other portions that are disordered. This figure was generated by Loic Salmon.

Figure 2

Molecular recognition mechanisms in conditionally disordered proteins. Two models, ‘conformational selection’ and ‘folding upon binding’, are currently being discussed to explain the observation that intrinsically disordered proteins regain structure in the presence of their binding partners. The conformational selection theory proposes that a small proportion of the intrinsically disordered protein population (shown in blue and purple) is in an appropriate configuration to interact with specific binding partners (shown in red). This interaction then shifts the equilibrium towards the binding-competent conformation. The folding upon binding model proposes that the intrinsically disordered region first binds to the partner and then subsequently refolds. As indicated by the central arrow, components of both mechanisms might contribute to the coupled folding/binding events of conditionally disordered proteins [25].

Figure 3

Mechanism of action of the flexible protein HdeA. At neutral pH (pH 7, left panel), HdeA exists as an inactive dimer. When exposed to acid (pH 2, center panel), HdeA, like many other proteins, unfolds and becomes more flexible. Although this unfolding process inactivates most other proteins, it triggers the activation of HdeA. The flexibility of HdeA allows it to mold itself to fit other proteins that have become aggregation-prone by acid-induced unfolding, thereby preventing their aggregation. HdeA slowly releases proteins after shifting to neutral pH (pH 7, right panel), keeping the concentration of aggregation-sensitive folding intermediates low.

Figure 4

Mechanism of Hsp33, a redox-regulated chaperone. Under non-stress conditions, Hsp33 is a chaperone-inactive, zinc-binding monomer (upper left). Upon exposure to oxidative stress conditions that cause protein unfolding (e.g., bleach stress, oxidative heat stress), Hsp33 undergoes massive conformational rearrangements, triggered by the formation of two intramolecular disulfide bonds and zinc release (orange ball). This leads to unfolding of the C-terminal redox switch domain (cyan and green), and the activation of Hsp33 as chaperone (dimerization of Hsp33 has been omitted for simplicity). The linker region (green) of Hsp33 appears to directly interact with secondary structure elements of newly unfolded client proteins (red), adopting a more stabilized conformation in this process (lower right). Return to reducing, non-stress conditions causes refolding of the Hsp33 zinc-binding domain (cyan), which appears to trigger conformational changes in the client proteins (orange), consistent with increased client unfolding (upper right). Client proteins are then released to cellular foldases (e.g., the DnaK-system), which are known to bind unstructured folding intermediates, and support the refolding of client proteins to their native state.