Review
Conditional disorder in chaperone action

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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.

Section snippets

Protein disorder: a fuzzy concept

Our view of proteins has been strongly shaped by the many beautiful structures that have been solved by X-ray crystallography. Looking at these structures, it is easy to forget that proteins often have very dynamic properties and regions that show considerable flexibility. Indeed, only 25% of crystal structures represent >95% of the complete molecule; all others have missing electron density for more than 5% of their sequence, usually because these regions take on multiple conformations [1].

Is protein disorder the default?

In viewing well-ordered crystal structures of proteins, it is easy to forget that disorder is the mathematical default for peptide sequences. Lau and Dill calculated that a maximum of only approximately 1 in 1010 random sequences are expected to fold into a defined structure [19]. The observation that the majority of proteins show at least some regions of ordered structure indicates that order is strongly selected for in evolution. This finding has led to the ‘form dictates function’ axiom,

Conditionally disordered proteins

In our view, the concept of conditional disorder provides an important way to reconcile the increasing evidence for the various roles that disorder plays in biological systems with the difficulty in understanding how a completely disordered protein can carry out a specific biological function. Conditionally disordered proteins can exist in a least two states, one that shows a high degree of flexibility and a second state where the protein is more ordered (Figure 2). The ability to study

Molecular chaperones: prototypes of proteins with multiple binding partners

Proteins that engage in multiple mutually exclusive transient interactions have on average a higher degree of disorder than non-hub proteins 34, 35. This also applies to molecular chaperones, which act to bind to a large variety of different protein folding intermediates to prevent their non-specific protein aggregation and facilitate protein folding both in vitro and in vivo. Their predicted degree of disorder ranges from 24% to 100% 36, 37. Folding chaperones (i.e., foldases), such as Hsp70 (

ATP-independent chaperones: activation by stress-induced unfolding

Chaperones that need to work independently of ATP, either because they work in ATP-free cellular compartments (e.g., periplasm) or they function specifically under ATP-depleted stress conditions (e.g., oxidative stress) [48], require alternative means to control substrate binding and release. Recent work has revealed several ATP-independent chaperones that use large-scale order-to-disorder transitions to trigger activation and client binding and employ disorder-to-order transitions to control

The role of intrinsic disorder in small heat shock proteins

Yeast Hsp26 is another example of an ATP-independent chaperone that utilizes specific stress conditions to trigger the conformational changes needed to activate its chaperone function. Hsp26, a member of the small heat shock protein (sHsp)/α-crystallin family, is chaperone-inactive at 25°C but readily interacts with client proteins upon incubation at heat shock temperatures [60]. Biophysical studies revealed that Hsp26 uses the folding status of a unique thermosensing middle domain, whose heat

Globally intrinsically disordered chaperones and molecular shields

Three other proteins with anti-aggregation activity (casein [65], late embryogenesis abundant (LEA) dehydration proteins [66], and α-synuclein [67]) appear to be globally disordered in vitro. Both casein and LEA proteins have been proposed to function as small ‘cageless’ chaperones, which act via transient hydrophobic interactions to shield aggregation-prone surfaces and increase refolding rates [68]. The LEA proteins are small highly hydrophilic proteins that appear to use physical

Concluding remarks

Establishing specific roles for disorder in protein function is experimentally very challenging. However, demonstrating that the disordered regions of a protein can assume multiple distinct conformations upon binding to different partner proteins would provide convincing evidence of an important functional role for disorder. The recent discovery of conditionally disordered molecular chaperones now provide us with excellent test cases, as these folding helper proteins can be studied in both

Acknowledgments

We are indebted to Drs Dana Reichmann, Linda Foit, and Loic Salmon for helping us preparing some of the figures of this manuscript, and for many useful comments on the manuscript. This work was supported by a National Institutes of Health RO1 GM065318 award (to U.J.). J.C.A.B. is a Howard Hughes Medical Institute Investigator.

Glossary

BSA
bovine serum albumin, a commonly used negative control protein in chaperone assays due to its very weak chaperone activity.
Chaperone
a protein that assists the non-covalent folding of proteins and the assembly or disassembly of other macromolecular structures. Its most common characteristic is its ability to dramatically reduce non-specific protein aggregation, a side reaction of protein folding and unfolding processes.
Conditionally disordered proteins
proteins that can exist in at least two

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      To our knowledge, our study represents the most comprehensive characterization of an ACD monomer to-date and outlines potentially general protocols and methods that can be used to investigate monomeric ACDs from other sHSPs. The conditional disorder that is encoded within ACD dimers adds to the growing significance of the role of intrinsic disorder within sHSPs and functional disorder within molecular chaperones [56,57]. In particular, conditional disorder and structural plasticity seem widespread within the family of ATP-independent chaperones [58].

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