Small heat-shock proteins: important players in regulating cellular proteostasis

Abstract

Small heat-shock proteins (sHsps) are a diverse family of intra-cellular molecular chaperone proteins that play a critical role in mitigating and preventing protein aggregation under stress conditions such as elevated temperature, oxidation and infection. In doing so, they assist in the maintenance of protein homeostasis (proteostasis) thereby avoiding the deleterious effects that result from loss of protein function and/or protein aggregation. The chaperone properties of sHsps are therefore employed extensively in many tissues to prevent the development of diseases associated with protein aggregation. Significant progress has been made of late in understanding the structure and chaperone mechanism of sHsps. In this review, we discuss some of these advances, with a focus on mammalian sHsp hetero-oligomerisation, the mechanism by which sHsps act as molecular chaperones to prevent both amorphous and fibrillar protein aggregation, and the role of post-translational modifications in sHsp chaperone function, particularly in the context of disease.

Fig. 1

a Schematic arrangement of the various structural regions in human αBc. The β-sheet-rich ACD is flanked by the relatively unstructured N- and C-terminal regions. The latter contains the highly conserved ‘IXI’ sequence and the unstructured and flexible C-terminal extension encompassing the last 12 amino acids. b Crystal structure of the Methanococcus jannaschii Hsp16.5 24-mer oligomer showing its large central cavity [20]. Each colour represents an individual subunit within the oligomer. Re-printed with permission from [20]. c X-ray crystal structure of the ACD and C-terminal region of αBc without its flexible C-terminal extension [12]. The αBc dimer is shown in which the six β-strands of each ACD are arranged in an immunoglobulin-like fold. The intra-dimer contacts arise between β-strands 6 and 7 of each subunit, arranged in an anti-parallel (AP) manner. The inter-dimer interaction is also shown in which an αBc peptide from the C-terminal region of one subunit encompassing a palindromic nine amino acid sequence (residues 156–164), including the ‘IXI’ sequence (IPI; residues 159–161), interacts with the fourth and eighth β-strands of the adjacent αBc subunit. Adapted and used with permission from [12]

Fig. 2

The chaperone mechanism of sHsps. Multiple partially folded protein intermediate states populate the folding/unfolding pathway of a protein. The mechanism by which sHsps, such as αBc, prevent target protein aggregation (either amorphous or fibrillar) is dictated by the conformational stability and exposed hydrophobicity of the precursor protein intermediates. High affinity interactions occur with highly destabilised intermediates (which exceed the threshold of binding) and these are sequestered into stable high-molecular-mass complexes. Target proteins in these complexes can be re-folded through the action of other ATP-dependent chaperones or shuttled for degradation or via chaperone-mediated autophagy. Alternatively, weak, transient interactions occur with more stable protein intermediates, which redirect them back to the folding pathway so as to facilitate their re-folding. sHsps can also interact with pre-fibrillar and fibrillar aggregates formed by target proteins. By binding to these species, sHsps stabilise them, preventing their further elongation and fibril fragmentation and secondary nucleation events, which can be the main source of toxic oligomeric species formed during amyloid fibrillar aggregation (adapted from [100])

Fig. 3

The effect of phosphorylation on sHsps in cells. Under basal conditions in the cell sHsps act to maintain proteostasis by buffering against protein aggregation of target proteins. Conditions of cellular stress (e.g. a change in pH, oxidative stress or increase in temperature) are sensed by the cell, triggering a signalling pathway that results in the activation of protein kinases, including mitogen activated protein (MAP) kinases which can phosphorylate sHsps, such as αBc and Hsp27. Phosphorylation of sHsps decreases their oligomeric size and causes some phosphorylated sHsps to be translocated to the nucleus. In doing so, phosphorylation boosts the chaperone activity of sHsps against target proteins in danger of aggregating and precipitating as a result of the cellular stress. Prolonged exposure to stress leads to the activation of the transcription factor heat-shock factor 1 (HSF1), which self-associates into trimers and then relocates to the nucleus, stimulating transcription of heat-shock response genes, including the sHsps Hsp27, αBc and Hsp22. Newly translated sHsps oligomerise with existing sHsps leading to the formation of a heterogeneous pool of sHsp oligomers (including phosphorylated and non-phosphorylated forms), which maximises their possible binding interactions with intra-cellular target proteins to prevent their aggregation. The Venn diagram in the centre represents the group of aggregation-prone target proteins with which the non-phosphorylated (blue), phosphorylated (red) or heterogeneous (purple) sHsp oligomers interact

Fig. 4

Homology model of the human αBc monomer showing approximate locations of naturally occurring, disease-causing mutations. Adapted and re-used with permission from [264]. Copyright (2014) American Chemical Society