A first line of stress defense: small heat shock proteins and their function in protein homeostasis

Abstract

Small heat shock proteins (sHsps) are virtually ubiquitous molecular chaperones that can prevent the irreversible aggregation of denaturing proteins. sHsps complex with a variety of non-native proteins in an ATP-independent manner and, in the context of the stress response, form a first line of defense against protein aggregation in order to maintain protein homeostasis. In vertebrates, they act to maintain the clarity of the eye lens, and in humans, sHsp mutations are linked to myopathies and neuropathies. Although found in all domains of life, sHsps are quite diverse and have evolved independently in metazoans, plants and fungi. sHsp monomers range in size from approximately 12 to 42kDa and are defined by a conserved β-sandwich α-crystallin domain, flanked by variable N- and C-terminal sequences. Most sHsps form large oligomeric ensembles with a broad distribution of different, sphere- or barrel-like oligomers, with the size and structure of the oligomers dictated by features of the N- and C-termini. The activity of sHsps is regulated by mechanisms that change the equilibrium distribution in tertiary features and/or quaternary structure of the sHsp ensembles. Cooperation and/or co-assembly between different sHsps in the same cellular compartment add an underexplored level of complexity to sHsp structure and function.

Keywords: molecular chaperones; protein aggregation; protein folding; stress response; α-crystallin.

Figure 1

(A) Domain organization of sHsps. N-terminal sequence (NTS; dark green, dashed line), α-crystallin domain (ACD; light green), C-terminal sequence (CTS; dark green with the conserved I-X-I motive in cylinder form, and remainder as a dotted line). As indicated, up to three phosphorylation sites exist in the NTS of some sHsps as discussed in the text. (B) Structure of a β6-swapped dimer of the ACD of M. jannaschii Hsp16.5 (X-ray crystallography, PDB: 1SHS; [45]). The ACDs of individual protomers are colored green and gray. (C) Structure of a β7-interface dimer of the ACD of human αB-crystallin (solid state NMR, PDB: 2KLR; [49]). (D) To scale comparison of the three available oligomeric crystal structures of sHsps. One dimeric building block is marked in green-cyan to highlight the variable interconnections of the dimers in the respective structures. MjHsp16.5; M. jannashii Hsp16.5 representing a 24mer [45]. The orange-red highlighted dimeric building block additionally highlights the equatorial protein axis forming an octahedron. SpHsp16; Schizosaccharomyces pombe Hsp16 representing a 16mer ellipsoid composed of two half-spheres of four dimers [46]. TaHsp16.9; Tritium aestivum (wheat) Hsp16.9 representing a 12mer of two stacked rings [40].

Figure 2

Model for the chaperone function of sHsps. Under stress conditions when substrate proteins are destabilized and begin to unfold, sHsps bind these partially unfolded substrates in an energy-independent manner and keep them in a folding-competent state. The physiologic ensemble of sHsp oligomers (grey) are activated (green) by a shift to a higher content of smaller species (often dimers). The substrate is stabilized by this activated ensemble of sHsps (green) and may reactivate spontaneously or is captured in stable sHsp/substrate complexes (of still enigmatic organization). Bound substrates are subsequently refolded by the ATP-dependent Hsp70 chaperone system (composed of Hsp70, Hsp40 and a nucleotide exchange factor; NEF) and may involve the Hsp100/ClpB chaperone system in cells and cellular compartments where it is found.

Figure 3

Comparison of types of interactions seen for different, cytosolic sHsp systems from bacteria to higher eukaryotes. The illustrated oligomers and sHsp-substrate complexes symbolize ensembles of oligomers as in Fig. 2. In a number of bacteria (e.g. Synechocystis sp. 6803) there is only a single sHsp (Hsp16.6) that is essential for heat tolerance and acts according to the mechanism described in Fig. 2 [96]. In other bacteria, such as E. coli, there are two (IbpA and IbpB) or more sHsps that form hetero-oligomers and function cooperatively [73, 128]. In yet other bacteria, like D. radiodurans, there are sHsps that work in parallel, independently of each other [33, 128]. In lower eukaryotes like baker`s yeast there are also two, oligomeric sHsps that act independently in parallel. In higher plants there are multiple sHsps classes, and each can have multiple members. Commonly members of the classes are oligomeric and from hetero-oligomers only within members of the same class [36]. The individual classes act in parallel, following in principle the general model (Fig. 2) with the exception that the Hsp100 chaperone system is not found in eukaryotes outside of plants, yeasts and parasitic protozoans. Variations in the spectrum of sHsps in other eukaryotes determine the extent to which sHsp coassembly occurs and the number of independent, parallel sHsp pathways that may be present.