Small heat shock proteins: Simplicity meets complexity

Abstract

Small heat shock proteins (sHsps) are a ubiquitous and ancient family of ATP-independent molecular chaperones. A key characteristic of sHsps is that they exist in ensembles of iso-energetic oligomeric species differing in size. This property arises from a unique mode of assembly involving several parts of the subunits in a flexible manner. Current evidence suggests that smaller oligomers are more active chaperones. Thus, a shift in the equilibrium of the sHsp ensemble allows regulating the chaperone activity. Different mechanisms have been identified that reversibly change the oligomer equilibrium. The promiscuous interaction with non-native proteins generates complexes that can form aggregate-like structures from which native proteins are restored by ATP-dependent chaperones such as Hsp70 family members. In recent years, this basic paradigm has been expanded, and new roles and new cofactors, as well as variations in structure and regulation of sHsps, have emerged.

Keywords: cell stress; crystallin; heat shock protein (HSP); molecular chaperone; non-native protein; oligomer dynamics; protein aggregation; protein folding; α-crystallin.

Figure 1.

A, domain organization of sHsps. NTR (sienna), ACD (gray), and CTR (green) with the conserved IX(I/V) motif are shown. B, immunoglobulin-like β-sandwich structure of the ACD. Note that ACDs of mammalian and higher eukaryotic sHsps (with the exception of plants) lack a distinct β6 strand but contain an extended β7 strand (referred to as β6 + 7 strand) (left). In ACDs of bacterial sHsps, for example, there is a distinct β6 strand located in the loop connecting β5 and β7 strands (right). C, structures of β7-interface ACD dimers (left) and β6-swapped ACD dimers (right) are shown. The ACDs of individual protomers are colored blue and gray. In human αB-crystallin (HSPB5) (left) (PDB code 2KLR) (62), the dimer interface is formed by the interaction of the extended β6 + 7 strands from neighboring protomers. The β7-interface is marked by a dashed line. In Hsp17.7 from D. radiodurans (right) (PDB code 4FEI) (54), the β6 strand exchanges between partner chains at the dimer interface. D, groove along the β7-interface (ACD groove) in bovine αA-crystallin (PDB code 3L1F) (58) with the bound 2-methyl-3,4-pentanediol molecules (yellow).

Figure 2.

Inter- and intramolecular interactions of sHsp N- and C-terminal regions. A, intermolecular binding of the C-terminal IX(I/V) motif of a protomer (shown in blue with the C-terminal region shown in green) into the hydrophobic groove formed by the β4 and β8 strands of another protomer (gray) of the adjacent dimer in human αB-crystallin (HSPB5) (PDB code 2KLR) (62) mediating dimer association. The IX(I/V) motif Ile-159–Pro-160–Ile-161 is shown as sticks (orange). B, in rat Hsp20 (PDB code 2WJ5) (57), the groove along the β7-interface (marked by a dashed line) is filled with C-terminal extensions (green, ball-and-stick representation) from other chains in the lattice. C, in human HspB6 (PDB code 5LTW) (30), the N-terminal IX(I/V) motif (Val-5–Pro-6–Val-7) (orange) of a protomer (only the residues Met-1–Arg-13 are shown, sienna) binds into the β4/β8 pocket of a partner chain (gray). D, binding of the hydrophobic N-terminal sequence ³⁵FNNIV³⁹ (sienna) of one protomer (gray) of an ACD dimer into the shared groove in C. elegans Sip1 (PDB code 4YDZ) (60). The β7-interface is marked by a dashed line. E, in S. solfataricus Hsp14.1 (A102D mutant) (PDB code 4YLB) (71), the NTR helices of adjacent dimers mediate dimer–dimer association through extensive hydrophobic interactions.

Figure 3.

Model for the chaperone function of sHsps. sHsps populate at equilibrium a wide variety of inter-converting oligomers (I). Under stress conditions, substrate proteins are destabilized and begin to unfold (II), and the sHsp ensemble becomes activated by remodeling of the ensemble composition in favor of smaller species (often dimers) with exposed substrate-binding sites (III). Activated sHsps bind early unfolding intermediates of substrate proteins in an energy-independent manner (IV) and stabilize them in sHsp/substrate complexes of different forms (V). Bound substrates may reactivate spontaneously or are subsequently refolded by the ATP-dependent Hsp70 chaperone system composed of Hsp70, Hsp40, and a nucleotide exchange factor (NEF)) (VI). The effective disassembling of insoluble substrate aggregates with incorporated sHsps and substrate refolding requires the concerted action of the Hsp70–Hsp100/ClpB bi-chaperone system.