As mentioned previously, Hsp27 belongs to the family of the α-crystallins as they share an α-crystalline domain. These molecular chaperones proteins act to prevent improper polypeptide association24 and are fundamental components of the protein cycling (like chaperoning misfolded proteins to the proteasome). They include the small heat-shock proteins (sHSPs), which are found in all organisms (as will see later on the species variation section of this work), and play an important role in preventing protein misfolding and aggregation. In general the sHSPs are capable of intercepting destabilized targets. αB-crystallin (ABC) is an abundant mammalian sHSP, whose expression is constitutive in most human tissues and up-regulated in a variety of pathological disorders as we will see later on the disease relevance section of this work. The α-crystallin domain contains several β-strands organized into two β-sheets responsible for dimer formation; the basic building block of most sHSPs (Figure 1). The amino-terminal extension modulates oligomerization, subunit dynamics and substrate binding, whereas the flexible carboxy-terminal extension promotes solubility, chaperoning and oligomerization25. The latter is by inter- subunit linkage26. This domain distinguishes sHSPs from other small proteins induced by heat and might have an important role in chaperon activity27. Numerous and in part controversial structural models have been proposed on the basis of known physico-chemical properties of α-crystallins (Figure 1). Microscopic examination of negatively stained samples revealed roughly spherical complexes for many members of the α-crystallin family, e.g., for mouse Hsp25, yeast Hsp26, human Hsp27, pea Hsp18.1 and Hsp17.7, Bradyrhizobium japonicum HspB and HspC, and Methanococcus jannaschii Hsp16.528,29,30 These studies have shown on reported cryo-EM images, nonameric triangles of Mycobacterium tuberculosis Hsp16.3, a structural organization presently unique among sHsps31. Three-dimensional re-construction of cryo-EM images of human β-crystallin, native bovine α-crystallin, human Hsp27, and M. jannaschii Hsp16.5 visualized spherical complexes with an inner cavity32,33.
The overall picture of the latter protein thus obtained closely matches its known crystal structure34. Whereas the ﬁrst 32 residues of M. jannaschii Hsp16.5 are highly disordered in the crystal, the structure from residue 33 onwards, including the entire α-crystallin domain (residues 46 arranged with octahedral symmetry in a spherical complex containing eight triangular and six square openings. The outer and inner diameters of the hollow sphere are around 120 and 65 Å, respectively. Each monomer consists of 10 β-strands and 2 short helices. The core of Hsp16.5 adopts an immunglobulin-like fold consisting of two β-sheets that are packed as parallel layers. 1, 7, 5, and 4 form one β-sheet, and 2, 3, 9, and 8, together with 6 of a neighboring subunit, form the other β–sheet. Each subunit in the complex makes extensive contacts with other subunits via hydrogen bonds as well as hydrophobic and ionic interactions. The short C-terminal extension is oriented toward the outside of the shell and interacts with 4 and 8 of a neighboring subunit. Although the size of the windows on the surface of the sphere would allow the passage principally of small molecules and unfolded polypeptide chains, it is unlikely that client proteins of Hsp16.5 are housed in the central cavity.