Structure, properties, and functions of the human small heat‐shock protein HSP22 (HspB8, H11, E2IG1): A critical review

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Abstract
The recently described human HSP22 belongs to the superfamily of small heat‐shock proteins containing a conservative α‐crystallin domain. HSP22 seems to be involved in regulation of cell proliferation, cardiac hypertrophy, apoptosis, and carcinogenesis, and expression of point mutants of HSP22 correlates with development of different neuromuscular diseases. Therefore, an investigation of the structure and properties of HSP22 is desirable for understanding its multiple functions. HSP22 seems to belong to the group of so‐called intrinsically disordered proteins and possesses a highly flexible structure. HSP22 tends to form small‐molecular‐mass oligomers and interacts with biological membranes and many different proteins, among them glycolytic enzymes and different protein kinases. HSP22 possesses chaperonelike activity and prevents aggregation of denatured proteins both in vitro and in vivo. Depending on the cell type and its expression, HSP22 might have either pro‐ or anti‐apoptotic effects. Chaperonelike activity seems to be important for antiapoptotic effects, whereas interaction with and regulation of certain protein kinases might be important for the proapoptotic effects of HSP22. Expression of K141N or K141E mutants of HSP22 correlates with development of distal hereditary motor neuropathy and/or Charcot‐Marie‐Tooth disease. These mutations destabilize the structure of HSP22, affect its interaction with other small heat‐shock proteins, and decrease its chaperonelike activity. HSP22 decreases or prevents aggregation of Huntingtin fragments and amyloid‐β peptide 1–40 of the Dutch type. Thus, HSP22 seems to play an important role in the nervous system, and further investigations are needed to understand the molecular mechanisms of its functioning.