Alpha-crystallin domain found in alphaA-crystallin (HspB4), alphaB-crystallin (HspB5), and the small heat shock protein (sHsp) HspB6, also known as Hsp20. sHsps are molecular chaperones that suppress protein aggregation and protect against cell stress, and are generally active as large oligomers consisting of multiple subunits. Alpha crystallin, an abundant protein in the mammalian lens, is a large (700 kDa) heteropolymer composed of HspB4 and HspB5, generally in a molar ratio of HspB4:HspB5 of 3:1. Only trace amounts of HspB4 are found in tissues other than the lens. HspB5 on the other hand is also expressed constitutively in other tissues including brain, heart, and type I and type IIa skeletal muscle fibers, and in several cancers including gliomas, renal cell carcinomas, basal-like and metaplastic breast carcinomas, and head and neck cancer. HspB5's functions include effects on the apoptotic pathway and on metastasis. Phosphorylation of HspB5 reduces its oligomerization and anti-apoptotic activities. HspB5 is protective in demyelinating disease such as multiple sclerosis (MS), being a negative regulator of inflammation. In early active MS lesions it is the most abundant gene transcript and an autoantigen, the immune response against it would disrupt its function and worsen inflammation and demyelination. Given as therapy for ongoing demyelinating disease it may counteract this effect. It is an autoantigen in the pathogenesis of various other inflammatory disorders including Lens-associated uveitis (LAU), and Behcet's disease. Mutations in HspB5 have been associated with diseases including dominant cataract and desmin-related myopathy. Mutations in HspB4 have been associated with Autosomal Dominant Congenital Cataract (ADCC). HspB6 (Hsp20) is ubiquitous and is involved in diverse functions including regulation of glucose transport and contraction of smooth muscle, in platelet aggregation, in cardioprotection, and in the prevention of apoptosis. It interacts with the universal scaffolding and adaptor protein 14-3-3, and also with the proapoptotic protein Bax.
Jiyao W et al.(2023). "The conserved domain database in 2023.",