Alpha-crystallin B chain is a

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

that in humans is encoded by the ''CRYAB''

gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

. It is part of the small heat shock protein family and functions as molecular chaperone that primarily binds misfolded proteins to prevent protein aggregation, as well as inhibit apoptosis and contribute to intracellular architecture. Post-translational modifications decrease the ability to chaperone. Mutations in ''CRYAB'' cause different cardiomyopathies, skeletal myopathies mainly myofibrillar myopathy, and also cataracts. In addition, defects in this gene/protein have been associated with cancer and neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease.

Structure

Crystallin In anatomy, a crystallin is a water-soluble structural protein found in the lens and the cornea of the eye accounting for the transparency of the structure. It has also been identified in other places such as the heart, and in aggressive breast c ...

s are separated into two classes: taxon-specific, or enzyme, and ubiquitous. The latter class constitutes the major proteins of vertebrate eye lens and maintains the transparency and refractive index of the lens. Since lens central fiber cells lose their nuclei during development, these crystallins are made and then retained throughout life, making them extremely stable proteins. Mammalian lens crystallins are divided into alpha, beta, and gamma families; beta and gamma crystallins are also considered as a superfamily. Alpha and beta families are further divided into acidic and basic groups. Seven protein regions exist in crystallins: four homologous motifs, a connecting peptide, and N- and C-terminal extensions. Alpha crystallins are composed of two gene products: alpha-A and alpha-B, for acidic and basic, respectively. These heterogeneous aggregates consist of 30–40 subunits; the alpha-A and alpha-B subunits have a 3:1 ratio, respectively.

Function

Alpha B chain crystallins (αBC) can be induced by heat shock, ischemia, and oxidation, and are members of the small

heat shock protein Heat shock proteins (HSP) are a family of proteins produced by cells in response to exposure to stressful conditions. They were first described in relation to heat shock, but are now known to also be expressed during other stresses including expo ...

(sHSP also known as the HSP20) family. They act as molecular chaperones although they do not renature proteins and release them in the fashion of a true chaperone; instead, they bind improperly folded proteins to prevent protein aggregation. Furthermore, αBC may confer stress resistance to cells by inhibiting the processing of the pro-apoptotic protein caspase-3. Two additional functions of alpha crystallins are an autokinase activity and participation in the intracellular architecture. Alpha-A and alpha-B gene products are differentially expressed; alpha-A is preferentially restricted to the lens and alpha-B is expressed widely in many tissues and organs. Elevated expression of alpha-B crystallin occurs in many neurological diseases; a missense mutation cosegregated in a family with a desmin-related myopathy.

Clinical significance

Although not yet clearly understood, defective chaperone activity is expected to trigger the accumulation of protein aggregates and underlie the development of α-crystallinopathy, or the failure of protein quality control, resulting in protein deposition diseases such as Alzheimer’s disease and Parkinson’s disease. Mutations in CRYAB could also cause restrictive cardiomyopathy. ER-anchored αBC can suppress aggregate formation mediated by the disease mutant. Thus, modulation of the micromilieu surrounding the ER membrane can serve as a potential target in developing pharmacological interventions for protein deposition disease. Though expressed highly in eye lens and muscle tissues, αBC can also be found in several types of cancer, among which head and neck squamous cell carcinoma (HNSCC) and breast carcinomas, as well as in patients with tuberous sclerosis. αBC expression is associated with metastasis formation in HNSCC and in breast carcinomas and in other types of cancer, expression is often correlated with poor prognosis as well. The expression of αBC can be increased during various stresses, like heat shock, osmotic stress or exposure to heavy metals, which then may lead to prolonged survival of cells under these conditions.

Interactions

CRYAB has been shown to interact with: * CRYAA, * CRYBB2, *

CRYGC Crystallin, gamma C, also known as CRYGC, is a protein which in humans is encoded by the ''CRYGC'' gene. Function Crystallins are separated into two classes: taxon-specific, or enzyme, and ubiquitous. The latter class constitutes the major prot ...

, *

HSPB2 Heat shock protein beta-2 is a protein that in humans is encoded by the ''HSPB2'' gene. Interactions HSPB2 has been shown to interact with: * CRYAB, * HSPB8, * Myotonic dystrophy protein kinase and * TRAF6 TRAF6 is a TRAF human protein. ...

, * Hsp27, and * PSMA3.

References

External links