Alpha A and Alpha B Crystallin are proteins that encoded by the genes CRYAA and CRYAB. Alpha A and B are a major protein component of the mammalian eye lens. Alpha A crystallin is mainly found on the lens of the eye with trace amounts in other tissues while Alpha B is essentially considered a ubiquitous protein. [1]
Function
This protein contributes to the transparency and refractive index of the eye lens. Alpha- Crystallin can act as chaperones preventing the aggregation of various proteins. Even though they act as molecular chaperones, they don't renature proteins and release them like true chaperones do; instead, they hold proteins in large soluble aggregates. [2]
Disease
Cataracts 9, multiple types
This disease is caused by mutations affecting the eye lens. One of those mutations is usually R116C where an arginine is mutated to a cysteine. R116C is generally linked to one form of autosomal congenital cataracts. Congenital cataracts refers to the opacification of the eye lens that occurs at birth while infantile cataracts refers to the opacification of eye lens that developed during the first year. [3] An opacification of the crystalline lens of the eye occurs, in most instances, may lead to impairment or blindness. Opacities vary in morphology it might be static or progressive. [4]
Myopathy, Miofibrillar
A group of chronic neuromuscular disorder characterized by the disintegration of the sarcomere Z disc and myofibrils. Myopathy is characterized by weakness of proximal and distal limbs, weakness of neck, hypertrophy cardiomyopathy, and cataracts in a subset of patients. [5]
Myopathy, myofibrillar, fatal infantile hypertonic, alpha-B crystallin
This is another form of myopathy myofibril, but it happens in infants. It is a chronic neuromuscular disorder characterized by disintegration of sarcomere Z disc and myofibrils. MFMFIH-CRYAB has onset on the first weeks of life after the neonatal period. Affected infants show rapid muscular rigidity of trunks and limbs which is associated with increasing respiratory difficulties resulting in death before the age of 3. [6]
Relevance
Alpha Crystallin A 1-172 is found at nearly 2 folds higher in diabetic lenses than an aged matched control lens. In humans, the alpha A gene is found in chromosome 21 and encodes for 173 amino acid residues while the alpha B gene is found in chromosome 11 and encodes for 175 amino acids.[1] In mammalian lens, the molar ratio between alpha A and alpha B is a 3 to 1 ratio. [7]
An increased level of Alpha Crystallin B chain has been associated with various diseases such Alexander's disease, Alzheimer's, and Parkinson's. [8]
Structural highlights
This structure highlight shows where is located in the A chain. A mutation of this amino acid to cysteine will lead to cataracts. [1]
The following structure shows where is located. The mutation of this amino acid in the B chain of crystalline gene decreased interaction with wild-type CRYAA and CRYAB. However, it increases interactions with CRBB2 and CRYGC leading to cytoplasmic aggregation. [9] Also, when Arg is mutated to Gly, it causes desmin- related myopathy, cardiomyopathy, and cataracts. Also, this mutation reduces chaperone protection activity and in some target proteins promoted aggregation. [1]
This highlight shows the location of the involved in inter-subunit bridging of Zn ions which enhances stability.[10] This is crucial as there is no protein turnover in the lens.
The following structure shows with all the chains that make it up.
