Alpha crystallin

From Proteopedia

Function

Alpha crystallin is known to be one of the primary structural proteins of the eye's lens, specifically crystallin types alpha (α), beta (β), and gamma (γ). While all types have essential roles in the eye, alpha-crystallin and its subunits make up 40% of the lens's protein composition ^[1]. These crystallin proteins create stability in the lens and have the ability to impact an individual's vision because they provide lens transparency. In order to retain their eye's transparency in the absence of protein turnover, or the replacing and reproduction of proteins within a cell as the proteins become broken down, the protein must retain some form of longevity. Its longevity is generally assumed to be correlated with the long-term retention of its . Its ability to maintain the structure for a prolonged period of time is related to the N-terminal residue as this terminal is correlated with protein life. Even though the N-terminal does impact protein longevity, it does not determine a protein's half-life completely. In alpha-crystallin, the N-terminus contains approximately 60 residues, some of which include isoleucine and alanine ^[2]. This means that the half-life of an alpha-crystallin protein, based on the N-terminal residues' contents, may be about 20 hours ^[3]. Its native structure can remain intact due to the efficient capture and refolding process of non-native proteins by the chaperones and the thermodynamic stability that the proteins have. Alpha-crystallins also provide vision clarity by regulating a proper refractive index within the eye. The lens contains a very high concentration of lens crystallins, primarily alpha-crystallin, and they remain densely packed within the space. This allows the lens to maintain a consistent refractive index over various distances, regardless of the light wavelength's location. They also have a particular arrangement of epithelial cells and fiber cells that lack organelles in their nucleases to reduce the blockage of light transmission to the eye ^[4]. Along with the contents and structure of the lens itself, the alpha-crystallin maintains the eye's clarity by acting as chaperone-like proteins in order to prevent aberrant, or standard deviating, protein interactions. The chaperone capabilities of these proteins also allow the prevention of non-native proteins becoming insoluble under stressful conditions like chemical reductions, oxidations, or elevation of temperatures. The combined abilities of the chaperone-like alpha-crystallin means that they can aid in the prevention of the formation of large, dense, light scattering masses. For example, one of the effects of old age is cataract formation, and alpha-crystallin is the known culprit for this development. As alpha-crystallin is depleted within the eye, its defensive efforts go with it, which allows these masses to take form. The alpha-crystallin within the eye is the primary protectant against the age-induced deterioration of the lens. It allows the effects of aging to be reduced for significant periods of time. Alpha-crystallin is also known to inhibit apoptosis from occurring, enhance the resistance of stress on cells, and protect the cytoskeleton. Clearly, this protein has many vital functions in the body, but primarily in the eye. Having a high alpha-crystallin index, or the amount of alpha-crystallin present in the space is crucial for proper eye health and maintenance.

Structural highlights

Alpha crystallin is made up of two subunits called alpha-A (αA-crystallin) and alpha-B (αB-crystallin). These subunits are present in a ratio of 3:1 alpha-A crystallin to alpha-B crystallin throughout the primary alpha-crystallin protein. This ratio of 3:1 is most optimal in terms of thermal stability for the alpha-crystallin. So much so that the alpha-crystallin is more thermally stable than either alpha-A or alpha-B crystallin alone. This optimum ratio also helps to explain the stoichiometric ratios of alpha-A and alpha-B crystallin subunits in a mammalian lens ^[5]. Even though both subunits are the most significant contributors to the alpha-crystallin protein, individually, both possess variable expression patterns that are unique to the tissue type. Some of these variations include different phenotypes, phosphorylation patterns, structural properties, and chaperone activities. Through extensive research, it has become clear that crystallins and their respective subunits have many regulatory and metabolic functions in different locations in the body, which gives them different expressions. For instance, subunit alpha-B crystallin can also be found in other tissues like the heart, skeletal muscles, skin, and brain. However, alpha-crystallin and both of its correlated subunits can be found in the eye, which will be the primary focus of this paper. Alpha-crystallin’s secondary structure is mostly beta-sheet arrangements. This means that rather than a helical shape, the amino acids are arranged in flat structures. The beta-sheet supramolecular structure that is held within the crystallin of the eye helps to create inherent stability. It also aids in the chaperone ability of alpha-crystallin as it is crucial for preventing fibril formation. However, over time, these very stable proteins that create the beta-sheet supramolecular structure undergo substantial modifications that come with aging processes. Due to this, the structures are led to destabilization within the lens. This leads to a denatured set of proteins that will aggregate and precipitate to form large light scattering masses. This will eventually lead to the loss of lens clarity which is a consequence of the natural aging process. In this time frame, the formation of cataracts may ensue ^[6]. Both subunits are polydisperse, oligomeric proteins that are made of flexible monomers with large surface areas. This means that they are very low molecular weight polymers made of a small number of repeat units ranging in various sizes. Based on their surrounding environment, their oligomeric properties will vary. Due to their oligomeric properties, each of these subunits is able to associate and form about 40 chain-long, large complexes in the shape of a sphere called oligomeric complexes. These complexes form when the monomers dimerize, causing the amino region and carboxyl regions of the alpha-crystallin to interact with residues from other corresponding regions of their neighboring subunits ^[7]. These spheres are so large and oppositely charged that they repel and distribute themselves across the lens cells. When combined to create alpha-crystallin, the proteins exist as globular aggregates whose quaternary structure is believed to behave as a protein micelle. However, high-resolution data about the quaternary and tertiary structure of alpha-crystallin is unavailable, which is likely due to the polydisperse nature of alpha-crystallin ^[8]. After 50 years of extensive study, the three-dimensional structure remains unknown because the protein is too large for NMR measurements and has yet to be obtained for X-ray studies. In humans, both of these crystalline forms are coded for on different chromosomes. While they may be coded differently, they possess about 55% sequence homology between themselves. This means that they are structurally similar due to the evolutionary pathway and ancestral history that they both underwent. Despite their similar makeups, they do have unique features about them. For example, alpha-B crystallin is stress-inducible while alpha-A crystallin is not. This implies that alpha-B crystallin has a distinct role in the eye's lens. Alpha-A crystallin is made of 173 amino acids arranged in . The molecular mass of the alpha-A subunit is 19.8 kDa, and the homooligomer weighs 660 kDa. On the other hand, Alpha-B crystallin has 165 amino acids arranged in seven beta-sheets, has a molecular mass of 20 kDa, and its homooligomer weight is 620 kDa. Together, the alpha-crystallin protein has , all of which are in the same position as the four metal-binding sites. They also contain . Within the crystallin is two tryptophan residues, Trp9 and Trp60, both of which can be found in the alpha-B crystallin subunit ^[9].

Evolutionary Development

Alpha crystallin is a product of evolutionary development as the protein was derived from a family of heat shock proteins (HSP) called small heat shock proteins (sHSP). Heat shock proteins are specialized proteins that are made when a cell is exposed to high temperatures, and it is present in all plants and animals. In a series of gene duplication and divergences, the initially small family of heat shock proteins was able to adapt and gain a new novel function. The component alpha-crystallin was also formed by gene recruitment which is the co-option of a particular gene for a different function as a result of mutation. This form of recruitment took place in the previously existing heat shock proteins. It allowed the newly formed alpha-crystallin to take on the characteristics and functions of the pre-existing proteins. For example, the small heat shock proteins were able to cradle other cells when enduring intense stress in order to prevent them from distorting their shape, which is a current function that the alpha-crystallin acts on in the eye's lens ^[10]. This is known to be true from a series of crystallin genes found in the specific family of small heat shock proteins. This set of crystallin genes also included regulatory elements that allow gene expression to be found in stressed cells and lenses. After several mutation events to the regulatory region, the production of heat-shock genes, a precursor to crystallin, was able to develop on the surface of the eye. After years of evolution, it transformed into the crystallin components and subunits we know today. Uniquely, the family of heat shock proteins from which the alpha-crystallin was derived and the alpha-crystallin in the eye today contain a core called the alpha-crystallin domain (ADC). This core can be found in the monomers of today's crystallin proteins and gives us conclusive evidence of the protein's evolutionary advancements. This center, or core, is approximately 90 amino acids long and has ends with variable hydrophobic N-terminal domains as well as variable hydrophilic C-terminal extensions. The alpha-crystallin domain is key to the formation of dimers which is the primary building block of oligomers. The N-terminal domain of the alpha-crystallin is not necessary for the protein to function as a chaperone, nor does it contribute to dimerization. It does, however, appear to be required for the formation of high-order aggregates ^[11]. The C-terminal extensions consist of about 25 residues that are crucial to alpha-crystallin as they are the primary contributors in its ability to act as a chaperone-like protein. The C-terminal also contains the IXI/V motif, which includes two isoleucine residues that are separated with an intervening residue. The IXI motif is able to promote chaperone action because it stimulates intersubunit interactions and provides chaperone site accessibility. Alpha crystallin also prevents denatured proteins from solidifying, which helps to retain the eye's transparency and prevent the formation of cataracts. It is also known to increase the cellular tolerance to stress. These features are very similar to the small heat shock proteins that range from 15 to 30 kDa, particularly in their C-terminal halves ^[12].

Reference

1. ↑ Andley UP. Crystallins in the eye: Function and pathology. Prog Retin Eye Res. 2007 Jan;26(1):78-98. doi: 10.1016/j.preteyeres.2006.10.003. PMID:17166758 doi:http://dx.doi.org/10.1016/j.preteyeres.2006.10.003
2. ↑ Bloemendal H, de Jong W, Jaenicke R, Lubsen NH, Slingsby C, Tardieu A. Ageing and vision: structure, stability and function of lens crystallins. Prog Biophys Mol Biol. 2004 Nov;86(3):407-85. doi:, 10.1016/j.pbiomolbio.2003.11.012. PMID:15302206 doi:http://dx.doi.org/10.1016/j.pbiomolbio.2003.11.012
3. ↑ Bachmair A, Finley D, Varshavsky A. In vivo half-life of a protein is a function of its amino-terminal residue. Science. 1986 Oct 10;234(4773):179-86. doi: 10.1126/science.3018930. PMID:3018930 doi:http://dx.doi.org/10.1126/science.3018930
4. ↑ Bloemendal H, de Jong W, Jaenicke R, Lubsen NH, Slingsby C, Tardieu A. Ageing and vision: structure, stability and function of lens crystallins. Prog Biophys Mol Biol. 2004 Nov;86(3):407-85. doi:, 10.1016/j.pbiomolbio.2003.11.012. PMID:15302206 doi:http://dx.doi.org/10.1016/j.pbiomolbio.2003.11.012
5. ↑ Horwitz J, Bova MP, Ding LL, Haley DA, Stewart PL. Lens alpha-crystallin: function and structure. Eye (Lond). 1999 Jun;13 ( Pt 3b):403-8. doi: 10.1038/eye.1999.114. PMID:10627817 doi:http://dx.doi.org/10.1038/eye.1999.114
6. ↑ Meehan S, Berry Y, Luisi B, Dobson CM, Carver JA, MacPhee CE. Amyloid fibril formation by lens crystallin proteins and its implications for cataract formation. J Biol Chem. 2004 Jan 30;279(5):3413-9. doi: 10.1074/jbc.M308203200. Epub 2003, Nov 13. PMID:14615485 doi:http://dx.doi.org/10.1074/jbc.M308203200
7. ↑ Selivanova OM, Galzitskaya OV. Structural and Functional Peculiarities of alpha-Crystallin. Biology (Basel). 2020 Apr 23;9(4). pii: biology9040085. doi:, 10.3390/biology9040085. PMID:32340218 doi:http://dx.doi.org/10.3390/biology9040085
8. ↑ Horwitz J. Alpha crystallin: the quest for a homogeneous quaternary structure. Exp Eye Res. 2009 Feb;88(2):190-4. doi: 10.1016/j.exer.2008.07.007. Epub 2008 Jul, 25. PMID:18703051 doi:http://dx.doi.org/10.1016/j.exer.2008.07.007
9. ↑ Liang JJ, Sun TX, Akhtar NJ. Spectral contribution of the individual tryptophan of alphaB-crystallin: a study by site-directed mutagenesis. Protein Sci. 1999 Dec;8(12):2761-4. doi: 10.1110/ps.8.12.2761. PMID:10631993 doi:http://dx.doi.org/10.1110/ps.8.12.2761
10. ↑ de Jong WW, Hendriks W, Mulders JW, Bloemendal H. Evolution of eye lens crystallins: the stress connection. Trends Biochem Sci. 1989 Sep;14(9):365-8. doi: 10.1016/0968-0004(89)90009-1. PMID:2688200 doi:http://dx.doi.org/10.1016/0968-0004(89)90009-1
11. ↑ doi: https://dx.doi.org/10.1016/S0021-9258(19)84762-2
12. ↑ Hejtmancik JF, Riazuddin SA, McGreal R, Liu W, Cvekl A, Shiels A. Lens Biology and Biochemistry. Prog Mol Biol Transl Sci. 2015;134:169-201. doi: 10.1016/bs.pmbts.2015.04.007., Epub 2015 Jun 4. PMID:26310155 doi:http://dx.doi.org/10.1016/bs.pmbts.2015.04.007