Crystal Clear: The Function of Crystallin Proteins

INTRODUCTION POST

CLOUDED VISION: AN INTRODUCTION TO CATARACTS

Eyes – we all have them and obviously use them daily, but do any of us know how they actually work? The pupils are the dark part of our eye which contracts and dilates in order to let light in, depending on the environment we’re in is dark or light. This serves to let the optimal amount of light in as well as protect our retina from damage. The iris is the colored part of our eye, and is actually the muscle which controls the width of the pupil. The light then hits the lens and changes shape, focusing the light onto the retina for conversion into electrical signals for  communication to the optic nerve and brain. The lens is the primary focus of this project, as I am interested in cataracts1.  All these structures are outlined in Figure 1.

As stated, the function of the lens is to change shape so it can focus the light onto the retina. It changes shape in order to accommodate for different distances, refracting the light and producing a sharp image formed on the retina1

The lens capsule surrounds the membrane and is mostly made up of collagen. Then, the cells of the lens epithelium allow for the passage of ions and liquids to enter the lens and keep osmotic homeostasis. Next is the layer of lens fiber cells which are made from the epithelium and virtually have no organelles when mature1

Figure 1. Structure of the eye as well as the cells of the lens.2

Over 90% of the proteins in the lens are characterized by crystallin proteins, as the lens is also referred to as the crystalline lens. These proteins aggregate in high molecular weights, but are soluble in the lens, contributing to the refraction of light and the transparency of the lens3. Three crystallins exist – α-, ꞵ-, and 𝛾-. These proteins were first isolated by a Swedish scientist in 18944. α-crystallins belong to their own family, and are found to be included in the class of molecular chaperones known as small heat shock proteins (sHSPs)5. Additionally, it functions as a structural protein to refract light. ꞵ- and 𝛾-crystallins are purely structural proteins, belonging to the same superfamily and possibly contributing to the water content of the lens6. Differentiated lens cells are unable to produce new crystallins, so the health of these macromolecules is directly correlated to ocular health. 

α-crystallins were found to share the α-crystallin/sHSP domain with the heat shock protein family, classified by mostly ꞵ sheets as well as a variable N terminus. This domain in α-crystallin is found in residues 64-144 in subunit A, and 79-169 in subunit B6. As molecular chaperones, they are responsible for protecting cellular components under stress8. They are involved in correcting misfolded proteins, or sending them completely for degradation. This occurs through the endoplasmic reticulum associated degradation pathway (ERAD). In this pathway, proteins present in the endoplasmic reticulum are transported to the cytosol where chaperones such as α-crystallin aid in their degradation. They are moved into the cytoplasm where they are ubiquitinated and degraded by the proteasome. Ubiquitination is a post-translational modification of proteins. Normally, the ubiquitin is activated with ATP by enzyme E1 and conjugated to the active site of an enzyme E2 which is then ligated to usually the lysine of a target protein by the last enzyme E3. The proteasome then recognizes the ubiquitin chain and degrades the protein at certain residues of the amino acid chain9.

The unfolded protein response is elicited via ER stressors, whereby it stops current translation, upregulates degradation, and causes the transcription of chaperones10.

α-crystallin additionally inhibits apoptosis of the cell, however the A and B subunits utilize different pathways11.

The structure of α-crystallin is made up of several oligomers, forming a micelle-like structure. The hydrophobic residues on the inside marked by mostly β-sheets as shown in Figure 2 are proposed to be sites for substrate binding, a conformational change needing to occur to expose the residues12. Important in α-crystallin as well as sHSPs is the C terminus, necessary for chaperone activity and interaction of the subunits. It has been shown that the subunit exchange of one monomer from the parent oligomer exposes the C terminus, allowing for substrate interaction13

Figure 2. Side view of the internal and external B-sheet arrangement of the 6T1R 16-mer of αA-crystallin14

Shown to be advantageous in chaperone activity is ATP. Binding of ATP to α-crystallin promotes the exposure of hydrophobic residues. This in turn leads to increased substrate refolding by holding the complex together. Additionally, the tighter structure prevents the cleavage of α-crystallin by trypsin, ensuring a longer life of protecting the cell. ATP binds to both αA-crystallin and αB-crystallin15. The binding sites in αB were proposed to be charged clusters Lys82, His83, Lys90, Lys92 and Arg116, His119, Arg120, Lys121, Arg123 which lie within the α-crystallin domain. Due to the homology of the domain, the residues in αA-crystallin correspond to Arg65, Arg68, Lys70, Lys78, His79 and Arg112, His115, Arg116, Arg117, Arg119

In the presence of zinc(II) ions, the chaperone activity of α-crystallin is similarly greatly increased as with ATP. The stability of α-crystallin was also enhanced, with zinc binding sites protecting it from proteolytic cleavage16. The protection of residues 50-119 increases the lifespan of α-crystallin chaperone activity, through binding with histidine residues. H79, H107 and H115 of αA-crystallin and H104, H111 and H119 of aB-crystallin are involved in binding of Zn2+. Some residue structure overlaps with substrate binding, as the 70-88 region of aA-crystallin and the 95-124 region of αB-crystallin were identified to be substrate binding sites by Sharma17

References

  1. Hejtmancik 2008
  2. Anatomy and Structure of the Adult Human Lens https://discovery.lifemapsc.com/library/images/the-anatomy-and-structure-of-the-adult-human-lens
  3. Hejtmancik 2016
  4. Morner 1894
  5. Ingolia 1982
  6. Bloemendal 2004
  7. Panda 2016
  8. Horwitz 1992
  9. Vembar 2008
  10. Hetz 2018
  11. Kesugi 2016
  12. Basha 2006
  13. Augusteyn 2004
  14. Kaiser 2019
  15. Biswas 2004
  16. Karmakar and Das 2012
  17. Sharma 2000

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