Membrane Structural Proteins and Cytoskeletal Proteins
The water-insoluble fraction of lens proteins can be divided into 2 fractions based on solubility in an 8 molar (M) solution of urea:
The urea-soluble fraction of the young lens contains cytoskeletal proteins that provide the structural framework of the lens cells.
The urea-insoluble fraction of the young lens contains the plasma membranes of the lens fiber cells.
In the urea-soluble fraction of the lens, the microfilaments and microtubules found are similar to those found in other cell types. However, the lens contains 2 types of intermediate filaments that are unusual: 1 class is made from the protein vimentin, which is not usually found in epithelial cells; the other class, the beaded filaments, is composed of the proteins phakinin and filensin, which are specific to the lens. Genetic disruption of the structure of the beaded filaments leads to disruption of the structure of the lens fiber cells and ultimately to the formation of a cataract.
In the urea-insoluble fraction of the lens, several proteins are associated with the fiber-cell plasma membranes. One of these makes up nearly 50% of the membrane proteins and is known as the major intrinsic protein (MIP; also known as aquaporin 0), a member of a class of proteins called aquaporins. Other members of the aquaporin family are found throughout the body, where they serve predominantly as water channels. MIP first appears in the lens just as the fibers begin to elongate. With increasing age, this protein, which has a molecular mass of 28 kDa, undergoes proteolytic cleavage, forming a protein fragment with a molecular mass of 22 kDa. When an individual reaches 20–30 years of age, the relative proportions of these 2 proteins become about equal. Over time, the protein fragment that started with the molecular mass of 22 kDa predominates in the lens nucleus.
Hejtmancik JF, Piatigorsky J. Lens proteins and their molecular biology. In: Albert D, Miller J, Azar D, Blodi B, eds. Albert & Jakobiec’s Principles and Practice of Ophthalmology. 3rd ed. Saunders; 2008: vol 1, chapter 105.
Excerpted from BCSC 2020-2021 series: Section 11 - Lens and Cataract. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.