8 julio, 2024

Structural proteins: functions, examples and characteristics

Structural proteins are important proteins present in all eukaryotic cells, that is, they are found in both animal and plant cells. These are part of highly diverse biological structures such as skin, hair, spider webs, silk, connective tissue, plant cell walls, etc.

Although the term «structural protein» is commonly used to refer to proteins such as collagen, keratin, and elastin, there are also important intracellular structural proteins that contribute to the maintenance of the internal structure of cells.

These proteins, belonging to the cytoskeleton, also control the subcellular location of the organelles and provide the transport and communication machinery between them.

Some structural proteins have been studied in great detail and have provided a deeper understanding of the overall protein structure. Examples of these are silk fibroin, collagen and others.

From the study of silk fibroin, for example, the secondary protein structure of β-folded sheets was described and, from the first studies carried out with collagen, the triple helix secondary structure was deduced.

Therefore, structural proteins are essential both within individual cells and in the tissues they comprise.

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functions

The functions of structural proteins are quite diverse, and depend, above all, on the type of protein in question. However, it could be said that its main function is to maintain the structural integrity of cells and, in a broader sense, the structure of the body.

As far as body structural proteins are concerned, keratin, for example, has functions in protection and coverage, in defense, in movement, among others.

The epidermis of the skin of mammals and other animals has a large number of filaments made up of keratin. This layer has functions in protecting the body against different types of stressful or harmful factors.

The spines and spikes, as well as the horns and beaks, claws and nails, which are keratinized tissues, perform functions in both protection and defense of the body.

Industrially, the wool and hair of many animals are exploited for the manufacture of garments and other types of clothing, so they have an additional importance, anthropocentrically speaking.

Cellular structural proteins

From the cellular point of view, structural proteins have transcendental functions, since they make up the internal framework that gives each cell its characteristic shape: the cytoskeleton.

As part of the cytoskeleton, structural proteins such as actin, tubulin, myosin, and others also participate in transport and internal communication functions, as well as in cell mobility events (in cells capable of moving).

The existence of cilia and flagella, for example, depend to a large extent on structural proteins that make up the thick and thin filaments, made up of actin and tubulin.

Examples of structural proteins and their characteristics

Since there is a great diversity of structural proteins, only examples of the most important and abundant among eukaryotic organisms will be given below.

Bacteria and other prokaryotes, along with viruses, also have important structural proteins in their cell bodies, however most attention is focused on eukaryotic cells.

-Actin

Actin is a protein that forms filaments (actin filaments) known as microfilaments. These microfilaments are very important in the cytoskeleton of all eukaryotic cells.

Actin filaments are two-chain helical polymers. These flexible structures are 5 to 9 nm in diameter and are organized as linear bundles, two-dimensional lattices, or three-dimensional gels.

Actin is distributed throughout the cell, however, it is particularly concentrated in a layer or cortex attached to the inner face of the plasma membrane, since it is a fundamental part of the cytoskeleton.

-Collagen

Collagen is a protein present in animals and is particularly abundant in mammals, which have at least 20 different genes that code for the various forms of this protein that can be found in their tissues.

It is found primarily in bone, tendon, and skin, where it constitutes more than 20% of the total protein mass of mammals (greater than any other protein).

In the connective tissues where it is found, collagen forms an important part of the fibrous portion of the extracellular matrix (which is also composed of ground substance), where it forms elastic fibers that withstand large tensile forces.

Structure of collagen fibers

Collagen fibers are composed of uniform subunits of tropocollagen molecules, which are 280 nm long and 1.5 nm in diameter. Each tropocollagen molecule is made up of three polypeptide chains known as alpha chains, which associate with each other as a triple helix.

Each of the alpha chains has around 1000 amino acid residues, where glycine, proline, hydroxyproline and hydroxylysine are very abundant (which is also true for other structural proteins such as keratin).

Depending on the type of collagen fiber considered, they are found in different places and have different properties and functions. Some are specific to bone and dentin, while others are part of cartilage and so on.

-Keratin

Keratin is the major structural protein of keratinocytes, one of the most abundant cell types in the epidermis. It is an insoluble fibrous protein that is also found in the cells and integuments of many animals.

After collagen, keratin is the second most abundant protein in the mammalian body. In addition to being a substantial part of the outermost layer of the skin, this is the main structural protein of hair and wool, nails, claws and hooves, feathers and horns.

In nature there are different types of keratins (in analogy to the different types of collagen), which have different functions. Alpha and beta keratins are the best known. The former form nails, horns, spikes, and epidermis of mammals, while the latter are abundant in beaks, scales, and feathers of reptiles and birds.

-Elastin

Elastin, another protein of animal origin, is a key component of the extracellular matrix and has important roles in the elasticity and resilience of many tissues in vertebrate animals.

These tissues include arteries, lungs, ligaments and tendons, skin, and elastic cartilage.

Elastin comprises more than 80% of the elastic fibers present in the extracellular matrix and is surrounded by microfibrils composed of various macromolecules. The structure of the matrices composed of these fibers varies between different tissues.

In the arteries, these elastic fibers are arranged in concentric rings around the arterial lumen; in the lungs, elastin fibers form a thin network throughout the organ, concentrating in areas such as the openings of the alveoli.

In tendons, elastin fibers are oriented parallel to the tissue organization and, in elastic cartilage, they are arranged in a three-dimensional honeycomb-like configuration.

-Extensins

Plant cell walls are mainly composed of cellulose, however, some of the proteins that are associated with this structure also have functional and structural relevance.

Extensins are one of the best known wall proteins and are characterized by the Ser-(Hyp)4 pentapeptide repeat sequence. They are rich in basic residues such as lysine, which contributes to their interaction with the other components in the cell wall.

Its function has to do with the hardening or strengthening of the walls. As with other structural proteins in animals, in plants there are different types of extensins, which are expressed by different types of cells (not all cells produce extensins).

In soybean, for example, extensins are produced by sclerenchyma cells, while in tobacco plants it has been shown that lateral roots have two layers of cells that express these proteins.

-Leaf

Cellular organelles also have their own structural proteins, which are responsible for maintaining their shape, their motility, and many other inherent physiological and metabolic processes.

The inner region of the nuclear membrane is associated with a structure known as the nuclear lamina, and both have a very special protein composition. Among the proteins that make up the nuclear lamina are proteins called lamins.

The sheets belong to the group of type V intermediate filaments and there are several types, the best known being A and B. These proteins can interact with each other or with other internal elements of the nucleus such as matrix proteins, the chromatin and the inner nuclear membrane.

References

Alberts, B., Dennis, B., Hopkin, K., Johnson, A., Lewis, J., Raff, M., … Walter, P. (2004). Essential Cell Biology. Abingdon: Garland Science, Taylor & Francis Group.
Gartner, L., & Hiatt, J. (2002). Text Atlas of Histology (2nd ed.). Mexico DF: McGraw-Hill Interamericana Publishers.
Gruenbaum, Y., Wilson, KL, Harel, A., Goldberg, M., & Cohen, M. (2000). Review: Nuclear Lamins—Structural Proteins with Fundamental Functions. Journal of Structural Biology, 129, 313–323.
Keller, B. (1993). Structural Cell Wall Proteins. Plant Physiology, 101, 1127–1130.
Mithieux, BSM, & Weiss, AS (2006). Elastin. Advances in Protein Chemistry, 70, 437–461.
Sun, T., Shih, C., & Green, H. (1979). Keratin cytoskeletons in epithelial cells of internal organs. Proc. Natl. Acad. Sci., 76(6), 2813–2817.
Wang, B., Yang, W., McKittrick, J., & Meyers, MA (2016). Keratin: Structure, mechanical properties, occurrence in biological organisms, and efforts at bioinspiration. Progress in Materials Science.

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