Integrins: Characteristics, Structure And Functions

The integrins are a large group or family of proteins, apparently unique to the animal kingdom cell surface. They are the main resource of cells to maintain interaction (in the form of adhesion) with other cells and with the cell matrix.

Its structure is made up of two subunits called alpha and beta. In mammals it is known that there are between 16-18 alpha units and 3-8 betas, which will act depending on their combination, and also on the physiological state of the cell or specific tissue.

Drawing of the molecular structure of the ITGB3 protein (beta 3 integrin). Taken and edited from: Emw [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)].

There are several proteins that have adhesive functions. However, the group of integrins is the one that is most distributed and interacts with all the key proteins of the cell matrix. Integrins participate in phagocytosis, cell migration, and wound healing, and are even highly studied for their participation in metastasis.

Article index

  • one

    Characteristics

  • two

    Structure

    • 2.1

      Alpha subunit

    • 2.2

      Beta subunit

  • 3

    Features

    • 3.1

      Attachment or coupling of the cell to the extracellular matrix

    • 3.2

      Signal transduction from the extracellular matrix to the cell

  • 4

    Integrins and cancer

  • 5

    Evolutionary perspective

  • 6

    References

Characteristics

They are proteins that are characterized by mechanically joining the cellular cytoskeleton of one cell to another and / or to the extracellular matrix (in a cell-cell and / or cell-matrix interaction). Biochemically they detect whether or not adhesion has taken place, and transduce cellular signals linking the extracellular environment with the intracellular one, in both directions.

They work or function with other receptors such as immunoglobillins, cadherin, selectins, and syndecands. Regarding the ligands of the integrins, these are constituted by fibronectin, fibrinogen, collagen and vitronectin, among others.

The binding of these to their ligands is due to extracellular divalent cations such as calcium or magnesium. The use of one or the other will depend on the specific integrin.

Integrins have an elongated shape ending in a globe-shaped head, which according to electron microscopy observations, projects more than 20 nanometers from the lipid bilayer.

Structure

Integrins are proteins that allow communication between cells.

Source: Berkshire Community College Bioscience Image Library [CC0]

Integrins are heterodimers, that is, they are molecules always made up of two proteins. Both proteins are considered subunits or protomers and are differentiated as alpha subunit and beta subunit. Both subunits are non-covalently linked. They have a molecular mass of between 90 to 160 kDa.

The number of alpha and beta subunits varies between different groups of organisms in the animal kingdom. In insects such as the fruit fly ( Drosophyla ), for example, there are 5 alpha and 2 beta subunits, while in nematode worms of the genus Caenorhabditis there are 2 alphas and one beta.

In mammals, the researchers suggest that there is a fixed number of subunits and their combinations; however, there is no consensus in the literature regarding this number. For example, some mention that there are 18 alpha subunits, 8 beta and 24 combinations, while others speak of 16 alpha and 8 beta for 22 combinations.

Each subunit has the following structure.

Alpha subunit

The alpha subunit has a structure with a β-helix domain of seven sheets or sheets that form the head, a domain in the thigh, two domains of the calf, a single transmembrane domain and also a short cytoplasmic tail that does not present enzymatic activity or binding to actin.

It presents chains with about 1000 to 1200 residues. It can bind divalent cations.

In mammals, which is where the integrins have been studied the most, the alpha subunits can be grouped according to whether or not they contain an inserted domain (alpha I).

With inserted domain Alpha I

The alpha I inserted domain consists of a 200 amino acid region. The presence of this domain in the integrins indicates that they are receptors for collagen and leukocytes.

No domain inserted

The alpha integrins that do not have the integrated domain are classified into 4 subfamilies, which we will see below.

PS1

Glycoprotein receptors, also called laminins, are vital for integrating muscle, kidney, and skin tissues.

PS2

This subfamily is the receptor for arginylglycylaspartic acid, also known as RGD or Arg-Gly-Asp.

PS3

This subfamily has been observed in invertebrates, particularly insects. Although little is known about it, there are studies evaluating its essential role in the functional activity of the CD11d leukocyte integrin gene in humans.

PS4

This subfamily is known as the alpha 4 / alpha 9 group and comprises the subunits with those same names.

Said subunits are capable of pairing with the beta 1 and beta 7 subunits. They also share ligands very similar to the alpha subunits that present the inserted alpha I domain, such as vascular cell adhesion molecules, blood soluble ligands, fibrinogen and others. including even pathogens.

Beta subunit

Structurally, the beta subunit consists of a head, a section called the stem / leg, a transmembrane domain, and a cytoplasmic tail. The head is composed of a beta I domain, which inserts into a hybrid domain that binds to the plexin-semaphore-integrin domain, also known as PSI.

The stem / leg section contains four modules equal to or very similar to the cysteine-rich integrin epidermal growth factor and, as already mentioned, a cytoplasmic tail. This cytoplasmic tail, as in the alpha subunit, has no enzymatic or actin-binding activity.

They have chains with a number of residues ranging between 760 and 790, and can bind, like the alpha subunits, bivalent cations.

Integrin signaling in epithelial cells. Taken and edited from K.murphy at English Wikipedia [Public Domain Domain].

Features

Integrins have multiple functions, however for which they are mainly known are those that we will see below.

Attachment or coupling of the cell to the extracellular matrix

The connection that exists between the cell and the extracellular matrix thanks to the integrins favors the resistance of the cell to mechanical pressure, preventing them from being torn from the matrix.

Several studies suggest that coupling to the cell matrix is ​​a basic requirement for the development of multicellular eukaryotic organisms.

Cell migration is a process in which integrins intervene by binding or coupling to different substrates. Thanks to this they intervene in the immune response and wound healing.

Signal transduction from the extracellular matrix to the cell

Integrins participate in the signal transduction process. This means that they intervene in the reception of information from the extracellular fluid, they encode it and then the alteration of intracellular molecules begins, as a response.

This signal transduction is involved in a large number of physiological processes such as programmed cell destruction, cell differentiation, meiosis and mitosis (cell division), and cell growth, among others.

Integrins and cancer

Several studies show that integrins play an important role in tumor development, especially in metastasis and angiogenesis. An example of this are the integrins αVβ3 and α1β1, among some others.

These integrins have been related to cancer growth, increased therapeutic resistance, and hematopoietic neoplasms.

Evolutionary perspective

An efficient adhesion between cells to form tissues was, without a doubt, a crucial characteristic that must have been present in the evolutionary development of multicellular organisms.

The emergence of the integrin family has been traced to the appearance of the metazoans about 600 million years ago.

A group of animals with ancestral histological characteristics are the poriferous, commonly called sea sponges. In these animals, cell adhesion occurs by an extracellular proteoglycan matrix. Receptors that bind to this matrix possess a typical integrin-binding motif.

In fact, in this animal group, genes related to specific subunits of some integrins have been identified.

In the course of evolution, the ancestor of the metazoans acquired an integrin and a binding domain to it that has been conserved over time in this immense animal group.

Structurally, we see the maximum complexity of the integrins in the group of vertebrates. There are different integrins that are not present in invertebrates, with new domains. Indeed, in humans more than 24 different functional integrins have been identified – while in the fruit fly Drosophila melanogaster there are only 5.

References

  1. Integrin. Navarra University Clinic. Recovered from cun.es.
  2. Accession. Atlas of plant and animal histology. Recovered from mmegias.webs.uvigo.es.
  3. B. Alberts, A. Johnson, J. Lewis, et al. (2002). Molecular Biology of the Cell. 4th edition. New York: Garland Science. Integrins. Recovered from ncbi.nlm.nih.gov.
  4. RL Anderson, TW Owens & J. Matthew (2014). Structural and mechanical functions of integrins. Biophysical Reviews.
  5. Integrity. Recovered from en.wikipedia.org.
  6. What is integrin? MBINFO. Recovered from mechanobio.info.
  7. S. Mac Fhearraigh & D. Bruce. The role of integrins in cell signaling. Recovered from abcam.com.
  8. AS Berghoff, O. Rajky, F. Winkler, R. Bartsch, J. Furtner, JA Hainfellner, SL Goodman, M. Weller, J. Schittenhelm, M. Preusser (2013). Invasion patterns in brain metastases of solid cancers. Neuro Oncology.

Related Articles

Leave a Reply

Your email address will not be published. Required fields are marked *

Back to top button