Plastin is part of a family of

actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ...

-bundling

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

s, specifically the

α-actinin Actinin is a microfilament protein. The functional protein is an anti-parallel dimer, which cross-links the thin filaments in adjacent sarcomeres, and therefore coordinates contractions between sarcomeres in the horizontal axis. Alpha-actinin is ...

family of actin-binding protein, which are found in many lifeforms, from

humans Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

and other

animals Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia (). With few exceptions, animals consume organic material, breathe oxygen, have myocytes and are able to move, can reproduce sexually, and grow from a ...

plants Plants are the eukaryotes that form the kingdom Plantae; they are predominantly photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with cyanobacteria to produce sugars f ...

and

yeasts Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constitu ...

. These proteins are known to

cross-link In chemistry and biology, a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...

actin filaments into bundles for various cell purposes.Members of plastin include: *

LCP1 Plastin-2 is a protein that in humans is encoded by the ''LCP1'' gene. Function Plastins are a family of actin-binding proteins that are conserved throughout eukaryote evolution and expressed in most tissues of higher eukaryotes. In humans, t ...

PLS1 Fimbrin also known as is plastin 1 is a protein that in humans is encoded by the PLS1 gene. Fimbrin is an actin cross-linking protein important in the formation of filopodia. Structure Fimbrin belongs to the calponin homology (biology), homolo ...

* PLS3

Structure

The structure of plastin has been evolutionarily maintained within organisms who utilize this protein, which includes humans and lower eukaryotic organisms. Plastin structures are known for their

EF-hand The EF hand is a helix–loop–helix structural domain or ''motif'' found in a large family of calcium-binding proteins. The EF-hand motif contains a helix–loop–helix topology, much like the spread thumb and forefinger of the human hand, in ...

Ca²⁺- and actin-binding domains that assist in assembling actin into higher-order bundles. Plastins have two actin-binding domains (ABD) in each of their polypeptide in which each ABD contains two of 125-residue calponin-homology (CH) components. This structure allows for plastins to cross-link actin filaments into their bundles in order to perform various tasks.

Function

Plastin, along with other actin-binding proteins, help stabilize and rearrange organization of actin cytoskeleton when present with external stimuli, cell migration, and cell adhesion. The EF-hand Ca²⁺-binding domains are important for the function of plastin as their activities are regulated by Ca²⁺. In mammals, three isoforms of plastin have been identified: L-plastin, found mostly within hematopoietic cells; T-plastin, found in cells of solid tissues; and lastly, I-plastin, expressed specifically in the small intestine, colon, and kidneys. Each of these isoforms of plastin have their own roles, dependent of the cell-type, in order to regulate the actin cytoskeleton.

L-plastin

L-plastin (leukocyte plastin, LPL, Plastin-2,

), are expressed in

hematopoietic Haematopoiesis (; ; also hematopoiesis in American English, sometimes h(a)emopoiesis) is the formation of blood cellular components. All cellular blood components are derived from haematopoietic stem cells. In a healthy adult human, roughly ten ...

cells and in various types of

leukocytes White blood cells (scientific name leukocytes), also called immune cells or immunocytes, are cells of the immune system that are involved in protecting the body against both infectious disease and foreign entities. White blood cells are genera ...

(i.e., T- and B-lymphocytes). They help to defend against foreign pathogens by using

phagocytosis Phagocytosis () is the process by which a cell (biology), cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs ph ...

. They help contribute to the motility of T-cells so that these cells are capable of activating during an immune response. L-plastins are known to be the only isoform within the three to undergo

phosphorylation In biochemistry, phosphorylation is described as the "transfer of a phosphate group" from a donor to an acceptor. A common phosphorylating agent (phosphate donor) is ATP and a common family of acceptor are alcohols: : This equation can be writ ...

during leukocyte activation via interleukin-1, -2, and phorbol myristate acetate. This, and the fact that plastins are Ca²⁺-dependent, allow L-plastins to be capable of signaling to leukocytes for rapid responses to stimuli. As a result, organisms or cells that are lacking L-plastins have a more difficult time responding to external stimuli that need an immunity activation.

T-Plastin

Also written PLS3 or Plastin-3, T-plastins are primarily found in solid tissues, within cells that are capable of replication (i.e.,

epithelial Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...

and

mesenchymal Mesenchyme () is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood, or bone. The interactions between mesenchyme and epithelium help to form nearly ever ...

cells). T-plastins are needed in order for cells to carry out proliferation and migration as well as membranes to protrude from cell to another cell and gap the distance within the

extracellular matrix In biology, the extracellular matrix (ECM), also called intercellular matrix (ICM), is a network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide structural and bio ...

(ECM). These proteins strengthen the actin-cytoskeleton network to accomplish cell migration and distribution. Without T-plastin, cells who migrate using the protrusion method would not be able to bypass the membrane tension that needs to be overcome in order for protrusion to occur.

I-Plastin

I-plastin (intestinal plastin, plastin-1,

) is localized to the intestinal epithelial cells, specifically in the intestinal

brush border A brush border (striated border or brush border membrane) is the microvillus-covered surface of simple cuboidal and simple columnar epithelium found in different parts of the body. Microvilli are approximately 100 nanometers in diameter and th ...

microvilli Microvilli (: microvillus) are microscopic cellular membrane protrusions that increase the surface area for diffusion and minimize any increase in volume, and are involved in a wide variety of functions, including absorption, secretion, cellula ...

. I-plastin help to stabilize the intestinal brush border microvilli and its function. Without I-plastin, it has been observed that there was decreased transepithelial resistance, increased cellular turnover, and increased sensitivity to specific diseases in the intestines. Phenotypically, microvilli in the intestines lacking this protein were found to be shorter, constricted, not have profound rootlets, and have increased fragility.

References

{{Reflist EF-hand-containing proteins Protein families Human proteins