Name: actin structure, isoforms, and roles
Uploaded: 2023-04-30T09:31:19.000Z
Duration: 1 min 26 s
Channel: JoVE

K12 Standards

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NC K-12 Science Standard Course of Study

9-12

Biology

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From Molecules to Organisms- Structures and Processes

LS.Bio.1.3

Introduction to Actin

In the cytoskeleton, actins are the building blocks of cytoskeletal microfilaments. Actin monomers have a round shape and are called globular or G-actin.
These monomers polymerize head-to-tail forming a tight, right-handed helical structure called filamentous or F-actin.
Each actin subunit has an outer and an inner domain joined by a smaller linker helix. Such an arrangement forms two clefts: the upper cleft binds an ATP and a magnesium ion. The lower hydrophobic cleft is specific for actin-binding proteins.
G-actin has a low ATPase activity that is enhanced in F-actin. When an ATP-G-Actin complex binds F-actin, ATP is hydrolyzed to ADP and phosphate, forming a highly stable filament.
The ATP-bound growing end of F-actin is called the plus-end, while the other end, bound to ADP,&#160; is the minus-end.
Actins have different isoforms, broadly classified into alpha, beta, and gamma. They are expressed in different cell types, such as &#945;-actin in contractile muscle fibers, &#946;-actin in cell cortex, and &#947;- actin in smooth muscle fibers.

introduction to actin

Actin is a highly conserved cytoskeletal protein found abundantly in eukaryotic cells. It constitutes 10% weight of the total cellular protein in muscle cells, while in non-muscle cells, it is lower and makes up around 1&#8211;5 percent of the total cell protein. Actin found in the unicellular amoebae and complex multicellular animals is around 80% similar, demonstrating their conservation over a billion years of evolution.&#160; Actin coding genes are conserved within species and across different species. For example, actins present in the yeast Saccharomyces cerevisiae and humans are 87% similar.
Actin was first discovered by W. D. Halliburton in 1887 in muscle extracts as a protein that can induce coagulation of the muscle plasma. It was later purified from muscle extract by Brun&#243; Ferenc Straub in 1942. Straub named it &#39;actin&#39; due to its ability to activate the motor protein myosin. In the early seventies, actin was also found in non-muscle cells and&#160;Acanthamoeba. In the late 1970s, Multiple actin isoforms with issue-specific expression were discovered by sequencing actins using amino acid hydrolysis. These isoforms were nearly identical, with only a few amino acid substitutions. They were classified into &#945;-, &#946;-, and &#947;-actins based on their isoelectric points. For example, birds and mammals have six actin isoforms expressed at different cell types. Actins expressed in skeletal muscle cells are classified as &#945;-skeletal-actin, in cardiac muscles as &#945;-cardiac-actin, and those in smooth muscles as &#945;-smooth-actin and &#947;-smooth-actin. Two other isoforms, &#946;-cyto-actin and &#947;-cyto-actin, are ubiquitously expressed in these organisms.
Subsequent research on these proteins led to the discovery of their role in various cellular functions such as muscle contraction, cell migration, cell adhesion, cell division, protein trafficking, and membrane organization. The first X-ray structure of monomeric G-actin associated with DNAse I was solved only in 1990, resulting in the first atomic model of actin filaments. Several other structures in complex with different proteins have been reported since then.

Actin is a highly conserved cytoskeletal protein found abundantly in eukaryotic cells. It constitutes 10% weight of the total cellular protein in muscle ...

Education

JoVE Core

Cell Biology

Unit 1

The Cytoskeleton I: Actin and Microfilaments

the-cytoskeleton-i-actin-and-microfilaments

KEY TERMS AND CONCEPTS

actin structure, isoforms, and roles

Actin is a major cytoskeletal protein in eukaryotic cells. In muscle cells, it can make up about 10% of the total cellular protein. In non-muscle cells, it is lower and usually makes up around 1 to 5 percent of total cell protein.

Actin is also highly conserved, which means it has changed very little over time. Actin in unicellular amoebae and in complex multicellular animals is about 80% similar. Actin genes are also conserved within a species and across different species. For example, actin in yeast Saccharomyces cerevisiae and humans is 87% similar.

Actin was first discovered by W. D. Halliburton in 1887 in muscle extracts. He found that it could induce coagulation of muscle plasma. Bruno Ferenc Straub purified it from muscle extract in 1942. Straub named the protein actin because it could activate the motor protein myosin.

In the early 1970s, actin was also found in non-muscle cells and in Acanthamoeba. In the late 1970s, researchers identified multiple actin isoforms, or closely related versions of the protein, by sequencing actins using amino acid hydrolysis. These isoforms were nearly identical and differed by only a few amino acid substitutions. They were grouped into alpha-, beta-, and gamma-actins based on their isoelectric points.

Birds and mammals have six actin isoforms expressed in different cell types. Skeletal muscle cells contain alpha-skeletal actin. Cardiac muscle cells contain alpha-cardiac actin. Smooth muscles contain alpha-smooth actin and gamma-smooth actin. Two other isoforms, beta-cyto-actin and gamma-cyto-actin, are found widely in these organisms.

Research on actin later showed that it helps with many cell functions. These include muscle contraction, cell migration, cell adhesion, cell division, protein trafficking, and membrane organization. The first X-ray structure of monomeric G-actin, a single actin subunit, bound to DNAse I was solved in 1990. That work led to the first atomic model of actin filaments. Since then, several more structures with different proteins have been reported.

Actin Structure, Isoforms, and Roles

introduction to the cytoskeleton

Cytoskeleton Structure and Cell Movement

The cytoskeleton is a network of protein filaments inside the cell. It gives cells shape, helps organize cell contents, and supports movement. Although the name suggests a fixed skeleton, it is a dynamic structure that can change quickly in response to signals or forces. Three main filament types make up the cytoskeleton: microfilaments, microtubules, and intermediate filaments. Each one has different assembly and disassembly behavior, mechanical strength, polarity, and associated motor...

adaptability of cytoskeletal filaments

Cytoskeletal Filaments in Cell Movement

Cytoskeletal filaments help cells change shape, divide, and move. Their flexibility lets them stay stable for hours during interphase or reorganize quickly during cell division. Strong regulation controls how these filaments assemble, fall apart, and form networks. Microfilaments and microtubules are dynamic, while intermediate filaments are considered more static. Microfilaments are spread throughout the cell body and can shift into different structures. Many proteins in the cell, including...

polarity of the cytoskeleton

How Cytoskeleton Polarity Guides Cell Movement

Cell polarity depends on the organization of the cytoskeleton and the uneven placement of cell parts. Two main factors help create this polarity. Mobile components such as regulatory molecules and subcellular structures can collect more on one side of the cell. Polar cytoskeletal filaments also line up along the cell’s axis of polarity. The interactions between cytoskeletal filaments help build and maintain this polarized shape. Several features also matter. These include the structure of the...

assembly of cytoskeletal filaments

Actin Bundles and Microtubule Cylinders

Cytoskeletal filaments are built from smaller protein subunits, but they can also join with similar filaments to form stronger cell structures. Microfilaments, which are made of actin monomers, depend on actin-binding proteins to form bundles and networks. Microtubules use microtubule-associated proteins, or MAPs, to form sturdy cylindrical structures. The protein partners for rope-like intermediate filament structures are still unknown. Actin bundles can be compact or loosely spaced. Compact...

cytoskeletal linker proteins - plakins

Plakins in Cell Junctions and Disease

Plakins are large cytoskeletal linker proteins that connect microtubules, microfilaments, intermediate filaments, and membrane protein complexes at cell junctions. They help organize the cytoskeleton by crosslinking its parts and by linking them to the cell matrix and cell adhesion complexes. These proteins also do more than provide structure. Plakins can interact with signal transducers, act as scaffolds for signaling complexes, and help control vesicle trafficking. They were first found with...

cytoskeletal accessory proteins

How Accessory Proteins Support the Cytoskeleton

Accessory proteins help the cytoskeleton work as a connected system. The cytoskeleton is a key cell structure with both structural and functional roles. Its filaments do not work well on their own, so accessory proteins assist with filament formation, stability, and function. These proteins also support communication between cytoskeletal filaments. They are found in both prokaryotes and eukaryotes, although bacterial accessory proteins differ a lot from one species to another. The evolutionary...

cytoskeletal proteins in bacteria

Bacterial Cytoskeleton and Cell Shape

Bacterial cytoskeletal proteins help bacteria keep their shape and divide correctly. They also help set cell polarity and move plasmids, which are small DNA circles, into place during cell division. Bacteria were once thought to be simple and randomly organized, but the discovery of cytoskeleton homologs changed that view. The main bacterial cytoskeletal proteins are actin-like and tubulin-like proteins. These proteins are more diverse in bacteria than in eukaryotic cells, and intermediate...

intracellular movement of viruses and bacteria

intracellular-movement-of-viruses-and-bacteria

How Viruses and Bacteria Ride the Cytoskeleton

Intracellular bacteria and viruses use the host cell cytoskeleton to move inside and between cells. These pathogens can travel from the cell surface to the nuclear region and back again. They may also spread across cells through cortical protrusions, which are cell membrane extensions at the edge of the cell. Several infectious agents use this strategy. Bacterial examples include Rickettsia, which can cause Rocky Mountain spotted fever and Mediterranean spotted fever, Chlamydia, which causes a...

studying the cytoskeleton

Visualizing Cytoskeleton Structure and Motion

The cytoskeleton can be studied with microscopy and biochemical methods. Electron microscopy helped reveal its architecture in the 1960s. It gives very high-resolution structural detail, but sample preparation can limit what is seen in biological samples. One electron microscopy method is rotary shadowing, also called metal replica. In this protocol, cytoskeletal filaments are fixed with chemicals such as glutaraldehyde or formaldehyde. The sample is then dehydrated with alcohol to a critical...

Actin is a major cytoskeletal protein in eukaryotic cells. In muscle cells, it can make up about 10% of the total cellular protein. In non-muscle cells, it is lower and usually makes up around 1 to 5 percent of total cell protein. Actin is also highly conserved, which means it has changed very little over time. Actin in unicellular amoebae and in complex multicellular animals is about 80% similar. Actin genes are also conserved within a species and across different species. For example, actin...

actin polymerization

Actin Filament Growth Stages

Actin filament growth starts when G-actin, or monomeric actin, links together to form F-actin, the filament form. This process builds the actin network in cells through head-to-tail binding between actin subunits. It moves through three stages: nucleation, elongation, and the steady-state phase. Nucleation is the first and hardest step. During this phase, three actin monomers come together to form a stable nucleus. That nucleus then serves as the start of a new actin filament. After...

actin treadmilling

Actin Filament Growth and Loss

Actin filaments grow and shrink at the same time in a process called treadmilling. In this process, G-actin monomers are added at one end of the filament and removed from the other end. The rate of each step depends on the amount of free G-actin in the cytosol, the fluid inside the cell. The plus end, also called the barbed end, usually adds actin faster. The minus end, also called the pointed end, usually loses actin faster. At steady state, the critical concentration is the level of free...

generation of straight or branched actin filaments

generation-of-straight-or-branched-actin-filaments

Actin Filament Branching and Straight Growth

Actin filaments can grow as either straight strands or branched networks. Two nucleating proteins control this shape choice: formins and the Arp2/3 complex. Formins build straight actin filaments, while the Arp2/3 complex builds branched ones. The Arp2/3 complex is made of seven subunits. Two of them, Arp2 and Arp3, are actin-like proteins, and five other subunits help keep them inactive until they are needed. Activation happens with help from other proteins called nucleation-promoting...

actin filament depolymerization

Actin Filament Breakdown at Cell Ends

Actin filaments are built from actin subunits, and they can break down from either end. This process is called depolymerization. It happens faster at the minus-end, also called the pointed end, where the subunits are usually in the ADP-actin form. Several proteins help speed up actin filament breakdown. The actin-depolymerizing factor, or ADF, and cofilin proteins bind to ADP-actin in a one-to-one ratio. When they bind, the filament twists, which creates stress and makes it brittle. That...

formation of higher-order actin filaments

formation-of-higher-order-actin-filaments

Actin Bundles and Cell Shape

Actin bundles and cross-links help shape cells by organizing F-actin into larger networks. G-actin monomers first polymerize into filamentous F-actin, and those filaments then assemble into higher-order structures that support cell function. These structures can help build lamellipodia and filopodia at the leading edge of a migrating cell. They also help form microvilli on the brush border of epithelial cells. Higher-order actin networks can form in two main ways. Straight F-actins may bundle...

overview of myosin structure and function

overview-of-myosin-structure-and-function

Myosin Types in Muscle and Cell Transport

Myosins are molecular motor proteins that help cells move materials and generate force. They were first identified in skeletal muscle, where they drive muscle contraction. They also help move molecules and vesicles inside cells. Myosin proteins are divided into 24 classes based on their domain sequence and organization. Six classes are especially well characterized: Myosin I, Myosin II, Myosin V, Myosin VI, Myosin VII, and Myosin X. These proteins also take part in contractile ring formation...

actin and myosin in muscle contraction

Sarcomere Structure in Skeletal Muscle

Sarcomere structure in skeletal muscle explains how actin and myosin help muscles contract. These contractile proteins are arranged in repeating units called sarcomeres, which are found in skeletal muscle tissue. Their repeated pattern gives skeletal muscle a striped, or striated, look under a microscope. The bands in a myofibril create this striped pattern. Dark A bands and light I bands repeat along the length of the myofibrils. When many myofibrils line up in a muscle cell, the whole cell...

the role of actin and myosin in non-muscle cells

the-role-of-actin-and-myosin-in-non-muscle-cells

How Actomyosin Bundles Shape Cell Movement

Actin and myosin form contractile bundles in non-muscle cells. These actomyosin structures help cells change shape, attach to surfaces, and divide. They were first observed in Dictyostelium and Acanthamoeba. Two common bundles are stress fibers and actomyosin adherence belts. These bundles are smaller and less organized than the ones found in muscle cells. Accessory proteins such as fascin, filamin, and fimbrin help hold them together, depending on where they are in the cell. Their formation...

key terms and concepts

Explore actin’s conservation, discovery, isoforms, and key cell roles, from muscle and non-muscle cells to the first X-ray structure of G-actin.

Actin Structure, Isoforms, and Roles | JoVE K12

cell biology

JoVE Core Cell Biology illustrates complex cellular processes in easy-to-understand yet comprehensive animated video lessons. This collection focuses on the essential components and functions of cells, including organelles and membranes, cellular transport, and cell energetics. It is complemented by scientist-in-action videos that demonstrate the application of cell biology in real-world research experiments.

Pennsylvania State Academic Standards

Life Science

Structure and Function

3.1.9-12.A

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Ohio’s Learning Standards and Model Curriculum

Cells

Cell structure and function

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HS+B.L1

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Alabama Course of Study Science

From Molecules to Organisms: Structures and Processes

9-12.B.1.1

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Actin Structure, Isoforms, and Roles

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