Name: actin filament branching and straight growth
Uploaded: 2023-04-30T09:31:33.000Z
Duration: 1 min 14 s
Channel: JoVE

Depending on their function, actins can form straight bundles or branched filaments.
Straight actin filaments are generated by the dimeric protein formin, whose FH1 domain acts as a binding site for the profilin-ATP-actin complex. The donut-shaped FH2 domain initiates actin nucleation with two actin monomers.
The FH2 domain receives one actin monomer at a time from the FH1 domain, generating a straight filament.
Branched actin filaments are generated by the Arp2/3 complex.
The Arp2/3 complex binds with CDC42-activated nucleation-promoting factor with attached actin monomer.
The binding of Arp2/3 with NPFs alters its conformation allowing it to bind to the sides of a pre-existing actin filament for elongation.
Upon binding to the actin filament, NPFs add actin monomers to the activated Arp2/3 complex for nucleation before dissociating.
The elongation of the branched filament proceeds at a seventy-degree angle, forming a Y-shaped network.

generation of straight or branched actin filaments

Generation of Straight or Branched Actin Filaments

The straight or branched structure formation of actin filaments is controlled by nucleating proteins such as the formins and Arp2/3 complex. Formin-mediated assembly results in straight filaments, whereas Arp2/3 protein complex-mediated assembly results in branched actin filaments.
Arp2/3 Complex
Arp2/3 complex is a seven-subunit complex consisting of two proteins similar to actin- Arp2 and Arp3, and five other subunits that help keep Arp2 and Arp3 inactive. When required, the complex is activated through conformational changes that occur with the synergistic help of other proteins, such as the regulatory nucleation promoting factors or NPFs, actin filaments, an actin monomer, and ATP. The complex nucleates the formation of a branch or a daughter filament on an existing actin filament. Arp2 and Arp3 have a conserved surface similar to actin and are the first two subunits at the pointed end of the branch. The presence of these subunits promotes the binding of actin monomers to the complex and initiates the formation of the branch.
Formins
Formins are homodimers of polypeptides that consist of characteristic domains such as the FH2 domain. FH2 domains associate with each other from head to tail to form a donut-shaped dimer. This dimer nucleates the formation of straight actin filaments at the required site. Formins are responsible for forming the contractile ring that separates daughter cells during cytokinesis and actin filaments bundles in filopodia. As the filament grows, formins are processively or continuously associated with the growing barbed end to protect it from capping proteins. Formins have several isomers that are function-specific in some cases and cannot be interchanged. For example, in fission yeast, three separate isoforms nucleate the formation of actin filaments used in the contractile ring, actin cables, and mating, respectively.

The straight or branched structure formation of actin filaments is controlled by nucleating proteins such as the formins and Arp2/3 complex. ...

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

Education

JoVE Core

Cell Biology

Unit 1

The Cytoskeleton I: Actin and Microfilaments

the-cytoskeleton-i-actin-and-microfilaments

KEY TERMS AND CONCEPTS

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 factors, or NPFs, along with actin filaments, an actin monomer, and ATP.

Once activated, the Arp2/3 complex starts a new branch, or daughter filament, on an existing actin filament. Arp2 and Arp3 sit at the pointed end of the branch and have a conserved surface similar to actin. This helps actin monomers bind to the complex and begin branch formation.

Formins work in a different way. They are homodimers, meaning two similar polypeptides pair together. Their FH2 domains connect head to tail and form a donut-shaped dimer that nucleates straight actin filaments at the right location.

Formins also stay attached to the growing barbed end of the filament as it lengthens. This processive binding protects the growing end from capping proteins. Formins help build the contractile ring used during cytokinesis and actin filament bundles in filopodia.

Some formin isoforms have specific jobs and cannot be swapped with one another. In fission yeast, three separate isoforms nucleate actin filaments for the contractile ring, actin cables, and mating. These examples show how actin assembly proteins help shape cell structure and cell division.

Actin Filament Branching and Straight Growth

actin-filament-branching-and-straight-growth

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...

introduction to actin

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...

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...

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

Learn how formins and the Arp2/3 complex shape actin filaments into straight strands or branched networks, and how these structures support cell division and movement.

Actin Filament Branching and Straight Growth | 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.

Actin Filament Branching and Straight Growth

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