biology

Stress fiber

Stress fiber is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Stress fiber rather than just read about it. In short: Stress fibers are contractile actin bundles found in non-muscle cells. They are composed of actin (microfilaments) and non-muscle myosin II (NMMII), and also contain various crosslinking proteins, such as α-actinin, to form a highly regulated actomyosin structure within non-muscle cells.

Key takeaways

Stress fiber belongs to biology; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Stress fiber to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Stress fiber from memory before moving on to harder problems.

Reference excerpt

Stress fibers are contractile actin bundles found in non-muscle cells. They are composed of actin (microfilaments) and non-muscle myosin II (NMMII), and also contain various crosslinking proteins, such as α-actinin, to form a highly regulated actomyosin structure within non-muscle cells. Stress fibers have been shown to play an important role in cellular contractility, providing force for a number of functions such as cell adhesion, migration and morphogenesis.

Structure Stress fibers are primarily composed of actin and myosin. Actin is a ~43kDa globular protein, and can polymerize to form long filamentous structures. These filaments are made of two strands of actin monomers (or protofilaments) wrapping around each other, to create a single actin filament. Because actin monomers are not symmetrical molecules, their filaments have polarity based upon the structure of the actin monomer, which will allow one end of the actin filament to polymerize faster than the other. The end that can polymerize faster is known as the plus-end, whereas the end that polymerizes slower is known as the minus-end. Stress fibers are usually composed of 10-30 actin filaments. Stress fibers are composed of antiparallel microfilaments: actin filaments are bundled along their length, and plus-ends and minus-ends co-mingle at each end of the bundle. The antiparallel arrangement of actin filaments within stress fibers is reinforced by α-actinin, an actin filament crosslinking protein which contains antiparallel actin-binding domains. These bundles are then cross-linked by NMMII to form stress fibers.

Assembly and regulation

The Rho family of GTPases regulate many aspects of actin cytoskeletal dynamics, including stress fiber formation. RhoA (sometimes referred to as just 'Rho') is responsible for the formation of stress fibers, and its activity in stress fiber formation was first discovered by Ridley and Hall in 1992. When bound to GTP, Rho activates Rho-associated coiled-coil forming kinase (ROCK) and mammalian homologue of Drosophila diaphanous (mDia). mDia is a formin, which nucleates and polymerizes long actin filaments. ROCK is a kinase that acts to phosphorylate MLCP (myosin-light-chain phosphatase), as well as the NMMII light chain, which inactivates MLCP and activates myosin. This will lead to the accumulation of activated myosin motor proteins, which bind the actin filaments that were polymerized by mDia, to create stress fibers. In addition, ROCK also phosphorylates and activates LIM-kinase. LIM-kinase will in turn phosphorylate and inactivate cofilin, which will prevent the breakdown and recycling of actin filaments, maintaining the integrity of the stress fibers.

Roles and associated proteins Stress fibers play the following roles in cellular functioning:

Adhesion Stress fibers are necessary for the formation and maintenance of cell-cell and cell-ECM adhesion, such as the formation of adherens junctions, tight junctions and focal adhesions.

Adherens junctions Adherens junctions are a type of cell-cell adhesion structure that is present in both motile and non-motile cells, which adhere cells together via the homophilic binding of cadherins and nexins. Stress fibers play an important role in the maintenance of cadherin-dependent and nexin-dependent cell-cell contacts, and the Rho-family GTPases have been found to regulate the structure and integrity of adherens junctions. α-catenin and β-catenin are integral components of adherens junctions, which bind together to produce cadherin-α-catenin-β-catenin complexes. Early studies showed that α-catenin could interact with actin filaments, leading to the belief that α-catenin links the actin cytoskeleton to adherens junctions. However it was later found that α-catenin can only bind F-actin when it is unbound by β-catenin and cadherin. Recently, α-catenin has been shown to associate with formins, EPLIN, and vinculin. EPLIN has been found to enhance the bundling and stabilization of actin filaments, and vinculin is involved in the linkage of adhesion molecules to the actin cytoskeleton. This may serve as a mechanism for how actin is recruited to adherens junctions.

Tight junctions Tight junctions, or zona occludens, are the most important cellular element for the formation of semi-permeable barriers within or between tissues. Tight junctions primarily consist of claudins and occludins, which are membrane proteins that form the cell-cell contact, as well as ZO-1, ZO-2 and ZO-3, which link tight junctions to the actin cytoskeleton. However, tight junctions have not been found to be directly linked to stress fibers, like they are for focal adhesions and adherens junctions.

Focal adhesions Focal adhesions are macromolecular assemblies that are used to connect cells to the ECM. They consist of three functional layers: an ECM-associated integrin layer, a membrane associated force transduction layer, and an actin layer, which is made up of actin stress fibers. As the naming or their layers implies, focal adhesions play a large role in mechanotransduction and cell migration. Focal adhesions are usually connected to stress fibers—in fact, stress fiber contractility is necessary for focal adhesion maintenance.

Migration

An essential feature of many cells is their ability to migrate towards certain mechanical (Durotaxis) or chemical (Chemotaxis) stimuli. Cell migration takes place through the concerted action of three Rho family GTPases: Rho, Rac, and Cdc42. When GTP-bound, Rac will cause the formation of lamellipodia, and Cdc42 will cause the formation of filopodia, thus promoting cell migration. In the migrating cell, there are three main types of stress fibers: ventral stress fibers, transverse arcs, and dorsal stress fibers. Ventral stress fibers are associated with focal adhesions at both ends, are located on the ventral surface of the cell, and function in adhesion and contraction. Transverse arcs are not directly linked to focal adhesions, and typically flow from the leading edge of the cell, back towards the cell centre. Dorsal stress fibers are located at the leading edge of the cell. They attach to focal adhesions on the ventral surface of the leading edge, and extend dorsally, towards the cell centre to attach to transverse arcs. During cell migration, actin filaments within stress fibers will be recycled by a process of retrograde actin flow. The mechanism of dissolution of the focal adhesion itself is poorly understood.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Stress fiber

Start with the simplest possible case. Write down what Stress fiber claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Stress fiber before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Stress fiber ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Stress fiber

In research: Stress fiber appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Stress fiber in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Stress fiber is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cells, Fibers, so understanding it makes those chapters shorter.
In everyday life: Look for Stress fiber outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Stress fiber in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Stress fiber means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Stress fiber out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Stress fiber in simple terms?

Why does Stress fiber matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Stress fiber?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Stress fiber.