Stathmin

Key takeaways

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

Reference excerpt

Stathmin, also known as metablastin and oncoprotein 18, is a protein that in humans is encoded by the STMN1 gene. Stathmin is a highly conserved 17 kDa protein that is crucial for the regulation of the cell cytoskeleton. Changes in the cytoskeleton are important because the cytoskeleton is a scaffold required for many cellular processes, such as cytoplasmic organization, cell division and cell motility. More specifically, stathmin is crucial in regulating the cell cycle. It is found solely in eukaryotes. Its function as an important regulatory protein of microtubule dynamics has been well-characterized. Eukaryotic microtubules are one of three major components of the cell's cytoskeleton. They are highly dynamic structures that continuously alternate between assembly and disassembly. Stathmin performs an important function in regulating rapid microtubule remodeling of the cytoskeleton in response to the cell's needs. Microtubules are cylindrical polymers of α,β-tubulin. Their assembly is in part determined by the concentration of free tubulin in the cytoplasm. At low concentrations of free tubulin, the growth rate at the microtubule ends is slowed and results in an increased rate of depolymerization (disassembly).

Structure Stathmin, and the related proteins SCG10 and XB3, contain a N-terminal domain (XB3 contains an additional N-terminal hydrophobic region), a 78 amino acid coiled-coil region, and a short C-terminal domain.

Function The function of Stathmin is to regulate the cytoskeleton of the cell. The cytoskeleton is made up of long hollow cylinders named microtubules. These microtubules are made up of alpha and beta tubulin heterodimers. The changes in cytoskeleton are known as microtubule dynamics; the addition of the tubulin subunits lead to polymerisation and their loss, depolymerisation. Stathmin regulates these by promoting depolymerization of microtubules or preventing polymerization of tubulin heterodimers. Additionally, Stathmin is thought to have a role in cell signaling pathway. Stathmin is a ubiquitous phosphorylated protein which makes it act as an intracellular relay for diverse regulatory pathways, functioning through a variety of second messengers. Its phosphorylation and gene expression are regulated throughout development and in response to extracellular signals regulating cell proliferation, differentiation and function.

Interactions

Stathmin interacts with two molecules of dimeric α,β-tubulin to form a tight ternary complex called the T2S complex. One mole of stathmin binds to two moles of tubulin dimers through the stathmin-like domain (SLD). When stathmin sequesters tubulin into the T2S complex, tubulin becomes non-polymerizable. Without tubulin polymerization, there is no microtubule assembly. Stathmin also promotes microtubule disassembly by acting directly on the microtubule ends. The rate of microtubule assembly is an important aspect of cell growth therefore associating regulation of stathmin with cell cycle progress. Regulation of stathmin is cell cycle dependent and controlled by the cell's protein kinases in response to specific cell signals. Phosphorylation at four serine residues on stathmin named Ser16, Ser25, Ser38 and Ser63 causes weakened stathmin-tubulin binding. Stathmin phosphorylation increases the concentration of tubulin available in the cytoplasm for microtubule assembly. For cells to assemble the mitotic spindle necessary for initiation of the mitotic phase of the cell cycle, stathmin phosphorylation must occur. Without microtubule growth and assembly, the mitotic spindle cannot form, and the cell cycle is arrested. At cytokinesis, the last phase of the cell cycle, rapid dephosphorylation of stathmin occurs to block the cell from entering back into the cell cycle until it is ready.

Clinical significance Stathmin's role in regulation of the cell cycle causes it to be an oncoprotein named oncoprotein 18 (op18). Stathmin (a.k.a. op18) can cause uncontrolled cell proliferation when mutated and not functioning properly. If stathmin is unable to bind to tubulin, it allows for constant microtubule assembly and therefore constant mitotic spindle assembly. With no regulation of the mitotic spindle, the cell cycle is capable of cycling uncontrollably resulting in the unregulated cell growth characteristic of cancer cells.

Role in social behaviour Mice without stathmin have deficiency in innate and learned fear. Stathmin−/− females do not assess threats well, leading to lack of innate parental care and adult social interactions. They lack motivation for retrieving pups and are unable to choose a safe location for nest-building. However, they have an enhancement in social interactions.

References

Illustrations

Worked examples

Example 1 — a first encounter with Stathmin

Start with the simplest possible case. Write down what Stathmin 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 Stathmin 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 Stathmin 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 Stathmin

In research: Stathmin 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 Stathmin 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: Stathmin is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cell cycle regulators, Cytoskeleton, Genes on human chromosome 1, so understanding it makes those chapters shorter.
In everyday life: Look for Stathmin 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.

How to study Stathmin in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Stathmin 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 Stathmin out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Stathmin in simple terms?

Stathmin, also known as metablastin and oncoprotein 18, is a protein that in humans is encoded by the STMN1 gene. Stathmin is a highly conserved 17 kDa protein that is crucial for the regulation of the cell cytoskeleton.

Why does Stathmin 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 Stathmin?

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 Stathmin.