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Kinesin

Kinesin 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 Kinesin rather than just read about it. In short: A kinesin is a protein complex belonging to a class of motor proteins found in eukaryotic cells. Kinesins move along microtubule (MT) filaments and are powered by the hydrolysis of adenosine triphosphate (ATP) (thus kinesins are ATPases, a type of enzyme).

Kinesin — main illustration
Kinesin — illustration

Key takeaways

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

Reference excerpt

A kinesin is a protein complex belonging to a class of motor proteins found in eukaryotic cells. Kinesins move along microtubule (MT) filaments and are powered by the hydrolysis of adenosine triphosphate (ATP) (thus kinesins are ATPases, a type of enzyme). The active movement of kinesins supports several cellular functions including mitosis, meiosis and transport of cellular cargo, such as in axonal transport, and intraflagellar transport. Most kinesins walk towards the plus end of a microtubule, which, in most cells, entails transporting cargo such as protein and membrane components from the center of the cell towards the periphery. This form of transport is known as anterograde transport. In contrast, dyneins are motor proteins that move toward the minus end of a microtubule in retrograde transport.

Discovery The first kinesins to be discovered were microtubule-based anterograde intracellular transport motors in 1985, based on their motility in cytoplasm extruded from the giant axon of the squid. The founding member of this superfamily, kinesin-1, was isolated as a heterotetrameric fast axonal organelle transport motor consisting of four parts: two identical motor subunits (called Kinesin Heavy Chain (KHC) molecules) and two other molecules each known as a Kinesin Light Chain (KLC). These were discovered via microtubule affinity purification from neuronal cell extracts. Subsequently, a different, heterotrimeric plus-end-directed MT-based motor named kinesin-2, consisting of two distinct KHC-related motor subunits and an accessory "KAP" subunit, was purified from echinoderm egg/embryo extracts and is best known for its role in transporting protein complexes (intraflagellar transport particles) along axonemes during ciliogenesis. Molecular genetic and genomic approaches have led to the recognition that the kinesins form a diverse superfamily of motors that are responsible for multiple intracellular motility events in eukaryotic cells. For example, the genomes of mammals encode more than 40 kinesin proteins, organized into at least 14 families named kinesin-1 through kinesin-14.

Structure

Overall structure Members of the kinesin superfamily vary in shape but the prototypical kinesin-1 motor consists of two Kinesin Heavy Chain (KHC) molecules which form a protein dimer (molecule pair) that binds two light chains (KLCs), which are unique for different cargos. The heavy chain of kinesin-1 comprises a globular head (the motor domain) at the amino terminal end connected via a short, flexible neck linker to the stalk – a long, central alpha-helical coiled coil domain – that ends in a carboxy terminal tail domain which associates with the light-chains. The stalks of two KHCs intertwine to form a coiled coil that directs dimerization of the two KHCs. In most cases transported cargo binds to the kinesin light chains, at the TPR motif sequence of the KLC, but in some cases cargo binds to the C-terminal domains of the heavy chains.

Kinesin motor domain

The head is the signature of kinesin and its amino acid sequence is well conserved among various kinesins. Each head has two separate binding sites: one for the microtubule and the other for ATP. ATP binding and hydrolysis as well as ADP release change the conformation of the microtubule-binding domains and the orientation of the neck linker with respect to the head; this results in the motion of the kinesin. Several structural elements in the head, including a central beta-sheet domain and the Switch I and II domains, have been implicated as mediating the interactions between the two binding sites and the neck domain. Kinesins are structurally related to G proteins, which hydrolyze GTP instead of ATP. Several structural elements are shared between the two families, notably the Switch I and Switch II domain.

Basic kinesin regulation Kinesins tend to have low basal enzymatic activity which becomes significant when microtubule-activated. In addition, many members of the kinesin superfamily can be self-inhibited by the binding of tail domain to the motor domain. Such self-inhibition can then be relieved via additional regulation such as binding to cargo, cargo adapters or other microtubule-associated proteins.

Cargo transport In the cell, small molecules, such as gases and glucose, diffuse to where they are needed. Large molecules synthesised in the cell body, intracellular components such as vesicles and organelles such as mitochondria are too large (and the cytosol too crowded) to be able to diffuse to their destinations. Motor proteins fulfill the role of transporting large cargo about the cell to their required destinations. Kinesins are motor proteins that transport such cargo by walking unidirectionally along microtubule tracks hydrolysing one molecule of adenosine triphosphate (ATP) at each step. It was thought that ATP hydrolysis powered each step, the energy released propelling the head forwards to the next binding site. However, it has been proposed that the head diffuses forward and the force of binding to the microtubule is what pulls the cargo along. In addition viruses, HIV for example, exploit kinesins to allow virus particle shuttling after assembly. There is significant evidence that cargoes in-vivo are transported by multiple motors in some cases by distinct forms of same-polarity kinesin motors that cooperate to optimize transport.

… excerpt ends here. Continue reading the full article.

Illustrations

Kinesin: The kinesin dimer (red) attaches to, and moves along, microtubules (blue and green).
The kinesin dimer (red) attaches to, and moves along, microtubules (blue and green).
Kinesin: Animation of kinesin "walking" on a microtubule. Protein domain dynamics can only now be seen by neutron spin echo spectroscopy.
Animation of kinesin "walking" on a microtubule. Protein domain dynamics can only now be seen by neutron spin echo spectroscopy.
Kinesin illustration
Kinesin: Mobile and self-inhibited conformations of kinesin-1. Self-inhibited conformation:IAK region of the tail (green) binds to motor domains (yellow and orange) to inhibit the enzymatic cycle of kinesin-1.Mobile conformation: Absent the tail binding, kinesin-1 motor domains (yellow and orange) can move freely along the microtubule(MT).[19] PDB 2Y65; PDB 2Y5W.
Mobile and self-inhibited conformations of kinesin-1. Self-inhibited conformation:IAK region of the tail (green) binds to motor domains (yellow and orange) to inhibit the enzymatic cycle of kinesin-1.Mobile conformation: Absent the tail binding, kinesin-1 motor domains (yellow and orange) can move freely along the microtubule(MT).[19] PDB 2Y65; PDB 2Y5W.
Kinesin: Detailed view of kinesin-1 self-inhibition (one of two possible conformations shown). Highlight: positively charged residues (blue) of the IAK region interact at multiple locations with negatively charged residues (red) of the motor domains[19] PDB 2Y65
Detailed view of kinesin-1 self-inhibition (one of two possible conformations shown). Highlight: positively charged residues (blue) of the IAK region interact at multiple locations with negatively charged residues (red) of the motor domains[19] PDB 2Y65

Worked examples

Example 1 — a first encounter with Kinesin

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

In research
Kinesin 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 Kinesin 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
Kinesin is common in secondary-school and first-year university syllabi. It links to neighbouring topics Motor proteins, so understanding it makes those chapters shorter.
In everyday life
Look for Kinesin 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 Kinesin in 20 minutes

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

Frequently asked questions

What is Kinesin in simple terms?

A kinesin is a protein complex belonging to a class of motor proteins found in eukaryotic cells. Kinesins move along microtubule (MT) filaments and are powered by the hydrolysis of adenosine triphosphate (ATP) (thus kinesins are ATPases, a type of enzyme).

Why does Kinesin 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 Kinesin?

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 Kinesin.

Tags

  • Motor proteins

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