A kinesin is a protein 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. [1] 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.
The first kinesins to be discovered were microtubule-based anterograde intracellular transport motors [2] in 1985, based on their motility in cytoplasm extruded from the giant axon of the squid. [3]
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. [4] 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 [5] and is best known for its role in transporting protein complexes (intraflagellar transport particles) along axonemes during ciliogenesis. [6] 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. [7] [8] [9] [10] For example, the genomes of mammals encode more than 40 kinesin proteins, [11] organized into at least 14 families named kinesin-1 through kinesin-14. [12]
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. [13]
Kinesin motor domain | |||||||||
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![]() Crystallographic structure of the human kinesin motor domain depicted as a rainbow colored cartoon (N-terminus = blue, C-terminus = red) complexed with ADP (stick diagram, carbon = white, oxygen = red, nitrogen = blue, phosphorus = orange) and a magnesium ion (grey sphere). [14] | |||||||||
Identifiers | |||||||||
Symbol | Kinesin motor domain | ||||||||
Pfam | PF00225 | ||||||||
InterPro | IPR001752 | ||||||||
SMART | SM00129 | ||||||||
PROSITE | PS50067 | ||||||||
SCOP2 | 1bg2 / SCOPe / SUPFAM | ||||||||
CDD | cd00106 | ||||||||
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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.
Kinesins tend to have low basal enzymatic activity which becomes significant when microtubule-activated. [16] In addition, many members of the kinesin superfamily can be self-inhibited by the binding of tail domain to the motor domain. [17] Such self-inhibition can then be relieved via additional regulation such as binding to cargo, cargo adapters or other microtubule-associated proteins. [18] [19] [20]
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. [21] It was thought that ATP hydrolysis powered each step, the energy released propelling the head forwards to the next binding site. [22] However, it has been proposed that the head diffuses forward and the force of binding to the microtubule is what pulls the cargo along. [23] In addition viruses, HIV for example, exploit kinesins to allow virus particle shuttling after assembly. [24]
There is significant evidence that cargoes in-vivo are transported by multiple motors. [25] [26] [27] [28]
Motor proteins travel in a specific direction along a microtubule. Microtubules are polar; meaning, the heads only bind to the microtubule in one orientation, while ATP binding gives each step its direction through a process known as neck linker zippering. [29]
It has been previously known that kinesin move cargo towards the plus (+) end of a microtubule, also known as anterograde transport/orthograde transport. [30] However, it has been recently discovered that in budding yeast cells kinesin Cin8 (a member of the Kinesin-5 family) can move toward the minus end as well, or retrograde transport. This means, these unique yeast kinesin homotetramers have the novel ability to move bi-directionally. [31] [32] [33] Kinesin, so far, has only been shown to move toward the minus end when in a group, with motors sliding in the antiparallel direction in an attempt to separate microtubules. [34] This dual directionality has been observed in identical conditions where free Cin8 molecules move towards the minus end, but cross-linking Cin8 move toward the plus ends of each cross-linked microtubule. One specific study tested the speed at which Cin8 motors moved, their results yielded a range of about 25-55 nm/s, in the direction of the spindle poles. [35] On an individual basis it has been found that by varying ionic conditions Cin8 motors can become as fast as 380 nm/s. [35] It is suggested that the bidirectionality of yeast kinesin-5 motors such as Cin8 and Cut7 is a result of coupling with other Cin8 motors and helps to fulfill the role of dynein in budding yeast, as opposed to the human homologue of these motors, the plus directed Eg5. [36] This discovery in kinesin-14 family proteins (such as Drosophila melanogaster NCD, budding yeast KAR3, and Arabidopsis thaliana ATK5) allows kinesin to walk in the opposite direction, toward microtubule minus end. [37] This is not typical of kinesin, rather, an exception to the normal direction of movement.
Another type of motor protein, known as dyneins, move towards the minus end of the microtubule. Thus, they transport cargo from the periphery of the cell towards the center. An example of this would be transport occurring from the terminal boutons of a neuronal axon to the cell body (soma). This is known as retrograde transport.
In 2023 direct visualization of kinesin "walking" along a microtubule in real-time was reported. [38] In a "hand-over-hand" mechanism, the kinesin heads step past one another, alternating the lead position. Thus in each step the leading head becomes the trailing head, while the trailing head becomes the leading head.
A number of theoretical models of the molecular motor protein kinesin have been proposed. [45] [46] [47] Many challenges are encountered in theoretical investigations given the remaining uncertainties about the roles of protein structures, the precise way energy from ATP is transformed into mechanical work, and the roles played by thermal fluctuations. This is a rather active area of research. There is a need especially for approaches which better make a link with the molecular architecture of the protein and data obtained from experimental investigations.
The single-molecule dynamics are already well described [48] but it seems that these nano scale machines typically work in large teams.
Single-molecule dynamics are based on the distinct chemical states of the motor and observations about its mechanical steps. [49] For small concentrations of adenosine diphosphate, the motor's behaviour is governed by the competition of two chemomechanical motor cycles which determine the motor's stall force. A third cycle becomes important for large ADP concentrations. [49] Models with a single cycle have been discussed too. Seiferth et al. demonstrated how quantities such as the velocity or the entropy production of a motor change when adjacent states are merged in a multi-cyclic model until eventually the number of cycles is reduced. [50]
Recent experimental research has shown that kinesins, while moving along microtubules, interact with each other, [51] [52] the interactions being short range and weak attractive (1.6±0.5 KBT). One model that has been developed takes into account these particle interactions, [48] where the dynamic rates change accordingly with the energy of interaction. If the energy is positive the rate of creating bonds (q) will be higher while the rate of breaking bonds (r) will be lower. One can understand that the rates of entrance and exit in the microtubule will be changed as well by the energy (See figure 1 in reference 30). If the second site is occupied the rate of entrance will be α*q and if the last but one site is occupied the rate of exit will be β*r. This theoretical approach agrees with the results of Monte Carlo simulations for this model, especially for the limiting case of very large negative energy. The normal totally asymmetric simple exclusion process for (or TASEP) results can be recovered from this model making the energy equal to zero.
In recent years, it has been found that microtubule-based molecular motors (including a number of kinesins) have a role in mitosis (cell division). Kinesins are important for proper spindle length and are involved in sliding microtubules apart within the spindle during prometaphase and metaphase, as well as depolymerizing microtubule minus ends at centrosomes during anaphase. [53] Specifically, Kinesin-5 family proteins act within the spindle to slide microtubules apart, while the Kinesin 13 family act to depolymerize microtubules.
Human kinesin superfamily members include the following proteins, which in the standardized nomenclature developed by the community of kinesin researchers, are organized into 14 families named kinesin-1 through kinesin-14: [12]
kinesin-1 light chains:
kinesin-2 associated protein:
Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nm and have an inner diameter between 11 and 15 nm. They are formed by the polymerization of a dimer of two globular proteins, alpha and beta tubulin into protofilaments that can then associate laterally to form a hollow tube, the microtubule. The most common form of a microtubule consists of 13 protofilaments in the tubular arrangement.
Dyneins are a family of cytoskeletal motor proteins that move along microtubules in cells. They convert the chemical energy stored in ATP to mechanical work. Dynein transports various cellular cargos, provides forces and displacements important in mitosis, and drives the beat of eukaryotic cilia and flagella. All of these functions rely on dynein's ability to move towards the minus-end of the microtubules, known as retrograde transport; thus, they are called "minus-end directed motors". In contrast, most kinesin motor proteins move toward the microtubules' plus-end, in what is called anterograde transport.
Molecular motors are natural (biological) or artificial molecular machines that are the essential agents of movement in living organisms. In general terms, a motor is a device that consumes energy in one form and converts it into motion or mechanical work; for example, many protein-based molecular motors harness the chemical free energy released by the hydrolysis of ATP in order to perform mechanical work. In terms of energetic efficiency, this type of motor can be superior to currently available man-made motors. One important difference between molecular motors and macroscopic motors is that molecular motors operate in the thermal bath, an environment in which the fluctuations due to thermal noise are significant.
Axonal transport, also called axoplasmic transport or axoplasmic flow, is a cellular process responsible for movement of mitochondria, lipids, synaptic vesicles, proteins, and other organelles to and from a neuron's cell body, through the cytoplasm of its axon called the axoplasm. Since some axons are on the order of meters long, neurons cannot rely on diffusion to carry products of the nucleus and organelles to the ends of their axons. Axonal transport is also responsible for moving molecules destined for degradation from the axon back to the cell body, where they are broken down by lysosomes.
Motor proteins are a class of molecular motors that can move along the cytoskeleton of cells. They convert chemical energy into mechanical work by the hydrolysis of ATP. Flagellar rotation, however, is powered by a proton pump.
The Kinesin-13 Family are a subfamily of motor proteins known as kinesins. Most kinesins transport materials or cargo around the cell while traversing along microtubule polymer tracks with the help of ATP-hydrolysis-created energy.
Molecular biophysics is a rapidly evolving interdisciplinary area of research that combines concepts in physics, chemistry, engineering, mathematics and biology. It seeks to understand biomolecular systems and explain biological function in terms of molecular structure, structural organization, and dynamic behaviour at various levels of complexity. This discipline covers topics such as the measurement of molecular forces, molecular associations, allosteric interactions, Brownian motion, and cable theory. Additional areas of study can be found on Outline of Biophysics. The discipline has required development of specialized equipment and procedures capable of imaging and manipulating minute living structures, as well as novel experimental approaches.
Dynactin is a 23 subunit protein complex that acts as a co-factor for the microtubule motor cytoplasmic dynein-1. It is built around a short filament of actin related protein-1 (Arp1).
Kinesin-like protein KIF3B is a protein that in humans is encoded by the KIF3B gene. KIF3B is an N-type protein that complexes with two other kinesin proteins to form two-headed anterograde motors. First, KIF3B forms a heterodimer with KIF3A ; (KIF3A/3B), that is membrane-bound and has ATPase activity. Then KIFAP3 binds to the tail domain to form a heterotrimeric motor. This motor has a plus end-directed microtubule sliding activity that exhibits a velocity of ~0.3 μm/s a. There are 14 kinesin protein families in the kinesin superfamily and KIF3B is part of the Kinesin-2 family, of kinesins that can all form heterotrimeric complexes. Expression of the three motor subunits is ubiquitous. The KIG3A/3B/KAP3 motors can transport 90 to 160 nm in diameter organelles.
Kinesin light chain 3 is a protein that in humans is encoded by the KLC3 gene.
Kinesin-like protein KIF1C is a protein that in humans is encoded by the KIF1C gene. Kif1C is a fast, plus-end directed microtubule motor. It takes processive 8nm steps along microtubules and can generate forces of up to 5 pN. Kif1C transports α5β1-integrins in human cells. Kif1C has been shown to be non-essential in mouse with other proteins able to perform the same function. However, mutations in KIF1C lead to spastic paraplegia and cerebellar dysfunction in humans. These mutations usually result in a total loss of the protein or (partial) loss of function, such as significant lower force output.
Kinesin-like protein KIF1A, also known as axonal transporter of synaptic vesicles or microtubule-based motor KIF1A, is a protein that in humans is encoded by the KIF1A gene.
Kinesin-like protein KIF11 is a molecular motor protein that is essential in mitosis. In humans it is coded for by the gene KIF11. Kinesin-like protein KIF11 is a member of the kinesin superfamily, which are nanomotors that move along microtubule tracks in the cell. Named from studies in the early days of discovery, it is also known as Kinesin-5, or as BimC, Eg5 or N-2, based on the founding members of this kinesin family.
Ronald David Vale ForMemRS is an American biochemist and cell biologist. He is a professor at the Department of Cellular and Molecular Pharmacology, University of California, San Francisco. His research is focused on motor proteins, particularly kinesin and dynein. He was awarded the Canada Gairdner International Award for Biomedical Research in 2019, the Shaw Prize in Life Science and Medicine in 2017 together with Ian Gibbons, and the Albert Lasker Award for Basic Medical Research in 2012 alongside Michael Sheetz and James Spudich. He is a fellow of the American Academy of Arts and Sciences and a member of the National Academy of Sciences. He was the president of the American Society for Cell Biology in 2012. He has also been an investigator at the Howard Hughes Medical Institute since 1995. In 2019, Vale was named executive director of the Janelia Research Campus and a vice president of HHMI; his appointment began in early 2020.
Intracellular transport is the movement of vesicles and substances within a cell. Intracellular transport is required for maintaining homeostasis within the cell by responding to physiological signals. Proteins synthesized in the cytosol are distributed to their respective organelles, according to their specific amino acid’s sorting sequence. Eukaryotic cells transport packets of components to particular intracellular locations by attaching them to molecular motors that haul them along microtubules and actin filaments. Since intracellular transport heavily relies on microtubules for movement, the components of the cytoskeleton play a vital role in trafficking vesicles between organelles and the plasma membrane by providing mechanical support. Through this pathway, it is possible to facilitate the movement of essential molecules such as membrane‐bounded vesicles and organelles, mRNA, and chromosomes.
Andrew P. Carter is a British structural biologist who works at the Medical Research Council (MRC) Laboratory of Molecular Biology (LMB) in Cambridge, UK. He is known for his work on the microtubule motor dynein.
Samara Reck-Peterson is an American cell biologist and biophysicist. She is a Professor of Cellular and Molecular Medicine and Cell and Developmental Biology at the University of California, San Diego and an Investigator of the Howard Hughes Medical Institute. She is known for her contributions to our understanding of how dynein, an exceptionally large motor protein that moves many intracellular cargos, works and is regulated. She developed one of the first systems to produce recombinant dynein and discovered that, unlike other cytoskeletal motors, dynein can take a wide variety of step sizes, forward and back and even sideways. She lives in San Diego, California.
Neurotubules are microtubules found in neurons in nervous tissues. Along with neurofilaments and microfilaments, they form the cytoskeleton of neurons. Neurotubules are undivided hollow cylinders that are made up of tubulin protein polymers and arrays parallel to the plasma membrane in neurons. Neurotubules have an outer diameter of about 23 nm and an inner diameter, also known as the central core, of about 12 nm. The wall of the neurotubules is about 5 nm in width. There is a non-opaque clear zone surrounding the neurotubule and it is about 40 nm in diameter. Like microtubules, neurotubules are greatly dynamic and the length of them can be adjusted by polymerization and depolymerization of tubulin.
Edwin W. Taylor is an adjunct professor of cell and developmental biology at Northwestern University. He was elected to the National Academy of Sciences in 2001. Taylor received a BA in physics and chemistry from the University of Toronto in 1952; an MSc in physical chemistry from McMaster University in 1955, and a PhD in biophysics from the University of Chicago in 1957. In 2001 Taylor was elected to the National Academy of Scineces in Cellular and Developmental Biology and Biochemistry.
J. Richard McIntosh is a Distinguished Professor Emeritus in Molecular, Cellular, and Developmental Biology at the University of Colorado Boulder. McIntosh first graduated from Harvard with a BA in Physics in 1961, and again with a Ph.D. in Biophysics in 1968. He began his teaching career at Harvard but has spent most of his career at the University of Colorado Boulder. At the University of Colorado Boulder, McIntosh taught biology courses at both the undergraduate and graduate levels. Additionally, he created an undergraduate course in the biology of cancer towards the last several years of his teaching career. McIntosh's research career looks at a variety of things, including different parts of mitosis, microtubules, and motor proteins.
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