M Type Kinesins
Encyclopedia
The Kinesin-13 Family are a subfamily of motor proteins known as kinesin
s. Most kinesins transport materials or cargo around the cell while traversing along microtubule
polymer
tracks with the help of ATP-hydrolysis
-created energy. They are easily identified by their three typical structural components including a highly conserved structural domain, catalytic core, and microtubule binding sites. The kinesin-13 family, unlike other kinesins, has an internally positioned motor domain. They were initially named KIF-M because of the unique location of their catalytic core in the middle of the polypeptide between the N-terminal globular domain and the C-terminal stalk but they are truly special due to their versatile nature. The Kinesin-13 family's molecular mechanism is less understood than other classes of kinesins which have their motor domains at one end of the molecule or the other. They are capable of traveling to both the minus and plus ends of microtubules whereas most motors are unidirectional. Thus they can catalytically depolymerize
a microtubule from both ends making it a very efficient process.
The exact mechanism of Kinesin-13 activated microtubule depolymerization remains unclear, however, recent biochemical and structural studies revealed some more detailed class specific features enabling researchers to formulate the following model. (Refer to Figure 6 in referenced Ogawa article for illustrated description.) The protein first contacts the side wall of a microtubule. This is not a stable interaction because the convex surface of the catalytic core does not fit to the flat surface of the straight microtubule protofilament. Steric hindrance between the molecule neck and adjacent protofilament further inhibits full contact between protein and the microtubule and only facilitates one-dimensional diffusion
along the microtubule. At this time, The protein's nucleotide binding pocket is trapped in an open state so that the structure is not hydrolyzing
ATP. Once the motor reaches the end of the microtubule, the protofilament spontaneously curves itself allowing motor to make full contact with the tubulin
subunit
. More MCAK molecules collectively bind to the curved region supporting the theory that they do not actively peel away the microtubule but they wait patiently for it to adopt this curved conformation. They stabilize the curved conformation by binding to the end of the microtubule and then catalyze depolymerization.
Kinesin
A kinesin is a protein belonging to a class of motor proteins found in eukaryotic cells. Kinesins move along microtubule filaments, and are powered by the hydrolysis of ATP . The active movement of kinesins supports several cellular functions including mitosis, meiosis and transport of cellular...
s. Most kinesins transport materials or cargo around the cell while traversing along microtubule
Microtubule
Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...
polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
tracks with the help of ATP-hydrolysis
ATP hydrolysis
ATP hydrolysis is the reaction by which chemical energy that has been stored and transported in the high-energy phosphoanhydridic bonds in ATP is released, for example in the muscles, to produce work. The product is ADP and an inorganic phosphate, orthophosphate...
-created energy. They are easily identified by their three typical structural components including a highly conserved structural domain, catalytic core, and microtubule binding sites. The kinesin-13 family, unlike other kinesins, has an internally positioned motor domain. They were initially named KIF-M because of the unique location of their catalytic core in the middle of the polypeptide between the N-terminal globular domain and the C-terminal stalk but they are truly special due to their versatile nature. The Kinesin-13 family's molecular mechanism is less understood than other classes of kinesins which have their motor domains at one end of the molecule or the other. They are capable of traveling to both the minus and plus ends of microtubules whereas most motors are unidirectional. Thus they can catalytically depolymerize
Polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
a microtubule from both ends making it a very efficient process.
The exact mechanism of Kinesin-13 activated microtubule depolymerization remains unclear, however, recent biochemical and structural studies revealed some more detailed class specific features enabling researchers to formulate the following model. (Refer to Figure 6 in referenced Ogawa article for illustrated description.) The protein first contacts the side wall of a microtubule. This is not a stable interaction because the convex surface of the catalytic core does not fit to the flat surface of the straight microtubule protofilament. Steric hindrance between the molecule neck and adjacent protofilament further inhibits full contact between protein and the microtubule and only facilitates one-dimensional diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
along the microtubule. At this time, The protein's nucleotide binding pocket is trapped in an open state so that the structure is not hydrolyzing
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
ATP. Once the motor reaches the end of the microtubule, the protofilament spontaneously curves itself allowing motor to make full contact with the tubulin
Tubulin
Tubulin is one of several members of a small family of globular proteins. The most common members of the tubulin family are α-tubulin and β-tubulin, the proteins that make up microtubules. Each has a molecular weight of approximately 55 kiloDaltons. Microtubules are assembled from dimers of α- and...
subunit
Protein subunit
In structural biology, a protein subunit or subunit protein is a single protein molecule that assembles with other protein molecules to form a protein complex: a multimeric or oligomeric protein. Many naturally occurring proteins and enzymes are multimeric...
. More MCAK molecules collectively bind to the curved region supporting the theory that they do not actively peel away the microtubule but they wait patiently for it to adopt this curved conformation. They stabilize the curved conformation by binding to the end of the microtubule and then catalyze depolymerization.