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A kinesin is a motor protein, or a protein that is capable of moving around on some cellular surfaces. Specifically, kinesins are able to "walk" along microtubules, filament-like protein structures within cells that are involved in a variety of cellular processes, ranging from cell division to transportation of components within cells. The process of kinesin movement requires energy that is provided through the chemical hydrolysis of adenosine triphosphate (ATP), an energy-rich molecule that powers many cellular processes. Kinesins moving along microtubules are responsible for such processes as cellular cargo transportation and the separation of daughter cells in cellular division.
While there are many different types of kinesins, they all tend to be relatively similar in both structure and function. A kinesin is generally composed of various protein domains that form two globular motor domains at the protein's "head" and a coiled-coil "tail." The head region attaches to a microtubule and is believed to "walk" in one direction along its length by placing one motor region in front of the other and repeating. The "tail" region attaches to some form of cellular cargo or to a cellular surface in order to transport a cellular component to a different location or to manipulate the cell itself during cell division.
Different organisms tend to have a variety of different types of kinesins. Humans have roughly 45 different kinesins while C. elegans, a nematode commonly used as a model organism for biological experiments, has 16. Different types of kinesins may have different motor regions, tail regions of different lengths, or any of a wide variety of other slight differences.
The most important feature of kinesin is its ability to "walk" along microtubules. Microtubules are polar in nature, meaning that they have distinct plus and minus ends. Most kinesins are only able to "walk" toward the plus end of a microtubule, though at least one type is able to move toward the minus end. The plus ends of microtubules tend to extend away from the middle of the cell, so most of the transportation conducted by kinesins is toward the edge of the cell.
The exact mechanism by which a kinesin is able to move along the length of a microtubule is not fully understood, though multiple theories have been proposed. The most widely-held theory is the "hand-over-hand" mechanism, in which the kinesin moves as if walking along the microtubule, placing one motor region in front of the other. Another less popular theory is that the kinesin moves like an inchworm — one of its motor regions always leads while the other follows.