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The protofilaments bundle parallel to one another with the same polarity, so, in a microtubule, there is one end, the (+) end, with only β-subunits exposed, while the other end, the (−) end, has only α-subunits exposed. While microtubule elongation can occur at both the (+) and (−) ends, it is significantly more rapid at the (+) end. [17]
In cell biology, microtubule nucleation is the event that initiates de novo formation of microtubules (MTs). These filaments of the cytoskeleton typically form through polymerization of α- and β- tubulin dimers, the basic building blocks of the microtubule, which initially interact to nucleate a seed from which the filament elongates.
Comparison of the architectures of a 5-protofilament bacterial microtubule (left; BtubA in dark blue; BtubB in light-blue) and a 13-protofilament eukaryotic microtubule (right; α-tubulin in white; β-tubulin in black). Seams and start-helices are indicated in green and red, respectively. [13]
Structure of a microtubule Microtubules in a gel-fixated cell Microtubules are hollow cylinders about 23 nm in diameter (lumen diameter of approximately 15 nm), most commonly comprising 13 protofilaments that, in turn, are polymers of alpha and beta tubulin .
Neurotubules are generally assembled by 13 protofilaments which are polymerized from tubulin dimers. As a tubulin dimer consists of one α-tubulin and one β-tubulin, one end of the neurotubule is exposed with the α-tubulin and the other end with β-tubulin, these two ends contribute to the polarity of the neurotubule – the plus (+) end and ...
These protofilaments form the backbone of the hollow, cylindrical microtubule, which is about 25 nanometers in diameter and varies from 200 nanometers to 25 micrometers in length. About 12–13 protofilaments arrange themselves in parallel to form a C-shaped protein sheet, which then curls around to give a pipe-like structure called the ...
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One model proposes that loss of the GTP-cap causes the GDP-containing protofilaments to shrink. Based on this GTP-cap model, catastrophe happens randomly. The model proposes that an increase in microtubule growth will correlate with a decrease in random catastrophe frequency or vice versa.