Microtubules are hollow tubular structures that are presentin almost all eukaryotic cells. Due to the fact that they play a role in a hugenumber of cellular processes they are a fascinating organelle to study, andhave been the subject of a huge amount of research in the past number of years.The first discovery of microtubules came over sixtyyears ago in 1954 by Fawcett and Porter’s use of transmission electronmicroscopy.
i As at that time specimens and samples couldnot be preserved well the research was hindered and a huge number of questionstriggered: “Which cells contain these microtubules? What are the components ofthese structures? How do these microtubules perform work in the cell? How arethey involved in cell division and movement? How do they assemble?” Thesequestions piqued the interest of many and the search for answers began.By 1963, Sabatini et al, having used more successfulstructural preservation techniques, had seen a universal presence ofmicrotubules in almost all eukaryotic cells.ii Thistriggered even further interest in these cellular components and their workingmechanisms and structure.The discovery of tubulin didn’t occur until yearslater in 1967, through the pioneering work by E.
W. Taylor.iii Thisdiscovery solved the mystery of the microtubule components. The mechanism of the self-assembly of these structureswas of huge interest, they are known as nature’s best example of a dynamic”smart” material. Microtubules show a spontaneous assembly from different partswhich a man-made machine would never be able to do.
Due to the numerous functions and attractivemechanisms of these microtubules, the scientific world was inspired to recreatethese structures from different materials. A further question was asked; “whatcan we learn from these extraordinary structures that can be utilised inmaterials science and bio nanotechnology?” This biomimicry has been testedusing a huge number of different materials. Mentioning them all would be animmeasurable task, so I focus on tubular structures that show self-assembly.These include some types of carbon nanotubes, organic tubules, DNA nanotubes, “annular-ring”microtubes and microtubules that use colloidal building blocks.In general, microtubes have an inner diameter in themicron region and nanotubes have a diameter in the order of nanometers.However, for cellular based microtubules the diameters are within the order ofnanometers. Typically they have an inner diameter of 12nm and an outer diameterof 24nm.
Although the naming of these structures can be slightly confusing, theuse of “micro” is merely to show the very small scale of these structures.I will focus on the growth mechanisms, structure,properties and applications of these biomimetic materials. Also, how theycompare to the original cellular based microtubule.MicrotubulesThecell’s cytoskeleton main functions are to maintain the shape and organisationof the cell’s components; to give a cell mechanical support; and to enablespecific functions, for example transport, movement and cell division. The cytoskeleton’scomponents can be classified into three different types of filament-like proteins:microtubules, intermediate filaments and actin filaments. Microtubules (MTs),which are of interest to us, are the largest of the three filaments with adiameter of around 24nm. Intermediate filaments have a diameter of around 10nmand actin filaments are the smallest with diameters of approximately 6nm.
Thescale of these filaments in relation to each other is shown in Figure 1.Microtubules consist of tubulin dimers and their structure will be discussed indetail below. Intermediate filaments are made up of proteins that vary fromcell to cell. They have a high tensile strength that can be utilised in thestrengthening of microtubules in the cell.iv Actinfilaments (also known as microfilaments) consist of free monomers of theglobular protein actin organised into long helical chains. These microfilamentsallow the cell to take on many different structures and are involved inmovement and cell division. Microtubules presence has been reported in almostall eukaryotic cells, with their absence only observed in Nanochlorumv, aspecies of algae.
Figure 1: Three filamentous structures present in cells; (a) actin filaments,(b)microtubules and (c)intermediate filaments. (Open University Course- A Tourof the Cell)Microtubulesare made up of tubulin monomers (? and ?) which dimerise to form heterodimerswhich then polymerise to form protofilaments. These protofilaments assembleinto sheets which curve to form the hollow tubular structure characteristic ofthese filaments. Thesmallest building block of MTs are tubulin monomers. Two almost identical structuresof the tubulin monomer exists, ?- and ?-tubulin, with each being composed of acore of 2? sheets surrounded by ? helices. Inthe first step of MT growth an ?-monomer and a ?-monomer dimerise together spontaneouslyto form a heterodimer, a protein comprising of two different macromolecules.
These heterodimers then polymerise end-to-end in a polar fashion to form a protofilamentwith a linear structure – each 5nm in diameter. The tubulin dimer and the sizeof the protofilament can be seen in Figure 1. Protofilaments assemble parallel to each otherand as lateral interactions occur a sheet is formed. This sheet hascharacteristic inward curvature which leads to the formation of a tubularstructure.
This structure indicates that a single microtubules has been formed.The length of the MT is extended by the addition of more heterodimers to thepositive end of the protofilament. The full schematic mechanism for theformation of microtubules is shown in Figure 2. Generallythe structure formed is a left-handed three-start helical structure.
The three-startnomenclature refers to the three tubulin monomers at the bottom of the helixthat are not interacting with a neighbouring protofilament. The left-handedhelix, shown by the anticlockwise twisting of the structure, is a requirementfor the self-assembly of microtubules.vi Microtubulesgenerally consist of 13 protofilaments invivo, however the actual amount can vary from 12 protofilaments,viiin nerve cells of crayfish, to 15 protofilaments, in cockroach epidermal cellsviii.The hollow centre of the MT structure has an approximate diameter of 12-14 nm. i Fawcett, D. W.
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