STRUCTURE AND FUNCTION OF MICROTUBULES NAME – RISHIKESH DASH ROLL NO. – 2017MBI026 CELL: Cell is basic structural, functional and biological unit of all living organism. CYTOSKELETON: It consist of network of protein filaments extending throughout the cytoplasm of eukaryotic cells. It provides a structural framework for the cell, serving as a scaffold that determines cell shape and general organization of the cytoplasm. It is composed of three principal type of filamentous structure i.e. • Microtubules • Microfilaments • Intermediate filaments MICROTUBULES: • Microtubules were first observed by DeRoberties and Franchi in 1953 in nerve fibers. • The term ‘microtubule’ was introduced by Slautterback in 1963. • These are the principal component of the cytoskeleton present in eukaryotic cells. • These are dynamic structure that undergo assembly and disassembly within the cell. • They function both to determine cell shape and in cell movement, the intercellular transport and the separation of chromosome during cell division. STRUCTURE OF MICROTUBULES: • Microtubules are hollow tubular structures that are assembled from the protein tubulin. • Microtubules are composed of a single type of globular protein called tubulin. • They have an outer diameter of 25nm. • The building blocks of microtubules are tubulin dimers consisting of two closely related 55kd polypeptides:α tubulin and β tubulin. • The wall of a microtubule is composed of globular protein i.e. tubulin arranged in longitudinal rows, termed protofilaments that are aligned parallel to the long axis of the tubule. • Non covalent interactions between adjacent protofilaments are found. • Microtubules are seen to consist of 13 protofilaments aligned side by side in a circular pattern within the cell. • Each protofilament is assembled from dimeric building blocks consisting of one α-subunit and one β-subunit in head-to-tail array. • Microtubules are polar structure with 2 distinct ends: a fast growing plus end and a slow growing minus end. • The plus end is terminated by a row of β-tubulin subunit and the minus end is terminated by a row of αtubulin subunit. POLYMERIZATION: • Tubulin dimers can polymerize as well as depolymerize and microtubules can undergo rapid cycles of assembly and disassembly. • Both α and β tubulin binds GTP which functions analogously to the ATP bound to actin to regulate polymerization. Treadmilling: • Assembly of tubulin dimers requires that a GTP molecule be bound to the β-subunit and sufficient concentration of Mg2+ at 37̊ C. • The different growth rates of the plus and minus end of microtubules is due to diff. required critical conc. of tubulins. • The critical conc. for the plus end is lower than the minus end. • When the free tubulin conc. is higher than the critical conc. for the plus end but lower than the critical conc. for the minus end, assembly will occur at plus end while disassembly takes place at the minus end. • The simultaneous assembly of tubulin molecules at the positive end and disassembly of tubulins from the minus end is known as treadmilling. • It keeps the polymer length unchanged. DYNAMIC INSTABILITY: • In microtubule, rapid GTP hydrolysis also results in behavior known as dynamic instability. • Here the individual microtubules alternates between cycles of growth and shrinkage. • Whether a microtubule grows or shrinks is determined by the rate of tubulin addition relative to the rate of GTP hydrolysis. • As long as new GTP bound tubulin molecules are added more rapidly than GTP is hydrolyzed, the microtubule retains a GTP cap at its plus end and microtubule growth continues. • If the rate of polymerization slows, the GTP bound to tubulin at the plus end of the microtubule will be hydrolyzed to GDP. If this occurs, the GDP bound tubulin will dissociate resulting in rapid depolymerization and shrinkage of the microtubule. • This switch from growth to shrinking is called a catastrophe. CENTROSOME: • It is principal microtubule organizing center in animal cells, which is generally located near the nucleus. • • • • The centrosome consists of two barrel shaped proteins called centrioles. 9 triplets of microtubule arranged side by side to form a centriole. The centriole is surrounded by dense granular(amorphous) material called pericentriolar material. Centrioles perform two functions: They are required for centrosome duplication and They recruit pericentriolar material for nucleation of microtubules. • During interphase, the centrosome matrix nucleates a cytoplasmic array of microtubules, with their fast growing plus ends projecting outward towards the cell periphery and minus ends associate with the centrosome. • The centrosome matrix contains the γ-tubulin ring complex which is the component mainly responsible for nucleating microtubules. • The α-tubulin of the heterodimer bind to a ring of γ-subunits. Pericentriolar Material Centrioles Triplet Microtubule Centrosome α-Tubulin γ-Tubulin γ-Tubulin Ring Complex Centrioles MICROTUBULE MOTOR PROTEINS: • Motor proteins are that proteins which move organelle and other cellular cargo on microtubule. • Mainly two motor proteins are associate with microtubule i.e. Kinesin and Dynein. Kinesin: • Kinesin is a tetramer, consist of two identical heavy chains and two identical light chains. Kinesin also have a pair of globular head that bind a microtubule. • Kinesins are plus end directing motor proteins. They move from minus end to plus end along with microtubule by generating a force by hydrolysing ATP molecule. • Cell organelle and cellular cargo are bind to light chain and move from centrosome towards the cell periphery. Dynein: • Dynein are composed of one or two or three heavy chains and a large and variable number of associated intermediate light chains. • They also have a stalk on head, heavy chains and light chains are connected by a stem. Stalk is bind to microtubule. • Dynein are minus end directing motor proteins. They move from Plus end to minus end along with microtubule by generating a force by hydrolysing ATP molecule. • They move vesicles and organelle from cell periphery towards the centrosome. • Dynein are the largest among the known motor proteins and they are also fastest among other motor proteins CILIA AND FLAGELLA: • Cilia and flagella are hairlike projections on the surface of the cells. Cilia are short while flagella are long in structure. • Cilia are about 2-10 μm in long while flagella are 10-20 μm in long. • Beating motion of cilia and flagella is different, cilia helps cell to move one place to another while flagella propel cell through fluid. • Cilia and flagella are microtubule based structure. Both share the same structure called an axoneme, which is constructed on basal body. • Axoneme is composed of microtubules and their associate proteins. • The microtubules are arranged in a characteristic "9 + 2" pattern in which a central pair of microtubules is surrounded by nine outer microtubule doublets. • The two fused microtubules of each outer doublet are distinct: One (called the A tubule) is a complete microtubule consisting of 13 protofilaments; the other (the B tubule) is incomplete, containing only 10 or 11 protofilaments fused to the A tubule. • The outer microtubule doublets are connected to the central pair by radial spokes and to each other by links of a protein called nexin. • The central microtubule are complete and covered by a sheath. The axoneme is surrounded by a membrane that continuous with the plasma membrane of a cell. • Axonemal dyneins are major motor protein present in cilia and flagella. • Dyneins are tightly anchored to the outer surface of the A tubules, with globular heads and stalks projecting towards the B tubule of the neighboring doublets. • Microtubules in adjacent outer doublets slide relatively to one another by hydrolyzing ATP molecules. • The sliding movement is converted to localized bending with the help of nexin bridges and radial spokes. • The bending takes the form of wave that begins at the base of flagella and proceeds towards the tip. FUNCTIONS OF MICROTUBULE: Cell shape: • • • • Long chain of tubulin pairs or dimers organize into a hollow tube shape. The shape of the tube give it strength. It can resist a lot of compression force and still maintains its hollow shape. This strength allows microtubules to maintain the overall shape of the cell. Intracellular transport: • • • • • Microtubule forms a dense network that stretch to every corner of the cell. This web network not only provides a cell structure but also functions as a transport system. The cell is full of organelles and molecules. Some structures within the cell need to move around to complete their function. Vesicles that bud off the Golgi hitch a ride on special motor proteins that travel along microtubules all over the cell. • Like a railroad track, the microtubules allow objects to move around inside a cell. • Motor proteins walk along microtubules pulling their cargo with them. • The Golgi vesicles, the cargo are pulled along microtubule tracks to the cell membrane. Chromosome movement: • Microtubules also play an important role in the movement of chromosome during cell division. • They anchor to structures called kinetochores on each chromosome and microtubules shorten to pull chromosome pair apart. REFERENCE: • Cooper, Geoffrey M, and Robert E. Hausman. The Cell: A Molecular Approach. Washington, D.C: ASM Press, 2009. • Videos and pictures were belongs to respected owner of the pictures and videos.
