Microtubules & Filaments

By:- Arvind Tewatia

Cytoskeleton•The major structural elements of the

cytosketeton are:- • microtubules,• microfilaments

•The range of techniques to understand the cytosketeton include fluorescence microscopy, digital video microscopy and electron microscopy.

Microtubules•Microtubules were first discovered by De

Roberties and Franchi (1953) •In the axons of medullated nerve fibres and were

named neurotubules. Porter(1955)•Term “Microtubules” was given by

Slautterback (1963)

•Microtubules are electron-microscopic structure.•Found only in Eukaryotic cellular structures.•But absent in mature mammalian erythrocytes,slim moulds and prokaryotes•Microtubules (MTs) are largest element of the cytoskeleton. •Cytoplasmic microtubules:- 1) maintain axons; 2) maintain shape; 3) orient cellulose microfibrils (in plants); 4) mitotic and meiotic spindles for chromosome movements; and 5) vesicle movement. •Axonemal microtubules:- highly organized, stable microtubules in specific movement associated subcellular structures (cilia, flagella and attachment basal bodies). The central shaft (axoneme) consists of a highly ordered bundle of axonemal MTs (see sperm tail axoneme).

Microtubule Structure

Microtubule Structure:- is made up of heterodimers of alpha-tubulin and beta-tubulin (form an alpha-beta-

heterodimer).

•An a, b-tubulin heterodimer is the basic structural unit of microtubules.

•The heterodimer does not come apart, once formed.

a b GTP GTP

tubulin heterodimer

•The alpha-tubulin and beta-tubulin molecules are 4-5 nm in diamater and 55 kDa, have almost identical shapes but only share 40% amino acid sequence identity.•A microtubule is a hollow cylinder of 13 protofilaments around a lumen with an outer diameter (25 nm), inner diameter (15 nm) and dimer width (8 nm).•Singlet microtubules are 13 protofilaments;doublets are 13 plus 10 (or 11)[in cilica or flagella] and •Triplets are 13 plus 10 (or 11) plus 10 (or 11) •[in basal bodies or centrioles].

•A microtubule is a hollow cylinder, about 24 nm in diameter. •Along the microtubule axis, tubulin heterodimers join end-to-end to form protofilaments, with alternating a & b subunits. •Staggered assembly of 13 protofilaments yields a helical arrangement of tubulin heterodimers in the cylinder wall.

seam

microtubule 3-start helix

b-GDP a-GTP

b-GTP a-GTP

+

Note :-Electron microscopy of microtubules decorated with motor protein heads indicate a "3-start helix.“Each turn of the helix spans 3 tubulin monomers (e.g., a, b, a).

•Microtubules form by addition of tubulin dimers at the ends. New microtubules form nucleation centres (oligomers) and grow by addition of subunits on either end: elongation. Critical concentration is when disassemby and assemby is exactly balanaced: treadmilling. The plus end: rapidly growing end; the minus end: the slower growing end. •Microtubule-organizing centre (MTOC) •serves as a site for initiation and as an anchor.•A centrosome (animals cells) has two centrioles surrounded by pericentriolarmaterial. •The microtubule polarity can vary with cell function.•Microtubule-associated proteins (MAPs) act as stabilizing and bundling proteins.

Centrioles at right angles, as in the centrosome.

cartwheel structure at one end

Microfilaments

Microfilaments were given by PALEVIZ et. al.(1974)

•Microfilaments (MFs) are smallest element of the

cytoskeleton.•Thread-like protein fibers•Found only in the cytoplasm of all eukaryotic cells•This are present in microvilli, muscle fibres. In muscle fibres these are called myofilaments •They are 3-7nm in Diameter •This are long, narrow, cylindrical, contractile & proteinous structure•These are absent in prokaryotes

Functions1) As cytoskeleton:- 1) Actin filaments are interlinked by actin-binding proteins like Fimbrin & Villin while these are linked to the micrevillin membrane by another actin-binding protein calmodulin 2) The arrangements of proteins make the microfilaments make a rigid structure which provide mechanical support to maintain the form of microvilli2)Movement of cell membrane:- MF act as a cell muscles & help in the cell membrane movement which also help in endocytosis through the cell membrane

3)In movement & locomotion :- Microfilaments of the myofibers of the muscles fibres help in movement & locomotion during Actomyosin complexes. These are formed b\w the heads of thick myosin and actin sites of thin actin filaments in the presence of energy released during hydrolysis of ATP by myosin & ATPase enzyme in presence of calcium ion released form yhe depolarized sarcoplasmic recticulam The head of myosin filaments undergo swivelling(rotaion) which pull the actin filament inward which inward over the myosin filament. This is called sliding-filament theory

Difference B\W Microtubules• These are non-contractile• A microtubule contain 13

protofilaments• They are hallow tubules• Microtubules are formed of

a & b tubulin proteins.• The diameter of

microtubule is 25 nm

Microfilaments• These are contractile.• These do not posses

longitudnal sub-units.• Microfilaments are solid

structures.• Microfilaments are mainly

made up of actin.• The diameter of a

miocrofilaments is 5-6 nm.•