Sliding Movement Mikrotubulus

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last change: 07/31/2003 20:50:00

Microtubules

Microtubules are components of all eucaryotic cells. They participate in a range of motionprocesses, like:

derthe movements of flagella and cilia,

themovements of chromosomesduring meiosis and mitosis and

thetransport of granules and vesicleswithin the cells that effects cell wall formation,shape and specialization of the cells.

Microtubules can form regular complex structures. All flagella and cilia of eucaryotes arecharacterized by the so-called '9+2' structure.

The microtubuli of plant cells in their interphase state are closely below the plasmalemma (theyare cortical) and their orientation correlates strongly with that of the cell wall's cellulose fibrils.1963 proved to be a decisive year for their research. For the first time was glutaraldehyde usedby D. B. SLAUTTERBACK as well as M. C. LEDBETTER and K. R. PORTER (Rockefeller
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University, New York) for the fixation of electron microscopic preparations. As a consequencewas the microtubuli structure stabilized and easier to describe. In the electron microscope dothey look like characteristic tubules with an outer diameter of 240 , and an inner one of 140 ,consequently are their walls 50 thick. Microtubules consist, as has been mentioned before, oftubuline subunits polymerized as helices. 13 tubuline dimers are required for one helical turn.

Cilia and flagella have two tubules: a complete A and an incomplete B tubule, which differ in thelateral bindings between the tubuline dimers (alpha - beta, beta - alpha in A; alpha - alpha, beta- beta in B).

Microtubuli are tubulesconsisting of polymerizedtubuline. Tubuline is aprotein dimer out of analpha and a beta subunit.The left tubule (surfaceview) is of the A type, the

one to the right is type B(according to R. E.STEPHENS, K. T. EDDS,1976).

Just as microfilaments are microtubuli, too, associated with numerous proteins. The best-knownis dynein (an ATPase) required for the energy transformations of movements. Furthermore existsa whole group of proteins collectively called MAPs (microtubule associated proteins), most of

which seem to have regulatory functions.Usually exists polymerized and depolymerized tubuline in the cytoplasm in parallel, while onlythe polmerized form is found in the flagella.Colchicine(the alkaloid of the naked ladyColchicum autumnale), a low temperature and excess calcium ions support depolymerization.The alkaloid of the periwinkle (Vinca rosea), calledvinblastine, precipitates tubuline.

It seems as if each cell would have control mechanisms at its disposal that regulate the initiationof microtubuli formation, their polymerization and depolymerization as well as their orientationwithin the cell. In contrast to microfilaments exist enough studies about their significance forplant cells. Antibodies against animal tubuline react with plant one, too. The close serologicalrelation points out that they changed only little during evolution and that their significance forthe cell has been established early. For reasons of simplicity is their part in flagellar and ciliamovements discussed first, afterwards is their role in the changes occurring during the cell cycleexplained.

Movements of Flagella and Cilia
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Strictly speaking are the cilia and flagellar movements of eucaryotes intracellular movements;for although a flagellum (or a cilium) seems to be an appendage of the cell, is it surrounded bythe plasma membrane. All dynein molecules along the whole length of the microtubule havecontinuously to be supplied with sufficient amounts of ATP. Both ion milieu and pH of theirsurrounding has to be right.

P. SATIR (1968, 1976 at that time at the University of California, Berkeley) described theprocess of movement as a sliding filament mechanism where the peripheral tubuline doubletsslide past each other. In the course of this contacts the dynein that is always anchored to the Atubule the B tubule of the neighbouring doublet with its tips.

This mechanism explains theforward and back strokesof the cilium but not the numerousvariations of cilia movementsfound mainly with protozoa. There exist pulling and pushingflagella, as well as tinsel-type flagella. There exist flagella that rotate around an imaginary axisand flexible flagella where the movement spreads wave-like along the axis. Many motile cellscan change into forward or reverse gear or can carry out more or less strong corrections of the

course.

Cilia differ from flagella only in their number per cell. They are usually quite short and coveroften the whole surface of a cell. Cilia are rare in plants, an often cited example are the zoosporesofVaucheria sessilis..

With algae (except red algae) are flagellated stages common. They are often found with thespermatozoids (male germ cells) of mosses and ferns. Early during the evolution of seed plantswere flagellated stages more and more driven out. Among the few still existing exceptions arethespermatozoids ofGingko bilobaand the cycads.

Movements are often controlled by extern signals. Many protists are attracted by certain sourcesof stimulation called taxis: light (phototactic behaviour) or certain chemicals (chemotacticbehaviour). Usually follows the motion a concentration or intensity gradient. If a threshold ofsensation is passed, begins a reverse reaction. During the last decades was signal recognition amuch-studied topic. We know, for example, that the carotenoids within thestigmaof some algae(Euglena, for example) are sensitive to blue light. Thechloroplast movementsof the algaMougeotia are controlled by thephytochrome systemand germ cells (of algae) react to speciesspecific sexual attractants. But how the perceived signal is converted and how signals of thesame or the opposite kind are co-ordinated in a directed movement is not even basicallyunderstood (black box).

The basis of many flagella is equipped with a complexly structured basal body. M.MELKONIAN (Botanisches Institut der Universitt Kln) analyzed the basal bodies of a numberof algae and found group-specific patterns. He rated these structures and their variations as traitsthat help to understand the family relations of the single groups of algae significantly.

Cytoplasmatic Microtubuli and Cell Morphogenesis
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It is long since known that microtubuli constitute the biggest part of the mitotic spindle and thatthey cause the chromosomal migrations. Their whereabouts during interphase remained amystery for long times. A lot of electron microscopic preparations fixed with glutaraldehydewere screened. And again and again did it show that microtubuli are found mainly in theperipherous plasma. In many cases corresponds their arrangement to the layering of the cellulose

microfibrills of the developing primary wall (J. D. PICKETT-HEAPS and D. H. NORTHCOTE,1966). The assumption that theirorientationdetermines the orientation of the wall's microfibrillssuggested itself. Despite several arguments against it, does it seem as if this assumption is right.But the existence of the microtubuli is neither cause nor prerequisite of cell wall formation: cellstreated with colchicine develop cell walls. Interestingly are the microfibrills of these wallsarranged statistically and not well-ordered. That is why it was at first assumed that themicrotubuli participate only in the transport of material required for cellulose synthesis. Anotherpossible assumption would be that they arrange the cellulose synthase molecules within theplasma membrane and that the specific cellulose fibril pattern is a consequence of this (B. A.PALEVITZ and P. K. HEPLER, 1976). Observations of several algae and of root hairs point atsuch a connection. Microtubuli within the plasma can be in contact with each other, with the

plasmalemma and/ or with the nuclear envelope.

In plasma tubules take microtubuli part in the shifting of secretory vesicles, organelles and othercellular particles. The tight network that surrounds the migrating generative nucleus is especiallyimpressive. The tip of the pollen tube (the apex) contains no microtubuli but an increasednumber of microfilaments.

A prerequisite of the division of isolated protoplasts is the formation of a cell wall. It is precededby the reorganization of the peripherous system of microtubuli in which the arrangement of themicrotubuli determines the shape of the developing cell. While the properties described so far areof a general nature, have microtubuli of specialized cells additional functions. They participate inthe control of cell wall thickening and in the lignin deposition of secondary cell walls. Treatmentwith colchicine prevents the development of the otherwise usual strip-shaped secondarythickenings instead develops - analogous to the effects on primary cell wall formation - anunorganized pattern of thickening elements (P. K. HEPLER and E. H. NEWCOMB, 1964, J.CRONSHAW, 1967).

M. M. FALCONER and R. W. SEAGULL (1985) analyzed the development of tracheids.Studies performed with the fluorescence microscope showed clearly that microtubuli occurwherever tracheary cell wall thickenings form. The pattern of these wall thickenings correspondsto the pattern of the microtubuli.

Differing wall thicknesses and changes of the cell's shape like those found in the development ofstomata could be caused by the uneven distribution of intracellular microtubuli duringmorphogenesis. They are more frequent at sides with thickened walls (the periclinal walls) thanat those with thin walls (the anticlinal walls).

The number of microtubuli correlates with an accumulation of dictyosomes so that it seems as ifthey were actually required for the lining up of material transport and the distribution of cell wallmaterial within the cell (B. A. PALEVITZ and P. K. HEPLER, 1976, B. GALATIS, 1980). In
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some algae (Hydrodictyon,PediastrumandStaurastrum, for example) do they influence thecell's shape and colony development (H. J. MARCHANT and J. PICKETT-HEAPS, 1974, H. J.MARCHANT, 1979). And inMicrasterias do they take part in the postmitotic migration of thenucleus (O. KIERMEYER, 1972), though they have no influence on the cell's shape. In contrastis the arrangement of the microtubuli in the flagellate Ochromonas that has no cell wall decisive

for the development of the specific cell shape; acorresponding order can be seen in the pellicleofEuglena, where the microtubuli participate in the constant change of the cells' shapes('metabolic movements').

Since both microtubuli and microfilaments partake in intracellular plasma movements, themigration of organelles and other particles arises the question which of the two systems isprimarily responsible for a certain movement. The answer is species specific. D. MENZEL(Universitt Heidelberg, 1987) discovered that the movements of organelles of the genusCaulerpa are caused by microtubuli, those ofAcetabulariaby microfilaments and that bothsystems are equally important inBryopsis.

The Mitotic Spindle

What happens in a plant cell when the preparations formitosis(ormeiosis) start? An especially striking feature of such cellsis the development of a preprophase band surrounding thenucleus. In its plane will later on develop the phragmoplast.The preprophase band forms by the shift of nearly all the cell'stubuline to this region, i.e. the tubular interphase cytosceletonis dismantled and a new one is built (P. K. HEPLER and D. H.

NORTHCOTE, 1967). Some cells like the much-studiedendosperm cells ofHaemanthus katherinae lack thepreprophase band. The question whether it is necessary inorder to move the nucleus into the right (central) position, orwhether the two processes are independent, was disputed for along time. Though it has not been settled yet, is theassumption that the two processes are independent of eachother nowadays favoured. It could also be that the position ofthe band determines the orientation of the mitotic spindle. Its

formation starts with the prophase. It is commonly agreed that the spindle consists of microtubulithough the question how it causes the chromosome movements is by far more difficult. Two

types of microtubuli exist: those that start at the poles and may stretch far beyond the equatorialplane are called polar microtubuli. They overlap so that it looks as if they would extend frompole to pole. Other microtubuli are attached to the chromosomes' kinetochores (= centromeres)and extend next to the poles or are associated with polar microtubuli. They are calledchromosomal or kinetochore-microtubuli.

Polar microtubuli initiate polymerization at the poles, while chromosomal ones start at thekinetochores.
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In animal cells can centrioles be found next to the cell poles. Condensation begins in theirvicinity. In most and especially in higher plants are they missing. They occur in some algae andappear in the meiosis but not the mitosis of theCharophyceae. At least in the development ofplant polar microtubuli are they therefore dispensable. It is not known if they are replaced bysome other structure

Numerous cytological results show that only kinetochore-attached chromosomes can be moved.This means that chromosomes must have a binding site specific for microtubuli (in chromosomeswith 'diffuse' kinetochores may it include the whole surface of the chromosome). The migrationtowards the poles is based on an interaction of polar and kinetochore-microtubuli. In allprobability are the chromosomes pulled and not pushed. The movements seems not to be basedon a simple sliding mechanism. Instead is the chromosomal movement during anaphase causedby a shortening of the kinetochore-microtubuli with simultaneous elongation of the polarmicrotubuli (McINTOSH and KOONCE, 1989).

The question of energy transduction remains to be settled. During telophase is the spindle

dismantled and the microtubuli gather in the phragmoplast area where the de novo organizationof theinterphase cytosceletonbegins. Microtubuli and microfilaments are in close vicinitytowards each other in the phragmoplast area. Whether the two systems co-operate cannot beanswered yet either.

Peter v. Sengbusch -Impressum

Protein motor berperan sebagai penggerak berbagai organela maupun protein dalam sel darisuatu tempat dalam sel menuju titik tujuan. Protein motor mampu merubah energi kimiawimenjadi energi mekanik sehingga proses pergerakan organela atau protein dapat berlangsung.

Vesikel, mitokondria, lisosom, kromosom dan beberapa filamen sitoskeleton yangpergerakannya dalam sel memanfaatkan peran protein motor tersebut1.Protein motor memanfaatkan sitoskeleton sebagai jalur pergerakannya. Dalam satu sel terdapatbeberapa puluh protein motor yang berbeda. Setiap protein motor empunyai peran spesifik untuksatu fungsi pada suatu daerah kerja tertentu. Terdapat tiga grup famili protein motor, yaknimiosin, kinesin dan dinein. Kinesin dan dinein bergerak sepanjang mikrotubulus, sedangkanmiosin merupakan motor penggerak disepanjang filamen aktin. Sejauh ini filamen antara takmemiliki protein motor. Diperlukan adanya energi untukmenjalankan fungsi protein motor.Terdapat dua proses yang berkelanjutan untuk membantu protein motor mampu menjalankanfungsinya.Kedua siklus tersebut adalah siklus kimiawi dan mekanik. Tahap proses kimiawidimulai dengan terjadinya ikatan ATP pada protein motor yang disusul dengan hidrolisis ATP,

melepaskan ADP dan Pi dari motor serta terikatnya kembali satu molekul ATP baru ke proteinmotor. Hidrolisis ATP akan menyediakan energi yang diperlukan untuk pergerakan proteinmotor tersebut1.

DINEINDinein merupakan protein motorik yang terdiri dari 2 kelas cytoplasmic dan Axonemal.Cytoplasmic dynein berperan dalam pergerakan organel yang bekerja secara bersamaan denganprotein dynactin sebagai penghubung dengan vesikel dan sebagai transport kearah retrograde
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mikrotubulus, sedangkan axonemal dinein berperan dalam pergerakan cilia dan flagella. dyneinterdiri dari 2 heavy chains bereaksi dengan Mikro Tubulus , 3 intermediate chains and 4 lightchains,2Peran dinein dalam pergerakan cilia dan flagella.Struktur motilitas dari flagella dan cilia terdiri dari microtubulus & dinein serupa rambut,

memiliki pergerakan seperti gerakan cambuk yang berguna dalam mendorong sel melewaticairan atau memindahkan cairan yang dilakukan silia beberapa sel dalam suatu jaringan.Pergerakan yang terjadi pada cilia dan flagella diakibatkan oleh adanya axonema yang terdiridari mikrotubulus dan protein yang terkait dengan proses sebagai berikut dimana sembilanmikrotubulus doublet yang tersusun dalam sebuah cincin yang mengelilingi sepasangmikrotubulus 9+2 array, disekeliling sepasang mikrotubulus . Pada posisi biasa sepanjang

mikrotubulus panjang, aksesori protein menghubungkan lintas mircotubules secara bersamaan.Dinein membentuk jembatan antara 2 mikrotubulus doublet yang berdekatkan disekitar lingkaranaksonema. Saat motor domain dari dinein teraktivasi, dinein melekat kesatu doblet untukmemulai berjalan di sepanjang mikrotubulus, kekuatan ini yang mengakibatkan doubletdoublet harus bergerak dan bergeser satu sama lainnya. Bagaimanapun juga dengan adanya

ikatan yang lain diantara mikrotubulusmikrotubulus doublet membatasi/menghentikanpergerakan ini, sehingga kekuatan dari dinein dikonversi menjadi biding motion. Energi yangdigunakan oleh dinein adalah ATP yang terdapat pada bagian heavy chain dari dinein, yangkemudian terhidrolisis menyebabkan energi sliding atau pergeseran tadi.2

KELAINAN PROTEIN MOTORSindrom Kartagener merupakan kumpulan dari gejala klinik yang terdiri dari situs inversus,bronchiectasis, dan sinusitis kronik. ditemukan oleh Kartagener pada tahun 1933. Pada tahun1970 diketahui bahwa pada penderita laki-laki sindrom ini terjadi gangguan pada pergerakanflagella sperma dan cilia saluran napas sehingga menyebabkan gejala bronkopulmoner daninfertilitas.3 Sindrom Kartagener merupakan salah satu jenis kelainan Primary CiliaryDyskinesia (PCD) atau dahulu dikenal dengan nama Immotile Cilia Syndrome (IMS). Semuabentuk PCD mempunyai karakteristik yang khas berupa dismotilitas atau imotilitas dari cilia/flagella pada saluran pernapasan , spermatozoa dan sel bersilia lainnya.3Situs inversus pada sindrom Kartagener dapat terjadi karena berbagai hal yang mempengaruhipada saat masa embrional. Pada masa embrional terdapat sebuah node /pengatur yang terdapatpada bagian distal embrio pada masa gastrulasi. Node ini penting dalam proses gastrulasi danpenentuan axis tubuh. Fungsi pengatur formasi axis ini terdapat pada lapisan endoderm bagianventral node. Pada bagian ventral ini terdapat cilia tunggal (monocilia) yang mengarahkanembrio ke cairan ekstraseluler. Monocilia mengatur aliran dari cairan ekstra embrional ke bagiankiri dari tubuh.3Apabila terjadi kelainan gen, monocilia tidak dapat berfungsi normal sehingga tidak dapatmengalirkan embrio ke axis yang benar. Kelainan gen juga dapat terjadi pada node yangmerupakan pengatur utama, sehingga tidak dapat menjalankan fungsinya dengan benar. Padakelainan gen sinyal dari node diinformasikan dengan salah ke bagian lateral mesoderm sehinggainformasi peletakan organ atau axis tubuh pun menjadi salah

Referensi1. Karp G.,2002. Cell and molecular biology. 3rd.edition. John Wiley & Son. New York.;pp: 333-96.dalam Ahmad Achmad R., Handono K., pengaruh hiperglemi terhadap peran


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sitoskeleton (cytoskeleton)sebagai jalur transduksi signal(signal transduction).2. Alberts, Jhonson, Lewis, Raff, Roberts,dan Walter. 2008. Molecular Biology of The Cell.Edisi 5. Garland Science.3. Geremek M., Witt M., 2004. Primary ciliary dyskinesia: genes, candidate genes andchromosomal regions . J. Appl. Genet. 45(3). pp. 347361.

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Sliding Movement Mikrotubulus

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