BIOL 200 (Section 921) Lecture # 11 [July 4, 2006]. UNIT 8: Cytoskeleton Reading : ECB, 2nd ed. Chap 17 . pp 573-606; Questions 17-1, 17-2, 17-12 to 17-23. ECB, 1st ed. Chap 16 . pp 513-542; Questions 16-1, 16-2, 16-10 to 16-21. UNIT 8: Cytoskeleton - Objectives.
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UNIT 8: Cytoskeleton
a cell’s shape, strength and movement [Fig. 1-27]
Actin Microfilaments- helical polymers involved in movement/shape
Microtubules-big hollow tubes support cell structures
Intermediate Filaments-tough ropes
the cytoplasm of the cell [Fig. 17-3]17_03_Interm_filaments.jpg
Intermediate keratin filaments (green,
fluorescent) from different cells are
connected through the desmosomes
A drawing from the electron
micrograph showing the bundles
of intermediate filaments through
Assembly of intermediate filaments involves coiled coil dimers[Fig. 17-4]
-The growing end of the microtubule (MT), at the top, has subunits arranged with the beta-tubulin on the outside. The subunits in the microtubule all show a uniform polarity
or fixed end
Fig. 16-11 Alberts MBOC- subunits arranged with the beta-tubulin on the outside. The subunits in the microtubule all show a uniform
GTP-bound tubulin packs efficiently into protofilament
GTP hydrolyzes to GDP in MT
GDP-bound tubulin bind less strongly to each other-depolymerize MT
The subunits arranged with the beta-tubulin on the outside. The subunits in the microtubule all show a uniform centrosomeis the major MT-organizing Center. It contains nucleating sites (rings of of γ-tubulin) which serve as starting point for growth of MTs [Fig. 17-11]17_11_centrosome.jpg
Centrioles are arrays of short MTs and are identical to basal bodies.
GTP cap leads to stability and growth of MTs subunits arranged with the beta-tubulin on the outside. The subunits in the microtubule all show a uniform
Dynamic instability (loss of
GTP cap) leads to MT shrinking
Sister chromatids separate at anaphase [Fig. 19-17] metaphase [Fig. 19-13]
Each microtubule filament grows and shrinks metaphase [Fig. 19-13]
independent of its neighbors [Fig.17-12]17_12_grows_shrinks.jpg
A model of microtubule assembly metaphase [Fig. 19-13]
[Becker et al. The World of the Cell]
The selective stabilization of MTs can metaphase [Fig. 19-13]
polarize a cell [Fig. 17-14]17_14_polarize_cell.jpg
Motor proteins [Dynein and Kinesin] transport vesicles along MTs in a nerve cell [Fig. 17-15]
Nerve cell polarity maintained by microtubules
Motor proteins [Dynein and Kinesin] move along MTs using their globular heads [Fig. 17-17]
Motor proteins transport their cargo their globular heads [Fig. 17-17]
along MTs [Fig. 17-18]17_18_motor_proteins.jpg
Kinesins move ER outward and Dyneins move Golgi inward to maintain cell structure [Fig. 17-23]
Kinesin walks along a MT [Fig. 17-22] maintain cell structure [Fig. 17-23]17_22_kinesin_moves.jpg
Moves in a
8 nm steps
Ciliated epithelium in airway [Fig. 17-24]
Flagella propel a sperm cell [Fig. 17-26]
MTs in a cilium or flagellum are arranged maintain cell structure [Fig. 17-23]
In a “9 + 2” array [Fig. 17-27]17_27_9_+_2_array.jpg
The movement of dynein causes bending of flagellum maintain cell structure [Fig. 17-23]17_28_dynein_flagell.jpg
Actin Filaments [Fig. 17-2] maintain cell structure [Fig. 17-23]17_02_03_protein_filament.jpg
Distribution of actin filaments in different cells maintain cell structure [Fig. 17-23]
Determines their shape and function [Fig. 17-29]17_29_Actin_filaments.jpg
Intestine (increase surface area)
and fingerlike (filipodia)
of a moving cell [important
in cell crawling, endo- and
during cell divn.
Two F-actin strands wind around each other to form an actin filament [Fig. 17-30]17_30_protein threads.jpg
Actin with bound ATP
Actin with bound ADP
Phalloidin: A cyclic peptide from the death
cap fungus, Amanita phalloides, inhibits
the depolymerization of actin, thereby
stabilizing actin microfilaments
Cytochalasin D: A fungal metabolite,
Inhibits the polymerization of actin
Actin-binding proteins regulate the behavior of actin filaments [Fig. 17-32]17_32_Actin_binding.jpg
(e.g. thymosin and profilin)
Actin polymerization pushes cell edge forward, contraction pulls cell body along [Fig. 17-33]
Actin in amoeboid movement of a fibroblast [Fig. 17-34] pulls cell body along [Fig. 17-33]
Filopodium grows by nucleation of actin microfilaments pulls cell body along [Fig. 17-33] [Fig. 16-29, ECB 1st ed.]
nucleation complex at PM
Association of actin and actin related proteins pushes forward lamellipodium17_36_actin_meshwork.jpg
The head group of myosin I walks towards
the plus end of the actin filament.
Myosin I: Move a vesicle relative to an actin filament.
Myosin I: Move an actin filament.
form myosin filaments [Fig. 17-40]17_40_Myosin_II.jpg
Bipolar myosin filament
Roles of actin-dependent motor protein, myosin II [Fig. 17-38
Myosin II: Regulate contraction – move actin filaments relative to each other.
The head group of myosin II walks towards the plus end of the actin filament.
Myofibrils made up of actin and myosin II packed into chains of sarcomeres [Fig. 17-42]
Muscle contraction depends on bundles
of actin and myosin
Sarcomeres (contractile units of muscle) are arrays of actin and myosin [Fig. 17-43]
Z disc: attachment points
For actin filaments
The myosin heads walk toward the plus end of the adjacent actin filament
driving a sliding motion during muscle contraction.
1. The Myosin head 17-44]
tightly locked onto an
2. ATP binds to the myosin
head. The Myosin head released from actin.
3. The myosin head displaced by 5 nm. ATP hydrolysis.
4. The myosin head attaches
to a new site on actin filament.
Pi released. Myosin head
regains its original
conformation (power stroke).
5. The myosin head is
again locked tightly to
the actin filament.17_45_myosin_walks.jpg
Modern microscopy techniques
Drugs and mutations to disrupt cytoskeletal structures
Becker et al. The World of the Cell