Biology 225

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Cytoskeleton

Back 1

a dynamic network of protein filaments (and associated proteins in eukaryotic cells). Composed of mictrotubules, microfilaments, and intermediate filaments. Capable of dramatic and rapid reorganization, and varios motile activites

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Functions of the cytoskeleton

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1. Structure and support
2. Intracellular transport
3. Contractility and motility
4. Spatial organization

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Microtubules

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assembled by polymerization of aB-tubulin heterodimers, can be found as singlets, doublets, or triplets. Diameter ~25 nm, wall thickness ~5 nm, 13 parallel protofilaments

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Protofilaments of microtubules

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are staggered by ~1 nm, creates a helical array, composed of alternating aB-tubulin heterodimers, each subunit repeats every 8 nm

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MTOCs

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microtubule organizing centres: the in vivo nucleation sites for MT polymerization. All MTOCs play similar roles and some examples are spindle poles, basal bodies, and centrosomes. All have gamma-tublin

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Spindle poles

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prior to mitosis, animal cells re-arrange their cytosolic MTs and duplicate their centrosomes to form two MTOCs. These are at opposite cell poles and are used to generate the mitotic spindle apparatus

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Basal bodies

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not present in plants or fungi, basal bodies are used in the generation of cilia and flagella, similar to structure in the centrioles that are often found in animal cell centrosomes, a single basal body can be found at the base of each cilium or flagellum

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Centrosomes

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most common MTOC of animals (not present in plants or fungi), located near the cell center, generates a radial array of MTs

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Colchicine

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a microtuble poison that binds tightly to free tubulin and prevents assembly

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colcemid

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reversible binds free tubulin and prevents assemble for cell cycle work

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taxol

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binds tightly to MTs and prevents disassemble. Can preferentially kill cancer cells.

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Myosins

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belong to a large superfamily of molecular motor proteins specific for F-actin

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Conventional (type 2) myosins

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-powers cardiac, skeletal, and smooth muscle
-dimer: 2 heavy chains and 2 pairs of light chains
-head groups are the motor domains

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Focal complexes

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When a cell is crawling along a substratum; the three contact points shown represent these. The cell is able to crawl by actin filament polymerization and depolymerization

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Three isoforms of actin

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α-actin, β-actin, and γ-actin

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α-actin

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major isoform of actin found in muscle cells

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β-actin, and γ-actin

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the isoforms that predominate in non-muscle cells, are spatially segregated in cells. γ-actin forms dense, compact multiple-filament stress fibres in the cell body where as β-actin forms a loose dendritic network in the lamenellopodia at the leading edge of the cell

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Arginylation

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the covalent addition of the amino acids arginine (Arg) to a protein, is a widespread posttranslational modification catalyzed by the enzyme arginyl-tRNA-protein transferase 1

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Arginine (Arg)

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is a positively charged (basic) amino acid, the covalent addition of an Arg residue imparts an extra positive charge to the target protein ( β-actin)

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Dyneins and kinesins

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Molecular motor proteins for microtubules

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Myosins

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Molecular motor proteins for microfilaments

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Are the 13 protofilaments of a MT held together by covalent bonds or noncovalent interactions?

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Noncovalent interactions

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What are some chemical ways of
inducing the disassembly of MTs?

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Treatment of cells with vinblastin, vincristine, podophyllotoxin, nocodazole, and cochicine

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Protofilaments

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The walls of microtubules are composed of these globular proteins arranged in longitudinal rows

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What are three nonchemical ways of inducing disassembly?

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Cold temperatures, hydrostatic pressure, and elevated Ca2+ concentration

Front 26

If you polymerized some MTs in vitro but then lowered the temperature, what would happen?
If you then raised the
temperature back to what it was, what would happen?

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They would disassemble, but if the temperature was raised back to what it was they would reassemble

Front 27

MTOCs

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Microtubule organization sites. Examples include spindle poles, basal bodies, and centrosomes.

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MAPs

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Microtubule associated proteins. Important in long-term stability and protection, many MAPs interconnect MT to help form bundles. Basically they increase MT stability, alter MT rigidity, and or influence MT assembly rate.

Front 29

Explain how MAPs can afford different stability

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Flagellar MTs are very stable to changes in pressure and temperature; presence of colchicine, where as spindle MTs are very sensitive to these conditions

Front 30

Three microtubule poisons

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1. Colchicine (binds tightly to free tubulin and prevents assembly)
2. Colcemid (reversibly binds free tubulin and prevents assembly)
3. Taxol (binds tightly to free MTs and prevents disassembly)

Front 31

Either colchicine or colcemid can be used to halt the progression of mammalian (and other) cells through the cell cycle by stopping them in metaphase of mitosis. If you wanted to synchronize cells in the cell cycle so that when
the drug was removed all of the cells would finish metaphase and proceed through mitosis, which of these two drugs would be best to use and why?

Back 31

Colcemid, because it reversibly binds. (Not sure though, double check answer)

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Function of microtubules

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1. Internal skeleton (scaffolding) providing structural support to the cell.
2. Organelle placement
3. Movement of organelles and vesicles
4. Motion of cilia and flagella
5. Active components of mitotic and meiotic machinery

Front 33

What molecular motor
proteins are responsible for
the motion of cilia? of
flagella? Do those motor
proteins walk along
microtubules, microfilaments,
or intermediate filaments?

Back 33

The machinary for ciliary and flagellar locomotion resides within the axoneme. Ciliary (or axonemal) dynein. No, they do not walk. The dynein arms release energy required for locomotion. They act as swinging cross-bridges that generate the forces required for ciliary or flagellar movement.

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Types of cargo transported by MTs

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1. Vessicles
2. Other organelles
3. Chromosomes
4. Other MTs

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Kinesins

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Are tetramers of 2 identical heavy chains and 2 identical light chains and they are (+) end walking

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Dyneins

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(~1.5 x 106 Da) in
comparison with kinesins (~380 kDa). Composed of 2 heavy chains, and a variety of intermediate and light chains for binding a variety of cargo. They walk toward the minus (-) end

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Microfilaments

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Composed of actin filaments (F-acting) and unpolymerized globular subunits (G-actin). A double right-handed helix of polymerized actin and strands are parallel. Exhibit structural polarity, diameter ~8nm, and pitch ~37nm

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Microfilament Polymerization

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occurs fastest at (+)-end, are ATP-binding with with ATPase activity, after polymerization ATP is hydrolyzed to ADP, which reduces stability.

Front 39

The mechanical cycle of dyneins and kinases

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Both employ ATP-dependent conformational changes, cross bridges are formed and borken between these molecular motar proteins and MTs:
The motor protein binds ATP, then hydrolyzes ATP, and then binds and sequentially releases the hydrolysis products (Pi and then ADP), and then cycle begins again with binding of a new ATP

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Treadmilling

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ocurres when a stead state has been reached in which the addition of actin subunits at the plus-end equals the release of subunits at the minus-end in actin microfilaments

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If the concentration of G-actin
was maintained at a
high level would
polymerization continue at
both ends of the filament?

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Not sure

Front 42

Cytochalasins

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microfilament poison, which promote depolarization of F-action by capping at the (+)-end. Basically, this prevents polimarization at the (+)-end and promotes depolimarization at the (-)-end

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Phalloidin

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a fungl cyclic peptide of the death cap mushroom, binds F-actin locking the subunits together and prevents depolymerization

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Latrunculin

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a microfilament poison from sponges that binds G-actin preventing its incorporation wtih F-actin

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Functions of microfilaments

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Ameboid movement, muscle contraction, cytoplasmic streaming, formation/structure of pseudopodia and microvilli, and cleavage furrow formation

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Myosins

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belong to a large superfamily of molecular motor proteins specific for F-actin. There are at least 18 different types from many different eukaryotes. All myosins (except myosin VI) walk towar the plus(+)-end of F-actin

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motor head domain of myosins have

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An actin binding site
ATP, ADP, and Pi binding sites
ATPase activity

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Unconventional myosins

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belong to type II (e.g. myosin II first discovered in skeletal muscle)
-Consists of two heavy chains and two light chains
-Head group of the two heavy chains powers the motor domain
-Powers cardiac, skeletal, and smooth muscle contractions

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What is the principle difference between focal adhesions and focal complexes?

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Focal adhesions are clusters of integrins that connect the ECM to the actin-containing microfilament system of the cytoskeleton, where as focal complexes show the adhesion of the lower surface of the lamellipodium to the substratum, an attachment that is mediated by integrins residing in the plasma membrane (the attachment points when a cell crawls over a substrate)

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Keritins

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Clase I and II of intermediate filaments, and belong to epithelial cells

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Intermediate filaments clase III

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Vimetins and related proteins of connective tissue, muscle tissue, and neuroglia

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Intermediate filaments clase IV

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neurofilaments of neurons

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Nuclear IFs

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Clase V... lamins of the nuclear lamina

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Sarcromere

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Contractile unit of a myofibril

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During skeletal muscle relaxation, calcium ions are

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pumped out of the synaptic terminal knobs of neurons, pumped into the sarcoplasmic reticulum, pumped out of the muscle cell cytosol into the extracellular fluid.

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In skeletal muscle, the myosin cross-bridging cycle occurs when

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cytosolic [calcium] is low, tropomyosin molecules move toward the grove of actin filments, and troponin C (TnC) molecules bind calcium