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BSCI330 > Lecture 18: The Cytoskeleton > Flashcards

Lecture 18: The Cytoskeleton Flashcards

1
Q

Cytoskeleton

A
  • A system of protein filaments that provides structure and mechanical support for the cell
  • Made up of three main types of fibers:
  • > Microtubules – 24 nm in diameter, polymers of tubulin (Largest in diameter)
  • > Intermediate filaments – average 10 nm in diameter, polymers of helical proteins (large family) (Called intermediate because they are intermediate in size compared to the other components)
  • > Microfilaments – 6 nm in diameter, polymers of actin (Smallest in diameter)
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2
Q

Cytoskeletal structures

A
  • Equilibrium between the small soluble subunits and the large filamentous fibers
  • Cytoskeletal structures are constructed by the polymerization of monomeric protein subunits through noncovalent attractions
  • Disassembly and reassembly allow for changes in cell shape and/or internal movements of organelles/vesicles relatively rapidly
  • > Provides for a lot of flexibility of the cytoskeleton
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3
Q

The nature of polymerization of cytoskeletal proteins

A
  • The polymerization of cytoskeletal monomers requires nucleoside triphosphates in the form of either GTP (binds tubulin) or ATP (binds actin) – we’ll abbreviate these NTP.
  • Cytoskeletal monomers containing NTP have higher affinity for their binding partners than do cytoskeletal monomers containing NDP.
  • There is a lot of intrinsic hydrolysis within these cytoskeleton components
  • Soluble subunits are mostly bound to the NTP form, while polymers are a mixture of NTP and NDP bind subunits
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4
Q

The nature of polymerization of cytoskeletal proteins #2

A
  • New filaments are added to the positive end, so subunits on the positive end are more recently added than the ones at the negative end
  • The longer the subunit has been attached to the polymer, the more likely it will have hydrolyzed the NTP to NDP
  • > Therefore hydrolysis moves from the negative end to the positive end
  • If loss of monomers at the negative end is slower than the gain of monomers at the positive end, then the polymer grows and the polymer will shrink if vice versa
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5
Q

Actin

A
  • Makes up microfilaments
  • Composed of a network of flexible filaments dispersed throughout a cell – highly concentrated just beneath the plasma membrane (‘cortex’).
  • Form the basis of cell shape and structure
  • Form the contractile rings of dividing cells
  • Aid in the contraction of muscle cells
  • Propel vesicles and other cellular compartments through the cytoplasm
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6
Q

Actin #2

A
  • Soluble, globular protein
  • > Most abundant protein in a typical eukaryotic cell
  • > Most highly conserved proteins among eukaryotes
  • Approx. 40 kDa M.W.
  • ATP-G actin monomers bind more tightly than ADP-G actin monomers
  • > ATP-G actin in filaments eventually hydrolyze into ADP-G actin
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7
Q

Globular actin (G actin)

A
  • Actin monomers

- Polymerize into F actin

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8
Q

Filamentous actin (F actin)

A
  • Actin polymers

- 2 strands of G-actin monomers wound together into a helical filament

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9
Q

“Treadmilling”

A
  • When addition at + end of microfilaments is equal to removal at - end
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10
Q

The nature of polymerization of cytoskeletal proteins - end vs + end (Actin)

A
  • Actin monomers bound to ATP are added to the plus end of the growing filament.
  • Actin-ADP monomers are lost from the depolymerizing minus end.
  • Actin exhibits dynamic instability called “treadmilling”
  • There are many proteins that control the polymerization and depolymerization of the actin cytoskeleton
  • > There are proteins that bundle the cytoskeleton in order to make thicker fibers
  • > There are also proteins that allow the fibers to interact with membranes
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11
Q

Arp2/3 complex

A
  • Regulates the initial steps of making a new fiber (nucleation)
  • One of the slow steps in generating a new fiber
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12
Q

Thymosins

A
  • Regulate binding to the monomers to reduce the amount of free monomers in solution
  • Increases the rate of depolymerization
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13
Q

Tropomodulin

A
  • Regulates binding to the ends of the polymers to stabilize them and inhibit growth
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14
Q

Profilin

A
  • Promote the extension of polymers

- Acts like a nucleotide exchange factor by exchanging ADP- for ATP-bound monomers

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15
Q

Cofilin

A
  • Interact with the fiber and promote depolymerization

- Can speed up the hydrolysis of ATP

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16
Q

Gelsolin

A
  • Can cut the fiber to decrease the viscosity

- Also provides more ends for depolymerization and polymerization to happen

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17
Q

Fimbrin

A
  • Crosslink the cytoskeleton to provide structural stability
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18
Q

Rho-GTP

A
  • Acts as a molecular switch to control actin polymerization dynamics by regulating the activity of actin-binding accessory proteins
  • Participates in actin bundling to create bundled stress fibers
  • Stress fibers contain actin/myosin
  • Helps cells respond to stretching and compression events (stress)
  • There’s regulated control of the activation of this GTPase
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19
Q

Rac-GTP

A
  • Participates in actin polymerization
  • Creates lamellipodia (sheet-like plasma membrane projections) and membrane ruffles
  • There’s regulated control of the activation of this GTPase
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20
Q

Cdc42-GTP

A
  • Regulates similar proteins as Rac-GTP and Rho-GTP
  • Creates filopodia (tube-like plasma membrane projections) and short cell protrusions called microspikes
  • There’s regulated control of the activation of this GTPase
21
Q

Microtubules Form:

A
  • A network of rigid tubules that radiate through the cytoplasm of all eukaryote cells
  • Mitotic spindles of dividing cells
  • The core of motile appendages: cilia and flagella
  • Not distributed everywhere but radiate from the cell
22
Q

Microtubules are formed from:

A
  • Heterodimers alpha and beta tubulin
  • Diverse family of soluble, globular proteins
  • Approx. 50 kDa M.W.
23
Q

Tubulin Structure

A
  • Both alpha and beta tubulin bind GTP
  • Only beta tubulin hydrolyzes GTP
  • Tubulin heterodimers polymerize into protofilaments, which assemble into tubules (microtubules)
  • 13 protofilaments are needed to form a full microtubule (forms a hollow structure with a lumen inside)
24
Q

Tubulin Structure #2

A
  • Beta tubulin faces the positive end while the alpha tubulin faces the negative end
  • To form a microtubule, first a protofilament must be formed, which is single polymerized chain that contains repeating beta and alpha subunits
  • Can extend or make the microtubule smaller by adding or removing the alpha and beta subunits
25
Q

The nature of polymerization of cytoskeletal proteins - end vs + end (Microtubules)

A
  • The plus end contains more GTP-bound dimers than the minus end
  • The plus end of the microtubules grows by the addition of tubulin dimers bound to GTP.
  • With time the GTP is hydrolyzed to GDP.
  • The minus end of the microtubules contains more GDP and grows more slowly.
  • Tubulin dimers are lost (slowly) from this end.
  • Normally the rate of polymerization at the plus end is more rapid than the rate of GTP hydrolysis so the plus end maintains a GTP cap (tubulin-GTP dimer).
  • If the rate of GTP hydrolysis exceeds the rate of polymerization, the GTP cap is lost and the plus end undergoes rapid depolymerization (catastrophe)
  • When there’s enough GTP in the plus end, more GTP can bind more rapidly (rescue)
26
Q

Microtubule Regulation: Stability

A
  • The growth or shrinkage of microtubules can be regulated by altering the balance between the addition and removal of tubulin dimers
27
Q

Stathmin

A
  • Binds microtubule subunits and prevents their assembly
28
Q

Kinesin 13

A
  • Interacts with tubules and enhances diassembly
29
Q

Katanin

A
  • Severs microtubules
30
Q

MAP (Microtubule associated protein)

A
  • Binds alongside tubules and stabilizes them
31
Q

XMAP

A
  • Stabilizes the positive end of tubules
32
Q

+TIPS

A
  • Associate with positive ends of tubules and links them to other structures (e.g. membranes)
33
Q

Microtubule Regulation: Orientation

A
  • How is it that microtubules assemble in specific locations and orientations in a cell?
  • Assembly of microtubules has two phases:
  • > Nucleation – small portion of tubule formed at the beginning
  • > Elongation - addition of tubulins and the GTP-cap
  • Microtubule Organizing Centers (MTOC’s) play a role in nucleation
34
Q

MTOC: Classic Example - Centrosomes

A
  • The pair of centrioles in a centrosome are at right angles to each other.
  • Each centriole contains nine fibrils in a pinwheel pattern, each composed of three microtubules A,B,C
  • One found in each animal cell.
  • Divides before cell division begins.
  • A complex structure: Two barrel-shaped centrioles & Surrounded by electron-dense centrosome matrix (CM)
  • In addition to alpha and beta tubulins, centriole microtubules contain a special ring of gamma tubulins at their ends that allows nucleation of new microtubules
  • Large numbers of microtubules converge onto the centrioles
35
Q

Myosins

A
  • Are a huge family of motor proteins that bind to actin microfilaments.
  • Family can be divided into “conventional” type II myosins and “un-conventional” myosins (14 types of the latter)
36
Q

Myosin II structure

A
  • Heteromer with 6 polypeptide chains – one pair of heavy and two pairs of light chains
  • Each heavy chain contains:
  • > An “S1” head w/ATP-ase activity
  • > A “neck” region
  • > A coiled-coil tail
37
Q

Myosin II

A
  • Myosin II molecules can associate into filaments
  • These filaments are highly stable in muscle cells, they form a basic structural unit of the contractile machinery.
  • In non-muscle cells, myosin II filaments are only formed transiently, as needed to move elements of the cytoplasm around
  • Myosin II can move actin filaments by attaching to actin filaments, moving (the “power stroke”) and then detaching
38
Q

Kinesins

A
  • The first kinesin was isolated as a motor protein responsible for moving vesicles and organelles along nerve axons from the cell body to the synaptic terminals.
  • > Composed of 2 light chains and two heavy chains.
  • > Heavy chains are entwined to create a stalk region made of coiled alpha-helices.
  • The two globular “heads” of the heavy chains:
  • > Have ATP-binding sites.
  • > Bind to microtubules initiating ATPase activity and the movement of the kinesin molecule (and cargo) along the microtubule
  • > Movement of kinesin only occurs in one direction from the (-) to the (+) end of the microtubules.
  • > How is this?
39
Q

Kinesins #2

A
  • The cargo is bound to the other end of the stalk, the tail region that also contains the light chains.
  • Cargos may include vesicles, protein complexes and organelles, which are bound to specific members of the kinesin family by adaptor proteins
40
Q

Dyneins

A
  • Another family of microtubule based motor proteins
  • Dimer of heavy chains with two ATP-binding “heads” and a stalk.
  • Intermediate and light chains surround the stalk. The light chains can be bound to a cargo through a complex of proteins called dynactin
  • Dynein molecules move along microtubule protofilaments in a similar manner to kinesin, but from the (+) end to the (-) end of the microtubule
41
Q

Intermediate Filaments

A
  • Composed of a group of related long helical proteins
  • Provide mechanical strength to cells
  • Not present in every cell type, or even every eukaryotic organism
  • Filaments form via coiled-coil interactions of a-helical proteins
  • Proteins first form dimers, which assemble in staggered fashion to form ropelike filaments
42
Q

Studying the Cytoskeleton: Manipulation

A

Various drugs can bind cytoskeletal proteins and affect their assembly/disassembly dynamics

43
Q

Latrunculin

A
  • Binds G-actin molecules and prevents assembly of filaments
44
Q

Phalloidin

A
  • Binds F-actin filaments and prevent disassembly
45
Q

Cytochalasins

A
  • Binds positive ends of filaments and supresses filament dynamics
46
Q

Nocodazole

A
  • Binds tubulin dimers and prevents microtubule assembly
47
Q

Taxol

A
  • Stabilizes microtubules and prevents disassembly
  • Can be used to inhibit cell division
  • Chemotherapy (cell division of other cells also affected –> hair loss)
48
Q

Colchicine

A
  • Copolymerizes into the microtubule latice, which suppresses microtubule dynamics

BSCI330 (12 decks)

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