Chapter 16- The Cytoskeleton Flashcards

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1
Q

Cytoskeleton

A

A system of protein filaments- basically acts like a skeleton for the cell. It gives the cell shape and integrity, supporting the membrane and helping the cell to withstand external forces. Proteins in the membrane are often bound to the cytoskeleton, anchoring the membrane in place. Rearrangements of the cytoskeleton allows the cell to change shape and facilitate movement (leukocyte crawling, phagocytosis, and muscle contraction). The cytoskeleton guides cell growth and is responsible for the cell’s spatial arrangement and movement of organelles or vesicles throughout the cell

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2
Q

3 filaments making up the cytoskeleton in animals

A
  1. Intermediate filaments
  2. Microtubules
  3. Actin filaments
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3
Q

Intermediate filaments

A

Provide mechanical strength

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4
Q

Microtubules

A

Position organelles and direct intracellular transport. They act as “tracks” for organelles or vesicles that are moving in the cell.

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5
Q

Actin filaments

A

Determine the shape of cell surface- actin is the filament that is positioned just under the membrane, in the cytoplasm. Therefore, rearrangements of actin will help to change the shape at the cell surface. Necessary for whole cell locomotion

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6
Q

Accessory proteins

A

These proteins are often associated with cytoskeleton filaments. They link cytoskeleton filaments to cell components and to each other. Accessory proteins are also responsible for the controlled assembly of filaments and bundling them together. Includes motor proteins, which are important for moving things like organelles and vesicles along microtubule tracks (a cytoskeleton filament).

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7
Q

Types of subunits

A

Filaments are formed from subunits. Actin and microtubules (tubulin) subunits are compact, globular (round), and soluble, and they come together to form larger filamentous polymers. Intermediate subunits are fibrous and come together to form an even larger fibrous structure.

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8
Q

Assembly of subunits

A

All 3 types of subunits will come together in helical assemblies. The subunits self-associate and form side to side and end to end contacts, forming the length of the filament. Differences in subunit structures and the strength of contacts create differences in the stability and mechanical properties of each type of filament

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9
Q

Subunit linkage

A

The linkage of subunits to form filaments is non-covalent, in contrast to other polymers like DNA, RNA, and proteins. However, these non-covalent linkages allow for rapid assembly and disassembly of subunits without the need to break covalent bonds- disrupting covalent bonds can be difficult. Accessory proteins regulate the building and dynamic behavior of filaments, responding to extracellular/intracellular signals. Filaments are constantly growing and shrinking.

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10
Q

Protofilaments

A

Subunits assemble end to end into long filament assemblies called protofilaments. The protofilaments will then associate laterally (side to side) to form a mature filament

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11
Q

Filament nucleus

A

Forms if enough subunits are present to form a large aggregate of the filament. Rapid filament elongation ensues

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12
Q

Nucleation

A

The initial rate of nucleus filament assembly, when the filament is growing in size

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13
Q

FtsZ

A

A bacterial tubulin (microtubule homolog) that also acts as a structural protein. It forms filaments that assemble into a circular Z-ring structure, which is important for forming a septum during cell division (binary fission)

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14
Q

MreB & Mbl

A

Bacterial homologs to actin in animal cells. Form dynamic patches that move along the length of the cell. Their function is unclear, but this is part of what gives B. subtilis structure. If the proteins are removed, bacteria will start to clump together and will not have a proper structure

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15
Q

ParM

A

A bacterial actin homolog. It binds to ParR proteins at the origin of replication in DNA- it helps to separate the circular bacterial chromosomes (original and replicated) to opposite parts of the cell prior to cell division

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16
Q

TubZ

A

A bacterial tubulin homolog. It serves the same function as ParM in other bacterial cells. TubZ is found in some bacterial species while ParM is found in other bacterial species

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17
Q

Crescentin

A

A bacterial intermediate filament homolog. Found in the species Caulobacter crescentus. Filamentous, provides strength & shape. Crescentin also gives these species a crescent shape

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18
Q

Structure of tubulin subunits

A

Heterodimer (made of α and β-tubulin). The heterodimer associates through non-covalent bonding. The α and β-tubulin each have a bound GTP molecule. Especially on the β subunit, GTP is important for the filament dynamics. It allows the filaments to grow and fall apart consistently

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19
Q

Structure of tubulin filaments

A

Consists of multiple heterodimers (α and β subunits) that associate end to end. The chain of numerous heterodimers builds to form a protofilament. However, there are lateral interactions between α-α domains and β-β domains that helps to form the mature filament. The mature filament contains 13 parallel protofilaments which are arranged around a hollow cylinder. The β-tubulin has an “up” position with the α-tubulin below it, giving the filaments a structural polarity

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20
Q

Structure of actin filaments

A

Actin monomers are true globular monomers, with an ATP binding site. Therefore, the monomers are bound to an ATP molecule. Actin protofilaments form when numerous actin monomers come together end to end. A mature filament is 2 parallel protofilaments that form a right handed helix. Actin filaments also have a polarity (directionality), with a minus and plus end. These filaments are flexible and more easily bent compared to microtubules. Many filaments are bundled and crosslinked together by accessory proteins, making them much stronger than individual filaments

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21
Q

Polarity of filaments

A

Actin and microtubules have a polarity due to their arrangement, they both have a plus end and minus end. The plus end is more dynamic and is where growth and shrinking can occur quickly. The minus end is more fully assembled, so growth and shrinkage occurs at a much slower rate. When filaments are referred to as growing or shrinking, that is always occurring at the plus end

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22
Q

Filament ATP/GTP hydrolysis

A

Tubulin and actin subunits catalyze GTP and ATP hydrolysis. Free energy released by hydrolysis stored in polymer lattice. Less free energy is needed for subunits to dissociate. There are 2 binding forms- T form (bound to ATP or GTP) or D form (ADP or GDP). The T form is more likely to grow- when GTP or ATP are bound to the plus end, the filament will be more likely to grow in size. The D form occurs when ATP or GTP is hydrolyzed (to ADP or GDP). It is more likely to shrink, and a higher concentration of monomers is needed for the D form to grow. There will also be an ATP or GTP cap on the growing plus end of the filament

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23
Q

Filament treadmilling

A

An intermediate monomer concentration occurs if higher than critical concentration for T form but lower than the critical concentration for D form. In this situation, filament treadmilling occurs. Subunits are recruited to plus end (T form) and shed from minus end (D form) in a balanced manner, so the filament is moving along like a treadmill. ATP/GTP hydrolysis must occur if there are D forms in the filament

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24
Q

Dynamic instability

A

Also called catastrophe and rescue. The idea that when ATP or GTP is at the plus end, we are growing the filament, but the filament is falling apart if ADP or GDP is at the plus end, and therefore the filament shrinks. The loss of the T form results in shrinking, gaining the T form results in growing- this cycle repeats over and over. The filaments are dynamic and are constantly growing and shrinking due to if ATP/GTP are present or hydrolyzed

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25
Q

Filament dynamics

A

Dynamic instability and treadmilling are common events in the cell. Microtubules switch between growth and shrinkage every few minutes. Likewise, actin filaments have rapid turnover. Individual filaments persist from a few tens of seconds to a few minutes

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26
Q

Where are intermediate filaments found?

A

All eukaryotic cells have actin filaments and microtubules. However, Intermediate filaments are found only in some metazoans- vertebrates, nematodes, and mollusks. These filaments also are not present in every cell. Oligodendrocytes (make myelin of CNS) lack intermediate filaments. They are more prevalent in cells subjected to mechanical stress, like epithelial cells. Similar to nuclear lamins in that they are structurally related

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27
Q

Formation of intermediate filaments (5 steps)

A
  1. Two α helical monomers come together and coil around one another. That forms a coiled-coil dimer
  2. 2 dimers associate in an antiparallel fashion, forming a staggered tetramer. Many tetramers come together to form an intermediate filament
  3. Two tetramers pack laterally to form a protofilament
  4. 8 parallel protofilaments are twisted into a ropelike filament
  5. In a mature intermediate filament, there are 32 individual α coils
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28
Q

Structure of intermediate filaments

A

In contrast to the other filaments, intermediate filament monomers are fibrous (not globular). Intermediate filaments are just extended α helical monomers. 2 coiled- coil dimers form a staggered tetramer. This tetramer is a subunit analogous to the tubulin α and beta subunits and actin subunits. Many tetramers come together to form an intermediate filament. There is no binding site for nucleoside triphosphate (no ATP or GTP will be bound). The 2 dimers point in opposite directions, so there is no polarity.

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29
Q

Intermediate filament dynamics

A

Intermediate filaments are easily bent and difficult to break. They can be dynamic and are probably regulated by phosphorylation, like lamins. When phosphorylated, lamins come apart, when they are dephosphorylated, lamins come back together. This is likely how intermediate filaments work

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30
Q

Types of intermediate filaments (4)

A

Actin and tubulin are mostly the same cell to cell, but there are many types of intermediate filaments.
1. Keratins
2. Neurofilaments
3. Nuclear lamins
4. Vimentin-like- Myocytes, astrocytes, Schwann cells (myelin for the PNS)

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31
Q

Keratins

A

There are 20 different types of keratin in human epithelial cells, and there are 10 different types specific to the hair and nails. Keratins are cross linked by disulfides. They are tough enough to survive the death of cells- make up the outer layer of the skin, hair, nails, claws, and scales

32
Q

Neurofilaments

A

Found along the axons of neurons. There are 3 types- NF-L, NF-M, and NF-H.

33
Q

gamma tubulin

A

g-tubulin is involved in nucleation (building tubules from scratch). Typically forms a structure called the g-tubulin ring complex (g-TuRC)- a large collection of gamma tubulin

34
Q

Microtubule-organizing center (MTOC)

A

Rich in g-TuRCs, it is a specific intracellular location from which microtubules are nucleated. Most animal cells have single, well-defined MTOC called the centrosome (located near the nucleus)

35
Q

Centrosome

A

Composed of a fibrous matrix with more than 50 gamma-TuRCs. Gamma tubulins nucleate the microtubules- microtubules emanate in an astral conformation from the gamma-TuRCs. The plus ends are the growing ends, and the ends that are farthest from the gamma-TuRCs. Centrosomes are nucleated at the minus end, and the plus ends grow toward the periphery. Centrosomes continuously grow and shrink by dynamic instability, probing the 3D volume of the cell

36
Q

Nucleation of actin

A

Actin nucleation occurs at or near the membrane, since actin is just underlying the membrane. The cell cortex is the area just underlying the cell membrane in the cytoplasm, and actin forms part of the cell cortex (the cortical cytoskeleton). The actin of the cortex determines the shape and movement of the cell surface (microvilli, phagocytic extensions). The nucleation of actin is regulated by external signals, it is catalyzed by actin-related proteins (ARP) complex and formins

37
Q

Formins

A

One of the actin nucleating complexes. Dimeric proteins, each of the monomers has a binding site for monomeric actin. Formins nucleate filament polymerization by capturing 2 monomers and moves along with the plus end to actively recruit actin monomers. It nucleates in situations where we nucleate parallel bundles of actin rather than a gel-like matrix. It is used for specialized structures like a cleavage furrow during cell division. It can also help to form structures like microvilli.

38
Q

Arp 2/3 complex

A

One of the actin nucleating complexes. Composed of 2 actin-like proteins and accessory proteins. It is activated by a protein called N-WASP and can begin nucleation at this point. Nucleation of an actin filament occurs from the minus end outward, analogous to gamma-TURC. The ARP 2/3 complex attaches to the side of another actin filament while bound to the minus end of a nucleated filament- this helps the complex to nucleate more effectively. In contrast to formins, the Arp 2/3 complex creates a treelike filament web

39
Q

How does Shigella hijack the Arp 2/3 complex during infection?

A

Shigella preferentially infects epithelial cells of the GI tract. It has a virulence factor (a protein called IcsA) that will bind to Arp 2/3. This allows Shigella to begin rearranging actin and use it for itself. It nucleates an actin tail that helps to propel the pathogen along, through the infected cell and into the next cell. Therefore, hijacking of the Arp 2/3 complex allows for cell to cell transmission

40
Q

How does Listeria hijack the Arp 2/3 complex during infection?

A

Listeria preferentially infects epithelial cells of the GI tract. A virulence factor protein called ActA helps to hijack host cell actin. Similar to Shigella, an actin tail is used for cell to cell movement. ActA is a molecular mimic of N-WASP, so ActA can bind directly to Arp 2/3 and activate it. However, Listeria actually produces less damage than Shigella

41
Q

Actin tail dynamics

A

The actin tail created by Shigella hijacks the actin filaments, which will form a tail structure. This structure helps to push the pathogen along. IscA helps to bind to the N-WASP of the host cell, which then activates the Arp 2/3 complex, allowing Shigella to hijack it. This is what allows the Shigella to spread from cell to cell and cause damage. The cells are killed when Shigella spreads to them. The damage causes breaks in the epithelial lining and results in a lot of the symptoms of Shigella infection

42
Q

Symptoms of Shigella

A

Mucous and blood in the stool- caused by breaks in the epithelial lining of the GI tract.

43
Q

Microtubule-associated proteins (MAPs)

A

Bind sides of microtubules, stabilizing an individual microtubule. It also facilitates interactions between neighboring microtubules- bundle formation. Can be long (MAP2) or short (tau) – influences spacing of microtubule bundles. Some MAPs bind ends, stimulating polymerization. Their activity is regulated by several kinases

44
Q

Tropomyosin

A

A protein that binds to an stabilizes actin. Elongated protein, binds to 7 adjacent actin subunits in 1 protofilament. By binding to the adjacent subunits, it stabilizes the actin filament

45
Q

Cofilin

A

A protein that destabilizes actin, it binds to actin and causes it to coil more tightly. The tight coiling weakens contacts between subunits that make up the actin filament. This eventually results in the actin filament falling apart.
Preferentially binds ADP-containing filaments and dismantles older actin filaments in cell

46
Q

Actin bundling proteins

A

Responsible for filament crosslinking. Results in parallel actin filaments- straight, stiff connections to actin. This kind of actin would be found in microvilli where the actin is kind of bundled together and makes a cellular extension. One of the bundling proteins associated with microvilli is called villin (monomer). Other examples are α-actinin (monomer) and fimbrin (dimer)

47
Q

Gel-forming proteins

A

Responsible for filament crosslinking. Create a flexible or stiff/bent connection in actin, making a gel or web-like matrix. Examples- filamin (dimer), spectrin or spectrin-like (tetramer)

48
Q

α-actinin

A

A monomer actin binding protein that bundles neighboring filaments of actin. Helps to form the contractile bundles that are necessary for muscle cells. This protein is more for transport and contraction

49
Q

Fimbrin

A

A dimer actin bundling protein. Forms parallel bundles but has more of a structural function- its tight packing prevents myosin-2 from entering the bundle

50
Q

Villin

A

A monomer actin bundling protein that forms tight bundles, like with microvilli (cellular extensions on GI epithelial cells). A sidearm portion of villin connects actin to the membrane

51
Q

Filamin

A

A dimer gel-forming protein. Cross-links actin into a 3-D network to form a matrix, which is found in a typical cell cytoskeleton. Necessary for neuronal migration

52
Q

Spectrin cytoskeleton in RBCs

A

A tetramer gel-forming protein. Heavily present in erythrocytes, and it gives them a discoid shape. The cross linking protein (spectrin) makes up a much larger part of the cytoskeleton than actin does. The proteins in the RBC membrane are anchored to the spectrin cytoskeleton. The band 3 protein in the RBC membrane interacts with the cytoplasmic protein ankyrin, which in turn binds to spectrin. Another RBC membrane protein called Glycophorin C binds to a cytoplasm protein called protein 4.1, which in turn binds to the junctional complex of the spectrin cytoskeleton. This is the part where we would find the actin.

53
Q

Spectrin structure

A

α spectrin forms a dimer with β spectrin (line up end-on-end), and the long strands of αβ spectrin form coiled tetramers. This forms a lattice-like cortical (underlying the membrane) network which is strong yet flexible.

54
Q

Flexibility of the spectrin cytoskeleton

A

Like all cytoskeletons, the spectrin cytoskeleton can be flexible. In the resting state, the RBC will have the regular disc shape. However, if the cytoskeleton is stretched, the RBC will exhibit changes in its morphology- it will have more of an oval shape. This is important because it allows RBCs to squeeze through small blood vessels like capillaries

55
Q

Spectrin anchoring

A

Band 3 binds to ankyrin in the cytoplasm, which in turn binds to spectrin. This anchors the membrane to the spectrin cytoskeleton. Most of the contacts are due to band 3, but some are due to glycophorin C in the membrane. This binds to protein 4.1, which in turn binds to the junctional complex.

56
Q

Cytoskeletal breakdown

A

The cytoskeleton begins to break down during processes such as cell death, and the membrane anchoring is lost. Sometimes, ankyrin degrades and the membrane loses its contacts with the cytoskeleton- this causes the membrane to form blebs and vesicles. Another way that breakdown could occur would be if spectrin degrades. This would also mean that the membrane is no longer anchored and the cell is undergoing cell death

57
Q

Junctions in the spectrin cytoskeleton

A

There are junctions between the spectrin coils which is where the actin is found, along with some tropomyosin. This is where the membrane protein glycoprotein C can come in contact with the cytoskeleton.

58
Q

Tracks for motor proteins

A

Actin and microtubules act as tracks for motor proteins. They use energy from repeated ATP hydrolysis to move, and proteins undergo repeated conformational changes to “walk” down the track. The motor proteins associate with filament tracks via a motor domain (called the head, although it looks like feet)- this binds/hydrolyzes ATP and produces movement as it undergoes conformational changes. The tail portion of the motor protein determines the identity of the cargo

59
Q

Why are tracks for motor proteins necessary?

A

They carry different cargo- many carry organelles to appropriate locations in the cell, like mitochondria, Golgi stacks, and vesicles. Others cause filament sliding-like muscle cells for muscle contraction. Also necessary for vesicular movement, like with the biosynthetic secretory pathway

60
Q

Myosins

A

Actin-based motor proteins. There are many types– prototypes are Myosin I and II

61
Q

Myosin 2

A

Functions in myocytes, it is a dimer coiled coil (2 heavy chains of alpha helices, wrapped around each other) with an N terminus head domain. 2 light chains are associated near the head group of each heavy chain

62
Q

Function of myosin 2 in muscle cells (5)

A
  1. Myosin 2 heads hydrolyze ATP, the hydrolysis and release of ADP produces conformational changes
  2. These conformational changes cause myosin 2 heads to “walk” toward the plus ends of the actin tracks
  3. The walking motion pulls the actin filaments, sliding them over one another
  4. This movement is driven by binding (ATP), hydrolysis of ATP to ADP, and release of ADP, over and over
  5. Sliding results in muscle contraction
63
Q

Form of myosin 2 in muscle cells

A

Tails of the coiled-coil myosin bundle with the many tails of other myosin 2 molecules, creating a thick bundle of myosin. Various myosin heads would stick out of the bundle. The heads are important because they hydrolyze ATP and undergo conformational changes. These heads are in contact with actin tracks. A thick filament would be formed with many myosin heads oriented in opposite directions. When the filament starts moving, some heads will “walk” to the left and others will “walk” to the right. The movement slides the actin filaments so they will undergo muscle contraction

64
Q

Conformational changes of myosin 2 heads (6)

A
  1. When the myosin head is attached to the actin filament, no nucleotide (ATP) is bound.
  2. Released- when ATP binds, the myosin head changes conformation and releases the actin filament
  3. Cocked- when ATP hydrolysis occurs, there is a conformational change in the lever arm. ADP and inorganic phosphate are bound
  4. The ADP bound head binds actin, and inorganic phosphate is released
  5. When ADP is released, it returns the lever arm to the original conformation (called a power stroke)
  6. The return of the lever arm to its original conformation is what will pull/slide the actin filaments over one another- muscle contraction occurs as thousands of myosin heads are changing conformation at once
65
Q

Kinesins

A

Microtubule motor proteins- they use microtubules as the track they walk on. Kinesins use ATP hydrolysis to produce conformational changes and walk towards the plus end of microtubules. They are similar to myosin 2 in that they are coiled-coil proteins and have 2 heads, but there are several varieties that differ

66
Q

Dyneins

A

Microtubule motor proteins that use ATP hydrolysis to walk towards the minus end of the microtubule. There are 2 varieties- cytoplasmic and axonemal. They are the largest and fastest molecular motors

67
Q

Cytoplasmic Dyneins

A

Similar to kinesins, have 2 “feet”. Heavy chain homodimers (2 heads) – vesicle trafficking, localization of Golgi near cell center

68
Q

Axonemal Dyneins

A

Works differently from cytoplasmic dyneins. Heterodimers or heterotrimers – drive beating of cilia & flagella

69
Q

Kinesins mechanism (4)

A
  1. When ADP is bound to the head, that means that the head can bind the microtubule, then the ADP is released
  2. Then, ATP binds to the head and it stays bound to the microtubule. A conformational change occurs in the neck linker (the “leg”)
  3. The conformational change pulls the trailing ADP head forward (ATP hydrolysis on orig. head)
  4. The cycle of binding in the ADP form, conformational change in the ATP form, and ATP hydrolysis repeats over and over
70
Q

Axonemal Dyneins structure

A

Contains heavy chains and light chains. The N-terminus of the heavy chain binds cargo or microtubules. The ATPase head of the axonemal dynein is ring shaped, and is where ATP hydrolysis occurs. There are 7 domains, with 6 of them being called triple A (AAA) domains. There is also one heavy chain C terminus domain. Between the 4th and 5th AAA domains, the heavy chain forms a coiled-coil stalk w/ microtubule-binding site at tip

71
Q

Axonemal dynein mechanism (3)

A
  1. When the dynein is ATP bound, it is detached from the microtubule
  2. ATP hydrolysis induces a conformational change that allows the stalk to bind to the microtubule. At this point, ADP and inorganic phosphate are still bound
  3. Once ADP and inorganic phosphate are released, there is another conformational change- this results in the power stroke, which slides the microtubule.
72
Q

Cytoplasmic vs Axonemal dynein

A

Cytoplasmic dynein works similarly, but has 2 “feet” like kinesin

73
Q

Mediation of the transport of membrane-enclosed organelles in the cell

A

Different tails and different motor proteins will associate with different cargo. They usually associate with cargo through membrane-associated motor receptors (in the organelle or the vesicle membrane). Then, the membrane associated motor receptors will interact with the tail portion of the motor protein

74
Q

Function of cytoplasmic dynein

A

Cytoplasmic dynein associates with a complex called dynactin- this is a mini cytoskeletal array that would be found on a vesicle or organelle. It mediates the attachment of dynein to organelles. Light chains of dynein can also interact w/ specific receptors

75
Q

Dynactin complex

A

The dynactin complex may enclose vesicle cargo. It has actin and spectrin making up part of its structure. Ankyrin anchors the complex to the membrane. This is how many cytoplasmic dyneins interact with their cargo