BI203 Lecture 19: Cytoskeleton - Actin Structure/Organization

Question 1

_____ - The structural scaffold that holds all of the cellular organelles in place.

Answer 1

cytoskeleton

Question 2

Three main types of filaments making up the cytoskeleton:

Answer 2

microtubules, intermediate filaments, microfilaments

Question 3

Cytoskeleton Major Functions:

1) Cell _____ /_____ and mechanical resistance to deformation.
2) Cell _____.
3) Cell _____ /_____.
4) Segregates _____ during cell division.
5) Scaffold for _____ and _____ transport.

Answer 3

shape, structure, movement, signaling, endocytosis, chromosomes, vesicle, organelle

Question 4

The cytoskeleton is a network of protein _____ extending throughout the cytoplasm of eukaryotic cells.

It provides a structural framework that determines cell _____, positions of _____, and general organization of the _____.

The cytoskeleton is also responsible for _____ of entire cells, and internal _____ of organelles and other structures.

It is not rigid, but is a _____ structure that is continually reorganized as cells move and change shape.

Answer 4

filaments, shape, organelles, cytoplasm, movement, transport, dynamic,

Question 5

The major cytoskeletal protein of most cells: ~_____% of total protein in the cell.

Answer 5

10%

Question 6

Actin polymerizes to form actin filaments, or _____.

Answer 6

microfilaments

Question 7

_____ polymerizes to form actin filaments (microfilaments).

Answer 7

actin

Question 8

_____ _____ monomers have tight binding sites that mediate head-to-tail interaction.

Answer 8

globular actin

Question 9

_____ _____ can grow at both ends and has polarity.

Answer 9

filamentous actin

Question 10

Each actin monomer (_____ _____) has tight binding sites that mediate head-to-tail interactions with two other actin monomers, to form filaments (_____ _____).

Answer 10

globular (G) actin, filamentous (F) actin

Question 11

All the actin monomers are oriented in the same direction, so actin filaments have _____.
This is important in their assembly and in establishing the direction of _____ movement relative to actin.

Answer 11

polarity, myosin

Question 12

_____ is the first step of actin polymerization — Dimers and trimers are formed, then monomers are added to either end.

*Note: Actin polymerization is _____; the filaments can be broken down when necessary.

Answer 12

nucleation, reversible

Question 13

_____ _____ regulate assembly and disassembly of actin filaments, cross-linking into bundles and networks, and associations with other cell structures.

Answer 13

actin-binding proteins

Question 14

Actin bound to _____ associates with the rapidly growing barbed end.
ATP is then hydrolyzed to ADP.
ADP-actin dissociates _____ rapidly from filaments than ATP-actin.
Results in _____, where _____ is added at the barbed end while _____ dissociates from the pointed end.

Answer 14

ATP, more, treadmilling, ATP-actin, ADP-actin

Question 15

_____ - bind ATP-actin and nucleate initial polymerization of long unbranched actin filaments.

Answer 15

formins

Question 16

_____ - binds actin monomers and stimulates exchange of bound ADP for ATP, increasing the local concentration of ATP-actin.

Answer 16

profilin

Question 17

_____ _____ (actin-related proteins) - initiates growth of branched actin filaments, important in driving cell movement at the plasma membrane.

Answer 17

Arp2/3 complex

Question 18

_____ stabilize actin filaments by binding lengthwise along the groove of the filament.
_____ _____ stabilize actin by binding to the barbed or pointed ends.

Answer 18

tropomyosins, capping proteins

Question 19

Other actin-binding proteins remodel or modify existing filaments.
_____ severs filaments, generating new ends which are then available for polymerization or depolymerization.

Answer 19

cofilin

Question 20

The actin-binding proteins act together to promote rapid turnover of filaments and remodeling of the cytoskeleton needed for cell _____ and changes in cell _____. Their activities are controlled by signaling mechanisms in response to environmental signals.

Answer 20

movement, shape

Question 21

Actin filaments are organized into: _____ _____ and _____ _____.

Answer 21

actin bundles, actin networks

Question 22

_____ _____ — filaments are cross-linked into parallel arrays.

Answer 22

actin bundles

Question 23

_____ _____ — filaments are cross-linked in arrays that form 3-D meshworks with the properties of semisolid gels.

Answer 23

actin networks

Question 24

Actin bundles attach to the plasma membrane and anchor the cell at regions of _____ _____.

Answer 24

cell-cell contact

Question 25

_____ _____ attach to the plasma membrane and anchor the cell at regions of cell-cell contact.

Answer 25

actin bundles

Question 26

Actin networks underly the plasma membrane and determine cell _____.

Answer 26

shape

Question 27

_____ _____ underly the plasma membrane and determine cell shape.

Answer 27

actin networks

Question 28

Cross-linking proteins have at least _____ domains which can bind actin (at the same time too).

Answer 28

2

Question 29

_____ _____ are small, rigid proteins that force filaments to align closely.
Proteins that organize actin networks are large, flexible proteins that cross-link perpendicular filaments.

Answer 29

actin-bundling proteins

Question 30

_____ _____ interact with actin filaments resulting in a spectrin-actin network to form the cortical cytoskeleton that controls cell shape.

Answer 30

spectrin tetramers

Question 31

Spectrin tetramers interact with actin filaments resulting in a _____ _____ to form the cortical cytoskeleton that controls cell shape.

Answer 31

spectrin-actin network

Question 32

_____ is a member of the calponin (calcium-binding protein) family of actin-binding proteins.
It is a tetramer of two polypeptides, α and β. The ends of the tetramers associate with short actin filaments, resulting in the spectrin-actin network.

Answer 32

spectrin

Question 33

_____ links the spectrin-actin network and the plasma membrane by binding to spectrin and a transmembrane protein (band 3).
Protein 4.1 is another link that binds spectrin-actin junctions and the transmembrane protein glycophorin.

Answer 33

ankyrin

Question 34

_____ in muscle cells links actin filaments to transmembrane proteins in the plasma membrane, which link to the extracellular matrix, helping maintain cell stability during muscle contraction.

Answer 34

dystrophin

Question 35

The _____ _____, encoding the dystrophin protein, is one of the longest human genes known, covering 2.3 megabases (0.08% of the human genome) at locus Xp21.
The primary transcript in muscle measures about 2,100 kilobases and takes 16 hours to transcribe.
The mature mRNA measures 14.0 kilobases.
The 79-exon muscle transcript codes for a protein of 3685 amino acid residues.

Answer 35

DMD gene

Question 36

_____ _____, an X-linked inherited disease, results in
progressive degeneration of skeletal muscle.

Dystrophin is absent or abnormal in patients with Duchenne’s or Becker’s _____ _____, respectively.

Answer 36

muscular dystrophy