Cell Biology: Cytoskeleton Functions and Filaments

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Questions and Answers

What is the role of the initiator proteins in the S Phase of cell division?

They separate chromosomes during mitosis.

They signal kinetochores for microtubule addition.

They load into origins and bind to them. (correct)

They break down the nuclear envelope.

Which microtubule type is associated with attaching to chromosomes during mitosis?

Kinetochore microtubules (correct)

Cytoplasmic microtubules

Astral microtubules

Interpolar microtubules

What occurs when chromatids are bound to different centrosomes during alignment?

Low tension leads to increased microtubule bonding.

High tension signals the addition of microtubules. (correct)

Low tension leads to rapid chromosomal separation.

High tension signals microtubule disassembly.

During chromosomal separation, what drives the initial movement towards centrosomes?

Plus end disassembly at the kinetochore. Signup and view all the answers

How does cytokinesis occur in a dividing cell?

Via actomyosin contraction and membrane insertion. Signup and view all the answers

What role does the cytoskeleton play in cellular structure?

It provides rigidity and organization to the cell. Signup and view all the answers

Which of the following statements accurately describes microtubules?

They are assembled from alpha and beta tubules and are polarized. Signup and view all the answers

What is the primary function of motor proteins in the cytoskeleton?

To convert chemical energy into mechanical work. Signup and view all the answers

What does 'treadmilling' in the context of filament dynamics refer to?

Adding to one end while removing from the other, maintaining the same length. Signup and view all the answers

How do capping proteins influence filament dynamics?

They prevent hydrolysis and stabilize filament growth. Signup and view all the answers

What is the significance of the GTP cap in microtubules?

It stabilizes microtubule structure and prevents curvature. Signup and view all the answers

Which protein complex is responsible for producing branched actin networks?

Arp 2/3 Signup and view all the answers

Calcium ions regulate muscle contraction primarily by affecting which protein?

Troponin Signup and view all the answers

Which type of cell junction is formed by cadherin clusters?

Adherens junctions Signup and view all the answers

What role do integrins play in cell adhesion?

They mediate the attachment of cells to the extracellular matrix. Signup and view all the answers

What triggers the contraction in skeletal muscle fibers?

The hydrolysis of ATP by myosin heads. Signup and view all the answers

Which of the following describes the behavior of intermediate filaments?

They provide flexible support and resist stretching. Signup and view all the answers

What constitutes the axoneme of cilia and flagella?

A core of microtubules arranged in a circular pattern. Signup and view all the answers

Study Notes

Cytoskeleton Major Functions

Gives structure and support to cytoskeleton
Helps organize inside cell
Resists forces
Network of tracks for organelle transport
Cell contraction
Cell movement and migration
Beating of the cilia
Separation of chromosomes
Splitting parents into daughter cells during cytokinesis

3 Types of Filaments

Actin

Plasma membrane
Shorter and smaller than other filaments
Drives contraction movements (like muscle)
Binds and hydrolyzes ATP
2 protofilaments
Polarized

Microtubules

Emanating out of a centrosome
Building blocks are alpha and beta tubules
Cytoplasm and nucleus
Flexible ropes resistant to stretching
Non-polarized
8 tetramers form filaments
Has lamins that organize nuclear pores and structure
Can bind with ATP or GTP
Formed from smaller subunits through noncovalent interactions

Intermediate Filaments

Not discussed in detail.

Filaments: Formation and Subunits

Protofilaments stabilize interior filaments
Protofilaments are asymmetric and polarized (Beta at the end and Alpha at the beginning, moving upwards)

Elongation and Treadmilling

Adding more than removing
Add to one end and remove at the other end
Staying the same length
Occurs when one end is largely ADP/GDP bound and the other is largely ATP/GTP bound

Microtubule Organizing Centers (MTOCs)

Nucleation occurs here
Polymerization of microtubules depends on nucleation and elongation
Beta combines GTP and hydrolyzes it into GDP

Concentrations

Each end of a filament has a different critical concentration
Critical concentration = rate of adding and subtracting subunits equal
If subunit concentration exceeds critical concentration, growth occurs
If below, disassembly occurs
Filament formation and concentration of free subunits influence association (Kon) and dissociation (Koff) rates
Plus end is more stable than the minus end

ATP/GTP Hydrolysis

Controls the concentrations
Beta binds GTP then to GDP to become a different form (D)
Hydrolysis causes conformational changes that decrease filament affinity
D forms are more likely to dissociate, T forms are more likely to be added

GTP Cap

Creates a cap on plus end - making it straight and stable
Prevents microtubule curvature that GDP subunits would have - keeping the shape round.
Catastrophe occurs when the cap is lost.

Motor Proteins

Convert chemical energy into mechanical energy
Have heads that bind to filaments and tails that bind to cargo
Binds to hydrolyze ATP
Carries organelles to destinations
Causes sliding of filaments

Motor Protein Types

Myosin = actin dependent
Kinesin = microtubule dependent, usually plus end directed
Dynein = microtubule dependent, usually minus end directed
Velocity = rate of physical displacement
Processivity = how long a motor protein stays active with a filament

Regulation of Filament Formation and Polymerization

Affected by proteins that bind free subunits
Proteins bind monomers and expose only the plus end binding side of actin monomer, promoting plus end assembly

ARP 2/3

Produces branched actin networks
Acts at minus end as nucleators

Formin

Functions at plus end of filament
Linear, unbranched actin filaments

Thymosin

Binds subunits to prevent assembly

Prolifin

Binds subunits and speeds up elongation to add filaments

Regulation of Filament After Formation

Cofilin
- Destabilizes bound actin filaments
- Induces a twist that loosens subunits
- Binds to ADP actin to promote turnover of older filaments

Capping Proteins

Stabilizes filaments
Prevents ATP/GTP hydrolysis
Capped on plus end after growth

Skeletal Muscles

Made of myofibrils
Myofibrils are made of sarcomeres
Actin outside Z disk, myosin between
Actin and anchoring stabilizer by capping proteins
Nebulin binds actin
Actin binding proteins stabilize filament
Stays actin bound for short periods.

Sarcomere and Accessory Proteins

Actin filaments anchor and stabilize by capping proteins
Nebulin binds actin
Stays actin bound for brief periods
Actin-binding proteins stabilize filaments

Sliding Model

Calcium is released, binds to troponin
Moves tropomyosin, myosin connects to actin
Myosin heads hydrolyze ATP, actin slides
Myosin head binds to ATP and detaches from actin

Calcium Regulates Contraction

Regulates position of tropomyosin in actin B
By binding to move it out of the way

Flagella and Cilia

Can propel cells through liquid media
Flagella is long filaments causing wave movement
Cilia are short filaments causing swimming power and recovery stroke

Axoneme

Inside of flagella and cilia
Connected in a circle
Activation of dynein, motor protein arms reach and attach to neighbor and tries to walk/slide, cross-linking proteins stop gliding.

Basal Bodies

Anchors flagella and cilia

ECM

Network of secreted proteins and polysaccharides, forms solid substrate for cell anchoring and crawling.
Glue that holds cells together

Collagen

Trimers, rod-like triple helix
Rigid fibrils, pack into thick filters
Resistant to pulling forces

Proteoglycans

Assembled into large complexes from core linkage
Has complex carbohydrates

Fibronectin

2 polypeptides linked by disulfide bridges
Binding sites for ECM proteins, receptors, brings together components

Laminin

3 Polypeptides linked by disulfide bonds
Binds to cell surface adhesion receptors and other ECM components.

Integrins (Mediate cell-ECM attachment)

Occurs at ECM binding sites and hemidesmosomes
Capable of inside-out signaling (active and inactive)
Outside-in signaling (affects cell differentiation)

Focal Adhesions

Sites that anchor cells to substratum (ECM)
Large clusters of integrins
Signaling hub
Capable of creating and responding to mechanical forces

Hemidesmosomes

Anchor basal surface of epithelial cells to basement membrane, large clusters of integrins.
Signaling hub
Resist forces
Intermediate filaments

Cadherins

Require Ca++ for binding
Mediate cell-cell interactions
Catentin tethers cadherins to the cytoskeleton

Cell-Cell Anchoring Junctions

Adherens junctions

Desmosomes

Links intermediate filaments

Tight Junctions

In epithelial tissues forming an impermeable barrier
Limits lateral diffusion within the plasma membrane
Claudins and occludins

Gap Junctions

Allows passage of small molecules
Link cell cytoplasms
Connexins proteins assemble to form transcellular pores

Adhesion and Traction

Protrusion is when cell extends membrane and attaches to substratum
Bundles of actin and myosin contract and squeeze cell forward
Adhesion on back breaks

FIllipodia and Lamellipodia

Filopodia: Spike out of plasma membrane, tight bundle of unbranched actin filaments.
Lamellipodia: ARP 3 nucleates actin at leading edge. Branched network leads to a broad flat membrane extension.

Cell Cycle

Cells duplicate genome and divide to create more cells.
When do cells divide? A buildup of factors hits a threshold, signaling health of the cell.
S Phase: Replication of entire genome; starts at origin of hundreds of different located genomes.

M-phase (Mitosis)

Spindle formation Microtubules and motor proteins Made from 2 centrosomes Separates chromosomes Astral microtubules contact cell cortex from centrosome Interpolar microtubules connect centrosomes Kinetochore attaches to chromosomes for movement.
Chromosomal Attachment and Tension Nuclear envelope breaks down Kinetochores on chromosomes mediate attachment to microtubules Can subtract or add subunits. Kinetochore signals to loosen/tighten bonds, depending on tension levels between chromatids.
Chromosomal Separation Microtubule disassembly and motor proteins, movement toward centrosomes Curvature of disassembly of microtubules, slides collar Strengthening dynein pulls of centrosomes toward cortex.
Cytokinesis Occurs through actomyosin contraction, membrane insertion Microtubule spindle signals where contraction will occur. Two sister chromatids pulled in opposite directions.

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Description

Explore the major functions and types of cytoskeletal filaments, including actin, microtubules, and intermediate filaments. This quiz focuses on their roles in cellular structure, support, and movement, as well as the details of filament formation and their subunits. Test your knowledge on how the cytoskeleton contributes to cell biology.