Test Ch 17 Flashcards
(16 cards)
Subunits Assembled to build each of the filaments
Intermediate Filaments: Alpha-helical protein subunits (e.g., keratin, lamins).
Microtubules: Alpha- and beta-tubulin dimers.
Actin Filaments: Globular actin (G-actin) monomers.
Structure and Assembly of Intermediate Filaments, Microtubules, and Actin Filaments
Intermediate Filaments: 10 nm diameter, rope-like structures made of alpha-helical proteins with different globular heads. They form a strong, stable network in the cytoplasm and nuclear lamina. Assembly is regulated by phosphorylation (especially for nuclear Lamins).
Microtubules: 25 nm diameter, hollow tubes composed of 13 protofilaments made from alpha- and beta-tubulin dimers. They grow from the microtubule organizing center (MTOC).
Actin Filaments: 7 nm diameter, composed of actin monomers (G-actin) that polymerize into filaments (F-actin). Polarized with plus and minus ends.
Functions of IF, Microtubules, and Actin Filaments
Intermediate Filaments: Structural support; cell integrity and nuclear envelope
Microtubules: Organize membrane
bound organelles
*Transport of vesicles
*Spindle apparatus for
pulling chromosomes
apart in mitosis
*Cilia and flagella
motility
Actin Filaments: Cell shape, movement, intracellular transport, cytokinesis, and muscle contraction.
Microtubule Organizing Center (MTOC) and 3 types
microtubules are: They’re tiny tubes inside cells that act like roads or skeletons.
They help with shape, movement, and cell division. Where microtubules grow
- Centrosome
Found near the nucleus in animal cells.
Has two centrioles (tiny barrel-shaped structures).
It’s the main MTOC when the cell isn’t dividing.
Minus end attached to gamma tubulin rings , growing end is the + end on the centrosome
- Spindle poles
Show up when a cell is dividing.
These help pull chromosomes apart.
They’re like two organizing centers at opposite ends of the cell during division.
- Basal bodies
Found at the base of cilia or flagella (tiny hairs or tails that help cells move).
They help microtubules grow into those structures.
Dynamic Instability in Microtubules
- Microtubules are built from tubulin.
Tubulin = the building blocks.
They use GTP (a “high-energy” molecule, like ATP) to build.
When tubulin has GTP, it can stick to the microtubule and make it grow — at the (+) end (the “growing end”).
✅ Tubulin + GTP → microtubule grows!
- GTP gets used up (“hydrolyzed”) to GDP.
After tubulin attaches, GTP breaks down into GDP (lower energy).
GDP-tubulin is weaker.
If there’s too much GDP-tubulin, the microtubule falls apart (disassembles).
⚡ GTP → GDP = microtubule becomes unstable and can shrink.
Drug, What it does, Simple meaning:
Colchicine / Nocodazole: binds to
tubulin dimers and prevents polymerization
Taxol: binds to microtubules and prevents depolymerization
Tubulin + GTP → Microtubule grows.
Tubulin + GDP → Microtubule shrinks.
Colchicine/Nocodazole = stop growth.
Taxol = stop shrinkage.
Treadmilling in Actin Filaments
- Actin filaments are built from actin proteins.
Actin is the building block (like tubulin for microtubules).
When actin is carrying ATP, it can stick to the (+) end and make the filament grow.
✅ Actin + ATP → filament grows at (+) end.
- ATP gets used up (“hydrolyzed”) to ADP.
After actin attaches, ATP breaks down into ADP (lower energy).
ADP-actin is weaker.
ADP-actin often falls off from the (-) end (the end that usually shrinks).
⚡ ATP → ADP = actin gets unstable and leaves from (-) end.
Drugs:
Cytochalasin: Blocks actin polymerization
Phalloidin: Freezes actin in place (prevents depolymerization)
Actin vs. Microtubule Polymerization
on google docs
Role of MAPs and Actin-Binding Proteins
MAPs (Microtubule-associated proteins): Stabilize MTs, prevent disassembly.
Actin-binding proteins: Regulate actin filament behavior (e.g., nucleation, severing, bundling).
Motor Proteins and comparison of Motor Proteins used with microtubules vs those use with actin filaments
Definition: Proteins that use ATP to move along cytoskeletal filaments.
MTs carry transport vesicles through the secretory and endocytosis routes
MTs also carry transport vesicles in neurons
Some produced in cell body, taken to axon termini(secretion)
Some taken in at the termini, returned to the cell body (endocytosis for lysosomal degradation)
MT-associated: Kinesin (+ end), Dynein (– end).
Actin-associated: Myosin (moves to + end).
Comparison:
MT motors: Long-distance transport.
Actin motors: Local movement, contraction.
Dynein and Kinesin in Vesicle Transport
Kinesin: Moves vesicles outward (to + end).
Dynein: Moves vesicles inward (to – end).
Both use ATP binding and hydrolysis for conformation changes and energy
Why 2 types?: Microtubules are polarized; each motor only moves in one direction, so two are needed.
Cilia and Flagella Structure
Basal body at base of cilia or flagella isMTOC
+ end capped at tip of cilia
Dynein is the motor protein
MTs arranged with 9 outer doubletsaround 2 central singlet tubules (9 + 2
arrangement)
Crosslinking prevent MTs from sliding past each other and produce bending
of structure
Role of Myosin I in Non-Muscle Cells
Myosin I: Does not form filaments. Functions in endocytosis, membrane tension, and intracellular trafficking.
Actin and Myosin II in Cytokinesis
Actin forms a contractile ring.
Myosin II slides actin filaments to pinch the cell in two during division.
Steps in Muscle Contraction
Nerve signal → acetylcholine release.
Na⁺ influx → depolarization.
Depolarization spreads via T-tubules.
Opens voltage gated Ca²⁺ channels in sarcoplasmic reticulum and releases Ca2+.
Ca²⁺ binds troponin changing conformation → moves tropomyosin off actin binding sites.
Myosin heads walk along actin using ATP hydrolysis energy → thus slides actin past mysoin → sarcomere shortens.
Ca²⁺ pumped back into SR → tropomysoin covers myosin binding sites on actin and muscle relaxes.
dynamic instability look on slide
GTP bound tubulin
polymerizes
beta tubulin binds to
GTP
GDP bound tubulin
depolymerizes
Disassembly inhibited
by MT associate proteins
(MAPs)
Both occur at the +
end preferentially
Cell type specific IF
Cytoplasmic IF:
-Keratin filaments found in epithelial cells
-vimentin and vimentin related filaments found in connective tissue, muscle cells, and glial cells
-Neurofilaments found in nerve cells
Nuclear IF:
Nuclear lamins found in all animal cells
