Lysosomes, Cytoskeleton, Microtubules, MAPs (Lecture 21) Flashcards
What inserts integral membrane proteins into the ER membrane?
What determines where a protein ends up in an amino acid sequence?
What transit through the Golgi Complex?
Getting proteins (and other components) to where they need to be is what for cell function?
What do COPI and COPII do?
Cotranslational Import inserts integral membrane proteins into the ER membrane.
Intrinsic information in the protein amino acid sequence determines where a protein ends up.
Proteins transit through the Golgi Complex (CGN to TGN) and get progressively modified.
Getting proteins (and other components) to where they need to be is critical for cell function; e.g., CFTR & cystic fibrosis.
COPI and COPII are coat proteins that help form transport vesicles, select cargo and direct them: ER ⇔ Golgi
What = endocytosis?
Plasma Membrane → Organelle
What is Lysosome Function
Lysosome Function
What is the degradation of internalized material?
- To recycle plasma membrane components and extracellular material
- To destroy pathogens e.g., bacteria & viruses (in phagocytic cells)
Lysosome Function
What is Autophagy?
How does it work?
Autophagy = organelle turnover
i.e., destruction and recycling of organelles
How?
- Lysosomes fuse with ER-derived autophagic vesicle
- forms autolysosome
- contents enzymatically digested and released
What are plant vacuoles?
- Fluid-filled, membrane-bound
- Can take up ~ 90% of cell volume
- Tonoplast
= vacuolar membrane- contains active transport systems that generate high interior [ion]
What are the functions of plant vacuoles?
-
intracellular digestion
- low pH, acid hydrolases (like lysosomes)
-
storage
- solutes and macromolecules
- chemical storage (no excretory system)
- isolate toxic compounds
- sequesters pigments (e.g. anthocyanin)
-
mechanical support; turgor pressure
- gives rigidity to plant - supports soft tissues
- stretches cell wall during growth
What is the cytoskeleton?
Cytoskeleton: Dynamic network of interconnected filaments and tubes that extends throughout the cytosol of eukaryotes
Functions:
- structural support
- spatial organization within the cell
- intracellular transport
- contractility and motility.
What are the functions of the cytoskeleton?
Functions:
- structural support
- spatial organization within the cell
- intracellular transport
- contractility and motility.
What are…
a. Microtubules
b. Microfilaments
c. Intermediate filaments
a. Microtubules
* A microtubule is a hollow tube formed from tubulin dimers
b. Microfilaments
* A microfilament is a double helix of actin monomers
c. Intermediate filaments
* An intermediate filament is a strong fiber composed of intermediate filament protein subunits
What are the structural components of the cytoskeleton?
- Microfilaments
- Microtubules
- Intermediate Filaments
What are Microtubules (MT)?
- largest cytoskeletal element (25 nm diameter)
- a polymer of proteins α-tubulin and β-tubulin
- 2 major types:
-
axonemal MT
- highly organized, stable
- part of structures (axoneme) involved in cell movement (e.g cilia, flagella)
-
cytoplasmic MT
- loosely organized, very dynamic l
- located in cytosol
-
axonemal MT
What is the structure of microtubules?
What are α and β heterodimers?
What are α and β tubulin heterodimers?
What is a tubulin polymer?
Microtubule Structure
What do α/β heterodimers form?
α/β heterodimers form long protofilaments
13 protofilaments form a longitudinal array creating a hollow cylinder
How are heterodimers aligned?
What are the two ends of the microtubule?
Why is structural polarity important?
Heterodimers are aligned in the same direction (head to tail) creating structural polarity*
MTs have a fast-growing ‘plus’ end and a slow-growing ‘minus’ end.
*structural polarity is important for MT growth/shrinkage and direction of movement of material along MT
Microtubules Undergo Dynamic Assembly and Disassembly
How does this work?
- in vivo, this leads to rapid turnover of most MTs within the cell (half-life is minutes)
- ‘dynamic instability’
- shrinkage can occur very rapidly at the ‘plus’ end (termed ‘catastrophe’)
- formation of MTs is regulated/ controlled
- Microtubule Organizing Centre (MTOC)
= central site of MT assembly
What are Microtubule-associated Proteins (MAPs)?
- several different proteins bind MTs to:
‣ modulate assembly, function
‣ mediate interactions with other cellular structures (e.g vesicles/organelles)
- stabilize MTs or stimulate assembly
MAPs are highly expressed in what?
MAPs are highly expressed in neurons
Microtubule-associated Proteins (MAPs)
What are the two classes of MAPs?
Two classes of MAPs
- Non-Motor MAPs:
* Control MT organization in the cytosol (e.g. Tau protein in neurons).
* Defective Tau protein → neurofibrillary tangles → Alzheimer’s disease. - Motor MAPs:
* Two main types: kinesin and dynein.
* Use ATP to generate force.
* Can move material along MT track.
* Can generate sliding force between MTs.
What are Dynein and Kinesin?
Dynein and Kinesin (MT-associated motor proteins)
- power intracellular transport
Dynein: minus end-directed
Kinesin: plus end-directed
Motor proteins and movement
What does Kinesin carry?
Kinesin carries cargo, such as vesicles, toward the plus ends of microtubules.
Dynein and Kinesin (MT-associated motor proteins)
power intracellular transport
Kinesin and Dynein at Work in Zebrafish Embryos
How do they work in the light and the dark?
- In the dark, melanin granules are dispersed outward by kinesin, causing the embryo to be darkly coloured.
- In the light, melanin granules are aggregated toward the center by dynein, causing the embryo to be lightly coloured.
