W5L1 Flashcards
Kinesin
MT + end motor protein
Many types (> 14 known classes)
Heavy chains: ATPase activity and bind MTs
Light chains recognize cargo
Anterograde movement
ATP hydrolysis causes conformational changes which results in movement
Most organelles only have either dynein or kinesin, except mitochondria bc it can move towards cell centre or away, wherever it is needed
Dynein
MT - end motor protein
ATPase activity
Interacts with another accessory protein, Dynactin, which binds to cargo
Retrograde movement
ATP hydrolysis causes conformational changes which results in movement
Most organelles only have either dynein or kinesin, except mitochondria bc it can move towards cell centre or away, wherever it is needed
Actin Cytoskeleton (Microfilaments)
Actin polymers
- monomeric Globular actin (G-actin) into Filamentous actin (F-Actin)
Double helix
Grows at both - and + ends (but faster at the + end)
ATP dependent
Polymerization/ Depolymerization
- Dependent on Profilin (polymerization) and Cofilin (depolymerization)
- Also dependent on capping proteins at - and + ends
Simple to very complex
- can bundle actin in complex way
Actin cytoskeleton - organization
Actin Organization
- Actin-Binding Proteins help actin bundle itself into diff cellular structures
Cellular Structures
1. Bundles (e.g., Fimbrin, a-actinin, etc.), which can be found in:
- Microvilli
- Filopodia
- Focal Adhesions
- Networks (e.g., Spectrin, Filamin)
- Inner Plasma membrane
- Leading edge of migrating cells - Support structures (e.g., Dystrophin)
- Plasma membrane actin linking protein
Actin cytoskeleton - structural and movement cells
A. Structure (epithelial cells)
- Microvilli
- in the gut - Adhesion belt
- Cadherins make adherens junction around the cell - Cell cortex
- cortical actin gives rigid epithelial cell
Note: the actin can remodel if the cell needs to migrate
B. Movement (mesenchymal cells)
- Filopodia
- Lamellapodium
- Cell cortex
- as the cell moves, the actin cortex squeezes one side to push the cell along
- squeeze on the lagging end - Stress fibers
C. Other
- Contractile ring
Branched Actin
Mesh/ Lattice
Branching is stimulated by adding laterally to existing actin
Nucleation and branching occurs with Actin Related Protein (Arp) 2/3 Complex
- creates a nucleation site for another actin to grow from
- at leading edge of migrating cells
- you need this to push at leading edge; you deform the whole membrane bc you want the thing to move
Motor proteins of actin cytoskeleton - Myosin
Myosin always goes towards positive end of actin
- Myosin II
- Myosin II is most abundant
- Heavy and light chains
- ATPase activity
- Ca+2
Function
- Membrane Association
- For muscle Contraction
- Transport
- Movement towards (+) end
- Myosin V
- for organelle transport to PM - Myosin I
- membrane association: actin cytoskeleton associates with PM
- endocytosis
Actin Cytoskeleton - Skeletal Muscle Sarcomere
Myosin II filaments are the A band
Z disk come closer together during contraction
- Z disks are bound to positive end of actin
Myosin walks towards positive end (towards Z disks), causing contraction of muscle
- activity is dependent on ATP and Ca2+
Actin Cytoskeleton - Myosin II during Cytokinesis
-
Myosin II is involved in contraction during cytokinesis
- actin wraps itself around centre of the cell to make contractile ring
- myosin will walk around contractile ring to cause it to get smaller and smaller, until you have cleavage furrow and 2 cells
Intermediate Filaments
- structure
Provide structural support for cell shape
Are 10 nm in diameter
Basic subunit: tetramer
- 4 polypeptides (or 2 dimers) come together to make the tetramer
- Many tetramers make a protofilament
- There are 4 protofilaments in a protofibril
- There are 4 protofibril in intermediate filament
Do not have
- Polarity
- Motor proteins
Mostly cytoplasmic
- Lamins are nuclear though
Intermediate Filaments - function
IFs provide structural support necessary for cell shape
- Keratins
- Link to Cadherins (desmosomes) and stabilize epithelial cells - Lamins
- Support nuclear membrane
- Network - linked to nuclear lamina
Classes of intermediate filaments
- Class I
- acidic keratins
- ep cells
- for tissue strength, integrity - Class II
- basic keratins
- ep cells
- for tissue strength, integrity - Class III
- desmin, vimentin
- muscle, mesenchymal cells
- for sarcomere
- for integrity and malleability of mesenchymal cells - Class IV
- neurofilaments
- in neurons
- for axon organization - Class V
- lamins
- in nucleus
- for nuclear structure and organization
Intermediate filaments other facts
Dynamic (although not as much as MTs or Actin)
- are a bit more stable than others
Are not polarized
- does not need to grow or shrink from particular end of a fiber
- can just mix and match anywhere, like lego bricks
Proteins within filament are exchanged
- Segments in an existing Intermediate filament can be taken out and replaced with new segments, without disrupting the rest of the filament
Need to disassemble during mitosis
Phosphorylation of Serine residues induces disassembly (and prevents reassembly)
Unlike MTs and Actin cytoskeleton (which are mainly regulated by binding and capping proteins), the regulation of Intermediate Filament stability is dependent on the opposing action of kinases and phosphatases
