Lecture 22 Flashcards
Actin associated motor protein
- like microtubules F-actin microfilaments are associated with motor proteins
Monomeric G-Actin Structure
- 4 subdomains
- divided by central cleft creating 2 kinda equal-sized lobes
Microtubule Organizing Center types
- basal bodies: associated w/ cilia and flagella
2. centrosome: associated w/ spindle formation
Microtubule Organizing Center
MOTC: central site of MT assembly
- only in eukaryotic cells
- Motor MAPs generate sliding force between MTs
Important during mitosis and chromosome segregation
Microfilaments (MF)
- thinnest cytoskeletal element- 8 nm
- polymer of actin protein
What is nucleation and polymerization?
- early nucleation steps of G-actin polymerization are slow
- ATP binding favors nucleation and stability of filaments
- monomers can be added at + and - end
- organization of these is regulated by actin-binding proteins
G-action → dimers → trimers → short filaments
Are F-Actin filaments polar or non-polar?
-polar: + and - ends
Microfilaments (MF) Functions
- maintenance of cell shape
- cell movement
- vesicle transport (plants)
- muscle contraction
- cytokinesis
F-Actin Filaments Structures
- 2 strands of subunits: 1 units= 28 g-actin subunits (14 each)
What is a lamellipodium?
actin projection on leading cell edge
What is a profilin?
actin-binding protein, enhances filaments growth
Vesicle transport by microfilament- based motors
- movement of pigment granules via cytoskeleton
- kinesin transfers vesicles to myosin motor protein
Actin nucleation by Arp2/3 complex
- helps in the creation of branch points for polymerization of new actin fibers
- F-actin networks highly branched (starts polymerization at branch points)
What are MF and Actin molecules?
Actin: central components of MFs
- in cells as monomer (g-actin, globular) or as polymer (f-actin, fibrous)
- enzyme binds and slowly hydrolyzes ATP
- double helix of actin monomers
- mass of globular actin molecule= 42 kDA
Motor proteins and their movement
- ATP binding to leading head induces a conformational change that swings trailing by 180 towards + microtubule end: force-generating step
- New leading head quickly binds to tubulin subunit and releases ADP (involves kinesin’s cargo forward)
- trailing head, ATP is hydrolyzed to ADP leading detachment form microtubule
- ATP binds to leading head to repeat reaction cycle
Directed cell mobility
- the coordinated activity of actin-binding proteins
- control microfilaments formation in a lamellipodium to allow directed cell movement
Intermediate Filament Strucutre
- 10-12 nm diameter
- exclusive to multicellular animal cells
- provide structural support and mechanical strength
- stable in comparison to MTs/microfilaments
- arrangement of fibrous a-helical proteins
- NOT polar (no + or - end)
- not used for transport
- composed of keratins in the cytoplasm
- composed of lamins in the nucleus
- abundant in axons of neurons
What are the 2 groups of Actin associated motor proteins
1) conventional myosins
- type 2
- primary motors for muscle contraction
2) unconventional myosins
- type 1 and III-XVIII
- generate force and contribute to motility in non-muscle cells
What is a phalloidin?
belongs to toxin class found in death cap mushroom - lethal after some days when injected into bloodstream
Is nucleation and polymerization reversible?
yes, ATP hydrolysis stimulated destabilization of polymer
- arranged in loose array network/tight bundles/cables/fibers
What is a capping protein?
blocks exchange of subunit at + end
What is myosins?
motor protein superfamily associated with microfilaments
- most myosin molecules move towards + end of microfilaments
- based contraction pulls trailing edge forward
What is colifin?
actin-binding protein, disassembles actin filaments
