Cytoskeleton Flashcards
What is the cytoskeleton
Network of protein fibers that extend throughout the cytoplasm
*Nomenclature is based on size
What are the functions of the cytoskeleton?
-Cell adhesion and movement (migration)
-Cell shape and structure
-Endocytosis/ exocytosis (secretion + recycling)
-Organelle/ protein transport
-Mitosis/ Cytokinesis
-Cilia and Flagella
-Muscle contraction
Etc.
What are the three types of cytoskeleton?
Microtubules, Actin (microfilaments), and intermediate filaments
What is the structure and diameter of microtubule (MT)?
Protofilaments
25 nm
What is the structure and diameter of actin?
Double helix
6-7 nm
What is the structure and diameter of intermediate filaments?
Two anti-parallel helices, form tetramers
10 nm
Describe microtubules
-subunit, etc
Polymer of alpha and beta tubulin dimer (basic subunit).
GTP-binding subunits come together in protofilaments (lines) to form hollow tubes
Explain the dynamic instability of microtubules
Balance between assembly (polymerization) and disassembly (catastrophe) and reassembly (rescue).
Dependent on subunit critical concentration and temperature (lower temp = more disassembly)
- also dependent on GTP-b-tubulin cap
What is MTOC? What is a major MTOC in cells?
Microtubule Organizing Center.
Centrosome is a major MTOC in cells (have multiple).
Microtubules radiate outward from the MTOC (negative end at MTOC), positive end grows away
What are MAPs?
Microtubule Associated Proteins
-can alter microtubule stability
-bundle microtubules
-can be regulated - phosphorylation promotes disassembly
*one way to stabilize microtubules
What are + TIPs? What is an example of a + TIP?
Possibly stabilize MTs to reduce catastrophe
e.g. EB1 (end binding protein 1) - shooting star proteins - seen in cell imaging falling-off then coming back
What are MT severing proteins?
Destabilize MTs
e.g. Katanin (like a sword that cut microtubules)
What is Kinesin?
What is it made up of?
MT + end motor protein (move out of cell toward + end) - Anterograde movement
Many types (> 14 known classes)
Made up of heavy chains (ATPase activity and bind MTs) and light chains (recognize cargo)
*ATP hydrolysis causes conformational changes which result in movement.
What is Dynein? How does it work?
MT - end motor protein - Retrograde movement
Interacts with another accessory protein, Dynactin which binds cargo
*ATP hydrolysis causes conformational changes which result in movement (ATPase activity)
Describe Microfilaments
Actin polymers - monomeric Globular (G-actin) into Filamentous (F-actin)
Double helix
Grows at - end and + end (but faster at + end)
ATP dependent
Can bundle Actin into diff cell structures - very complex.
What is Actin (Microfilament) dependent on for polymerization/ depolymerization?
Dependent on Profilin (polymerization) and Cofilin (depolymerization).
Also dependent on capping proteins at - and + ends.
What is Actin organized by?
Actin-binding proteins
What cell structures / bundles (e.g. Fimbrin, a-actinin, etc.) can actin form?
Microvilli
Filopodia
Focal adhesion
What is lamellapodium?
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
What is the motor protein that works with the Actin cytoskeleton?
Myosin
-Myosin II most abundant
-Heavy and light chains
-ATPase activity
-Ca+2
What is the function of Myosin?
Actin motor protein
-Membrane association, endocytosis
-Contraction (muscle)
-Organelle transport
Movement towards (+) end
Describe intermediate filaments
Provide structural support for cell shape
Basic subunit: Tetramer
2 dimers come together (4 polypeptides), to form a tetramer - forms protofilament, 4 protofilaments in a protofibril, 4 protofibrils in intermediate filament.
Mostly cytoplasmic
Do not have: polarity, or motor proteins
Lamins are nuclear
What is the function of intermediate filaments?
IFs provide structural support necessary for cell shape
What are Keratins?
Intermediate filaments
Link to Cadherins (desmosomes) and stabilize epithelial cells
What are Lamins?
Intermediate filaments
Support nuclear membrane
Network - linked to nuclear lamina
What is vimentin?
Intermediate filaments
Found in mesenchymal cells
Provides stability to mesenchymal - migrating cells (e.g. through uneven terrain)
How are intermediate filaments regulated?
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.
*Need to dissasemble during mitosis - phosphorylation of serine residues induces disassembly (and prevents reassembly)
How is protein exchange different in intermediate filaments than actin and microtubules?
Segments in an existing intermediate filament can be taken out and replaced with new segments. E.g. if want to change direction can just replace parts in the middle unlike MT and actin where you need to completely rebuild
