Cytoskeleton II Flashcards
What are the basic building blocks of microfilaments?
- globular actin (G-actin) monomers assemble to make two-stranded, helical filaments (F-actin)
- most contain actin-binding proteins
What are microfilaments critical for?
- cell shape
- movement
- polarity
Are microfilaments polarized?
yes (due to orientation in the filament and molecular asymmetry of the subunits)
What does phalloidin do?
- from the highly toxic death cap mushroom
- binds and stabilizes F-actin (causing a net increase in actin polymerization)
Where does actin filament nucleation typically occur?
plasma membrane
What catalyzes nucleation?
- actin-related protein complex (ARP or Arp2/3) - nucleates microfilament polymerization from the minus end, allowing rapid elongation at the plus end. Results in branched actin filaments
- Forming - leads to parallel actin bundle formation
Actin plays a key role in ___________ of epithelial cells.
polarization
What are some of actin’s key functions?
- polarizing epithelial cells
- anchoring proteins that are involved in tight junction (TJ) and adherens junction (AJ) formation
- apical microvilli formation
- forms the core of brush border microvilli
What happens when there is a decreased association of adherens junction proteins (cadherins and catenins) with actin?
- leads to internalization of cadherins and loss of cell-cell adhesion
- prerequisite for epithelial-to-mesenchimal (EMT) transition and cancer formation
How do microfilaments form brush border microvilli?
- tight microfilament bundle forms the core
- all actin plus-ends are anchored in the apical protein cap of the microvillus
- actin bundles are held together by cross-linking proteins villain and fimbrin
In what disease is loss of microvilli observed?
microvilli inclusion disease
To what protein family do actin binding motor proteins belong?
Myosin
Describe the structure of myosin proteins (heavy chain).
- structurally related to kinesins
- head region (ATPase activity and actin binding sites)
- tail region (involved in binding to other molecules)
Describe the structure of myosin II.
- hetero-oligomers involve two heavy chains and two light chains
- coiled tails bundle with other myosin molecules to form bipolar assemblies (several hundred myosin)
What helps stabilize the plasma membrane and determine the shape and movement of the cell surface?
microfibril-rich layer underlying the plasma membrane
What do flipodia and lamellipodia do?
can adhere or detach to/from a cell substratum via appropriate adhesion molecules
What are stress fibers made from and what do they do?
- made from F-actin
- inserted into large adhesive patches (focal adhesions)
- viewed in cell cultures
- focal-adhesion-anchored cells exhibit low motility
What large family are the signaling molecules for actin organization (and thus cell shape)?
Ras GTPase
What does Rho do?
formation of stress fibers and focal adhesions
What does Rac do?
formation of veils
What does Cdc42 do?
protrusion of filopodia
How do neural cells reach destinations in many parts of the body?
axons have what’s called a nerve growth cone that moves like an amoeboid cell (the nerve cell body typically remains stationary)
What do Rho GTPases respond to in order to change the actin cytoskeleton of a cell?
- chemokinesis - random migratory activity
- chemotaxis - directed migration (can be positive or negative)
What are the four steps an amoeboid cell goes through during locomotion?
- protrusion (of lamellipodia, fillopodia)
- attachment
- traction
- detachment
What drives protrusion of fillopodia and lamellipodia?
- polymerization of actin meshwork at the leading edge (plus ends are facing forward)
- this is controlled by Rac and involves WASp (Wiskott-Aldrich syndrome protein) -> stimulates Arp2/3 complex
What does actin-related protein complex (ARP or Arp2/3) do?
nucleates microfilament polymerization from the minus end, allowing rapid elongation at the plus end. Results in branched actin filaments
What does Forming do?
leads to parallel actin bundle formation
How is attachment and traction achieved by amoeboid cell protrusions?
- they form attachments with the substratum and pull the cell forward
- generated by myosin in conjunction with actin microfilaments
Wiskott-Aldrich syndrome
- X linked immunodeficiency disease
- results from WASp mutations
- symptoms include thrombocytopenia (from defective lamellipodia/platelet formation) and recurrent infections (macrophages and neutrophil leukocytes are migration and chemotaxis deficient)
Lissencephaly
- severe defect of brain development - smooth cortical surface
- loss of function of n-cofilin, actin filament depolymerizing factor -> lack of neuronal migration -> mental retardation
How does actin play a role in cytokinesis (last stages of cell division)?
formation and contraction of actomyosin ring drives the formation of the cleavage furrow and separation of the daughter cells
