Cytoskeleton Flashcards
Describe four characteristics of microfilaments.
- made from actin filaments
- 5-7 nm in diameter
- double helix
- polar (asymmetric)
- plus end and minus end
What are the subunits of microfilaments?
free actin monomers (45kDa globular protein subunit) aka G-actin (globular actin)
Describe the mechanism by which microfilaments are assembled.
- start with building block of actin monomers
- each monomer can bind ADP or ATP as cofactor for enzymatic activity
- stacking of subunits causes displacement that creates the appearance of double stranded helix
- As ATP is hydrolyzed to form polymer, conformation changes
- ADP bound to subunit is weaker than ATP so subunits fall off at - end
- higher conc of G-actin required to keep filament growing
- protein binding at - end to stabilize filament
What role do actin-binding proteins play in regard to microfilament assembly?
- ABP’s help regulate polymerization of actin filaments (and rate, depending on conc of G-actin)
- responsible for filament organization
Distinguish between the + end and - end of actin filaments/microfilaments.
+ end: fast-growing end, where polymerization occurs (requires ATP)
- end: slow-growing end, where ADP is bound to actin (break down of filament)
Different characteristics of actin filaments depend on what proteins?
Characteristics of actin filaments depends on what ABP is present.
- actin-bundling proteins
- actin filament-severing proteins
- actin-capping proteins
- actin cross-linking proteins (i.e. filamin)
- actin motor proteins hydrolyzes ATP to allow for movement along the actin filament from - end to + end (i.e. myosin)
Describe the mechanism by which profilin stabilizes actin filaments.
- binds to subunit at + end
- helps present subunits for polymerization by catalyzing exchange of ATP & ADP
How does branching occur in actin filaments?
- branching is catalyzed by Arp complex which allows for elongation of “daughter” actin filament off “mother” filament
- important for phagocytosis (bringing material into cell using plasma membrane), need rapid polymerization of actin
What role does formin play in the growth of microfilaments?
- catalyzes growth of microfilaments
- recruits new subunits to create rapid growth of filaments
- polymerizes and depolymerizes under specific conditions
Name 5 functions of microfilaments
- cell division (contractile ring separates dividing cells)
- endocytosis/exocytosis/transport
- cell shape change (gives structure to epithelium in cell-cell junctions)
- cell motility (stress fibers)
- structural –> microvilli
Explain how stress fibers aid in cell attachment and mobility.
aggregates of actin filaments form to create stress fibers which are bound to the cellular matrix
What effect would removing a capping protein from a microfilament have on intercellular junctions such as a zonula adherens?
- capping proteins work to stabilize the - end of a microfilament so that it does not depolymerize
- removing the caps destabilizes the - end of microfilaments, allow them to disassemble themselves and zonula adherens
What are the differences and similarities between stereocilia and microvilli?
- both stereocilia and microvilli are made of an actin core
- in the case of stereocilia, they are essentially elongated microvilli that allow for movement
- the main function of microvilli is to increase surface area for absorption rather than movement
Describe five characteristics of microtubules.
- 24 nm tube
- 13 rows of subunits (protofilaments)
- rigid
- polar/asymmetric (ends are distinguishable from one another)
- all microtubules originate from same point within cell
What molecules form the subunit of microtubules?
- alpha tubulin and beta tubulin form a heterodimer (110 kDa, 8 nm long)
- beta end is the + end, has enzymatic activity (binds to both GDP and GTP)
- alpha subunit always contains inactive GTP
Identify the only place where the heterodimer can interact with proteins in the cytosol.
- alpha unit contains binding site with highly negative charge
- limits activity within cell
What is required for the assembly of microtubules?
- GTP
- originates from centrioles
How do tubulin heterodimer subunits add to make microtubules?
- one at a time (protofilaments do not assemble individually
- heterodimers constantly adding on to + end with GTP form on end
What causes subunits to fall off microtubules? From where?
- when GTP hydrolyzes to GDP form
- GDP can’t remain on filament
- dynamic instability –> forming or depolymerizing very quickly
What is the difference between centrioles, MTOC, and centrosomes?
MTOC = microtubule organizing center
- centrioles = paired cylinders built from nine microtubule triplets, arranged perpendicular to each other, found close to the nucleus
- centrosome (aka MTOC) = two centrioles plus gamma tubulin ring complex & pericentriolar material, disappears when centrioles are not present
- both can only form at certain places in the cell
- all ends at centriole/centrosome are - ends
What is the difference between centrioles and basal bodies?
basal bodies come from centrioles when they have migrated to the apical surface of a cell to form microtubules for cilia formation
What is the organization of microtubules that make up cilia and flagella?
9+2 configuration
What determines the shape of cells?
- shape of cells determined by the contact microtubules make at the surface
- microtubules disappear and reform until they find something to stabilize them
dynein
- acts as motor to make cilia move
- in 9+2 configuration, each pair of microtubules is bound to two dynein proteins
- have ATP motor complex which moves it from + to - end during intracellular transport
- cargo is determined by what it is interacting with at the + end
