1.1 Actin and Actin Motors Flashcards
What are the functions of the cytoskeleton (4)?
- provides structural support, shape to the cell
- helps provide internal organization
- enables cell movement
- enables intracellular transport
What are the three filament types?
- actin filaments
- microtubules
- intermediate filaments
What is the function of actin filaments?
determine the shape of the cell surface, and needed for whole-cell locomotion
What is the function of microtubules?
determine the position of organelles, and direct intracellular transport
What is the function of intermediate filaments?
provide mechanical strength
To complete their respective tasks, each filaments interact with [ ]. These proteins will [ ]. These accessory proteins include [ ].
- hundreds of different accessory proteins
- link filaments to each other, or to other cellular components
- motor proteins
Motor proteins utilizes ATP hydrolysis to [ ].
move various structures along the filaments, or to move the filaments themselves
The cytoskeleton can remain [ ] but is quite [ ].
- stable
- dynamic
Two examples of cell types with stable and differentiated morphology:
- neurons
- intestinal epithelium
Stable cytoskeletal structures like [ ] must be maintained throughout the life of the cell.
microvilli
[ ] cells turnover every few days.
intestinal epithelial
What are stereocilia?
cells that last the life of the organism, unless damaged. If damaged, they are lost forever. This can result in a loss of hearing
Cytoskeletal filaments are [ ].
protein polymers
[ ] is specific to the type of filament.
protein subunit
Each protein is [ ] in length, but form filaments that can span [ ].
- only a few nanometers in length
- hundreds of micrometers (1000x)
Subunits of actin filaments and microtubules are also enzymes that [ ].
hydrolyze ATP and GTP, respectively
What are the subunits for actin filaments?
globular actin or G-actin
What are the three isoforms of actin?
- alpha-actin (only in muscle cells)
- beta-actin
- gamma-actin (beta and gamma found together in almost all non-muscle cells)
G-actin is [ ].
asymmetrical
When G-actin subunits assemble, they arrange themselves [ ].
head-to-tail, forming a polar filament (F-actin)
Actin filaments form a tight [ ].
right-hand helix
What is the growth rate in F-actin (2)?
- faster-growing “plus end”
- slower-growing “minus end”
Individual filaments are very [ ].
flexible
What is filament nucleation?
several subunit-to-subunit interactions are required to acheive stability
Nucleation phase is also called [ ].
“Lag phase”
Instability of oligmers makes nucleation inefficient. However, [ ] gradually yield more stable nuclei.
formation of oligomers
The cell can bypass the lag phase by using [ ] of preexisting filament fragments.
performed seeds
What are the three phases of filament formation?
- nucleation
- elongation
- steady state
What is the concentration of subunits at the steady state?
at this state, the rate of subunit addition equals that of subunit disassociation
What is the critical concentration?
the concentration of free subunits at this steady state (Cc = koff/kon)
What is the different in critical concentration in vitro and in vivo?
the concentration is higher in vivo than in vitro
The plus and minus ends have the [ ] affinity for actin subunits that are bound to an [ ].
- same
- ATP or ADP
The koff and kon rates at the plus end are [ ] than at the minus end
much greater
Rate of hydrolysis increases when [ ].
actin is incorporated into filaments. The ADP remains bound to actin subunit
What are the two types of filament structures?
- ATP-bound T form
- ADP-bound D form
ATP hydrolysis results in [ ], but the energy is not lost. The energy is stored in [ ].
- free energy release
- filament polymers
Free energy change of subunit dissociation of a D form filament becomes more [ ] than the T form. The D form therefore has a [ ] koff/kon ratio than the T form.
- negative
- greater
The Cc (D form) is [ ] than Cc (T form). Consequently, at certain concentrations, the D form will [ ] while the T form will [ ].
- greater
- shrink
- grow
In living cells, most actin subunits are in the [ ]. Concentration of free [ ] is about 10x higher than [ ].
- T form
- ATP
- ADP
The [ ] actin subunits remains in the [ ], more likely actin is to [ ].
- longer
- filament
- hydrolyze the ATP
When subunit concentration < Cc, hydrolysis of ATP may occur [ ] another subunit is added. The end would be converted from [ ] to [ ] form, and would dissociate, causing filament to [ ].
- before
- T
- D
- shrink
When subunit concentration > Cc, addition would [ ] hydrolysis on both ends. Subunit addition at [ ] > than [ ]. Minus end is more likely to convert to [ ], while plus end remains in [ ].
- outpace
- plus
- minus
- D form
- T form
What is treadmilling?
growth at plus end equals shrinking at minus end
What gives actin cytoskeleton its contractile feature?
myosin, which generates the force for muscle contraction
What is the structure of myosin?
formed from two heavy chains and two sets of two light chains
What is the structure of the heavy chain (2)?
- globular head at the N-terminus where force is generated and ATP is hydrolyzed by the head group
- Long alpha-helical chain forms coiled-coil with another heavy chain to facilitate dimerization
Two light chains bind near the [ ] on each heavy chain.
N-terminal head domain
What are tail-tail interactions?
each coiled-coil bundles with the tails of other myosin proteins
What are thick filaments?
formed from tail-tail interactions. These have hundreds of myosin heads in each filament, oriented in opposite directions. but same on the same bare zone
What are the actin and myosin interactions in skeletal muscle (3)?
- actin surrounds myosin
- ATP hydrolysis causes myosin head group to “walk” towrads the plus end of actin
- opposing orientation of head groups causes the shortening of muscle (contraction)
During development, many separate cells fuse to form [ ].
large single muscle cells (separate nuclei are retained in the single cell)
What are myofibrils?
the basic contractile unit of muscle cell. Each is typically the length of the cell. Contains repeated units called sarcomeres
What are sarcomeres?
ordered arrangement of thick and thin filaments (myosin and actin)
Actin are attached by their [ ] to the [ ].
- plus end
- Z disc
Actin-myosin assemblies can be used in non-muscle cells. This could allow for (3):
- cell morphology
- aid in cell division
- facilitate cell migrations
In non-muscle cells, the formation is [ ].
transient
