Motor Proteins Flashcards
Cytoskeletal motor proteins associate with their filament tracks through
a “head” region, or motor domain, that binds and hydrolyzed ATP to drive conformational changes as it “walks” along the filament.
What does the regions of filament determine
The head determines the filament and direction of movement, while the tail determines the cargo.
Groups of cytoskeletal motor proteins
-myosins
-kinesins
-dyneins
First motor protein identified and involved in muscle contraction
Myosin II (a signal myosin motor)
Myosin II structure
C terminal tail that is associated with the cargo, we have 2 coils of alpha helices, we also have an N terminal region head region
Each myosin head binds and hydrolyzes ATP to walk toward the _____ end of an actin filament
plus
What myosin doesnt move toward the plus end of actin
Myosin IV
Myosin V role
involved in vesicle and organelle transport
role of Myosin I
involved in intracellular organization including the protrusions of actin-rich structures at the cell surface (microvilli)
role Myosin II
involved in muscle and non-muscle cell contraction and cytokinesis.
Two types of microtubule motor proteins
-kinesin
-dyenins
Similarities between kinesin and myosin II
have 2 heavy chains and 2 light chains per active motor (2 globular head motor domains and an elongate tail for heavy chain dimerization)
Most kinesins have a binding site in the tail for either a ___________or another ________
-membrane-enclosed organelle or another microtubule
Motor proteins differ in (3)
-the type of filament they associate with
-the direction they move
-the cargo they carry
Kinesin 1 structure
has the globular motor heads that will walk along the MT at the N terminal, it is coiled into a dimer and has a c terminal end which will bind to the cargo
What is the “cargo”
the cargo is usually organelles that need to move within the cell, a big one is mitochondria
Kinesin 3 structure and involved in what
Looks very different, exists as a monomer and is also involved in moving organells around the cell
Kinesin 5 structure and function
most similar to a myosin fiber, but it only forms this tetramer, but a long fiber (like myosin does), it has a similar function to myosin in that it is responsible for sliding one MT relative to another MT generating tension
Kinesin 13 structure and function
actually has the motor domain in the middle (not on the N terminal like the rest) and it is thought to play a role in catastrophe but we don’t really know how it does this.
Which kinesin has motor domain in middle
13
Dyneins
a family of minus-end microtubule motors unrelated to kinesin superfamily
Two major braches of dyneins
1) cytoplasmic dyneins
2)axonemal dyneins
What are cytoplansmic dyneins
heavy chain homodimers with two large motor proteins as heads
What are axonemal dyneins
heterodimers and heterotrimers with 2 or 3 motor proteins as heads.
What are cytoplasmic dyneins involved in
vesicular trafficking and Golgi apparatus positioning (therefore MTOC positioning as well)
What are axonemal dyneins involved in
the beating of flagella and cilia
What are the largest and fastest molecular motors
Axonemal dyneins
Are the motor domains of myosin or kinesins substantially larger
Myosins
Differences between myosins and kinesins
These two motors track along different filaments, have different kinetic properties and no detectable a.a. similarities
Similarities between myosin and kinesin
Kinesin has linker protein and myosin has long arm, both function for cargo binding and bind ATP, both have similar sites that bind to respective cytoskeletal element (MF or MT)
Motor proteins generate force by
Coupling ATP hydrolysis to conformational change
What ends do kinesin, myosin and dynein move towards
Kinesin and myosin move towards plus end, dynein moves towards minus end
How do cytoskeletal motor proteins differ from GTP binding proteins
they must be propelled forward in a single direction along a filament
Myosin motor protein cycle attached (beginning) phase
(how it moves across MT)
Myosin head lacks bound nucleotide, locked tightly onto actin filament.
What happens in the attached (beginning) phase of myosin motor protein cycle in an actively contracting muscle
This state is very short lived, being rapidly terminated by binding of ATP molecule
Myosin motor protein cycle released phase
-ATP molecule binds large cleft on “back” of the head
-causes slight conformation change of domains making up actin binding site
-reduces affinity of head for actin, allows it to move along filament
Myosin motor protein cycle “cocked” phase
-Cleft closes around ATP molecule
-triggers large shape change, causes head to be displaced along filament
-ATP hydrolysis occurs, ADP and inorganic phosphate (Pi) produced remain tightly bound to protein
Myosin motor protein “force generating” phase
-Weak binding of myosin head to new actin filament site causes release of the produced inorganic phosphate
-Release triggers “power-stroke”
-During power stroke, head loses its bound ADP, returns to start of cycle
Power stroke (during force generating stage)
force-generating change in shape during which the head regains its original conformation
Myosin motor protein cycle attached (END) phase
-Myosin head is again locked tightly to actin filament in a rigor confirmation
-The head has moved to a new position on actin filament
What happens at the start of each kinesin motor proteins cycle “step” (how it moves along MT)
-Rear head is bound to MT and ATP (meaning tightly bound) (lagging head)
-Front head to MT and ADP (meaning loosely bound) (leading head)
2nd part of kinesin motor proteins cycle step (after “start of each step”)
ATP in the lagging head is hydrolyzed to ADP, causing lagging head to now have weak association with MT as well
3rd/final part of kinesin motor proteins cycle step (after hydrolyzed)
-ATP replaced ADP on the front head and Pi is released from the lagging head
-causes a change in orientation from rear pointing to forward pointing by shifting the neck
-Pull lagging head forward, bind to MT and complete step
When head is bound to ATP it will be ____ bound to MT
tightly
When head is bound to ADP it will be _____ bound to MT
weakly
3 elements of dynein molecule
-a stalk (binds the MT)
-ring (motor protein)
-tail (attached to cargo or another molecule)
How many ATP bind sites does dynein have and what happens when it is bound to ATP
-4 ATP bind sites
-Stalk detaches from MT when bound to ATP
Dynein- rapid hydrolysis of ATP to ADP
Causes dynein to attach to MT , then release of ADP and Pi causing a conformational change in form of rotating motor head
= POWER STROKE moving dynein along MT
Myosin, kinesin and dynein similarities and differences - binding
-Myosin without any nucleotide is tightly bound to its actin track.
-Kinesin forms a tight association with microtubules when ATP is bound.
-Dyneins are similar to myosin in that the nucleotide-free state is tightly bound to microtubules
Kinesin and myosin similarities and differences- travelling along MT
-Kinesins move in a highly processive manner, traveling for hundreds of ATPase cycles along a microtubule without dissociating.
-Myosin II makes one or two steps along an actin filament before letting go.
Why do kinesin and myosin travel differently across MT for their biological roles
A small number of kinesin molecules must be able to transport an organelle all the way down a nerve axon, -Myosin acts in a huge array of myosin molecules and must quickly get out of the way of the other molecules after its power stroke.
Myosin in inactive state
tail region has folded up on itself, so it cannot attached to the filament and it cannot stack its tails into a large filament, (b/c light chains not phosphorylated)
Mysoin in an active state
phosphorylation by MLCK, it phosphorylates on the light chains causing confirmation changes, making it longer and unfolded, but you also expose the actin binding site on the myosin head - (can now associate w actin)
Sliding of _______ and __________ cause muscles to contract
Myosin II and actin filaments
The bulk of the cytoplasm inside cells is made up of
myofibrils
Myofibrils are the
contractile element
Myofibrils consists of small contractile units called
Sarcomeres
Each sarcomere is formed of parallel, partly overlapping _____ and _____ filaments.
Thin, thick
What are thin filaments composed of and what are they attached to
Thin filaments are composed of actin and associated proteins and are attached at their plus ends to a Z disc at each end of the sarcomere
What is sarcomere shortening caused by
myosin filaments walking toward the plus end of the two sets of thin filaments of opposite orientations, driven by dozens of independent myosin heads
The “bare zone”
Middle or M line, where invidividual myosin fibers come together
Sarcomere accessory proteins
CapZ - plus end cap
Nebulin - ruler
Tropomodulin - minus end cap
Titin - spring
Nebulin role
ruler- gives information about how big a sarcomere is at any point in time
Titin role in muscle contraction
myosin heads walk along the actin filaments, compressing titin which is like a spring, titin will store the energy and then use it to push the myosin fiber back once the contraction is over (the calcium is removed from the cell).
What does capping proteins do for sarcomeres
capping proteins extend the half-life of the actin subunits (several days in muscle, several minutes in most other cell types).
Nerve cells signal to myofibrils to contract by increasing cytosolic ______
Ca2+
Calcium muscle contraction 2 step event - step 1
-calcium in extracell. space and voltage gated calcium channel on mem.
-AP causes channels to open and SMALL release of calcium into cytosol
-not enough to trigger muscle contraction
Calcium muscle contraction 2 step event - step 2
-calcium binds to calcium release channel on sarcoplasmic reticulum
-causes it to release calcium stroes which then allows myosin to bind to actin = muscle contraction
Calcium binding to troponin C causes what
causes topomyosin to dissociate from actin filament and reveal myosin binding sites on actin filament
-allows myosin to bind to actin and muscle contraction to occur
What happens to calcium once muscle contraction is over
-calcium taken back into sarcoplasmic reticulum
-allows for muscle to relax
What does storing large amount of calcium in SR allow for
quick contraction and relaxation of the muscle and ensures that it is always available to cycle
How does adrenaline regulate contractile activity of smooth muscle
Adrenaline causes an intracellular increase in cAMP which activates protein kinase A (PKA). PKA phosphorylates and inactivates MLCK
What causes dilated cardiomyopathy and what does this often result in
Minor missense mutations in the cardiac actin gene causes dilated cardiomyopathy that frequently results in early heart failure.
