Lecture 2: Cytoskeleton and Motor Proteins Flashcards
What machinery do animal cells use to generate movement?
- cytoskeleton
- motor proteins
What is the cytoskeleton?
protein-based intracellular network
What are motor proteins?
enzymes that use energy from ATP
What are the 4 general types of cellular movement?
- polymerization
- mobile motor
- mobile cytoskeleton
- mobile motor and cytoskeleton
4 General Types of Cellular Movement
a) Polymerization
reorganization of the cytoskeletal network (actin polymerization)
- growth of the cytoskeleton in one region of the cell pushes the plasma membrane outward
- know where to move based on signals and receptors
- used in amoeboid movement
4 General Types of Cellular Movement
b) Mobile Motor
motor proteins βwalkβ along relatively fixed elements of the cytoskeleton
- can be used to transport cargo throughout the cell β cargo is bound to the cytoskeleton with a motor protein, which hydrolyzes ATP, allowing it to walk along the cytoskeleton
4 General Types of Cellular Movement
c) Mobile Cytoskeleton
motor proteins attached to the cell membrane pull on the cytoskeleton, moving an element of the cytoskeleton
- ratcheting action of motor proteins is important for pulling cytoskeleton in one direction
4 General Types of Cellular Movement
d) Mobile Motor and Cytoskeleton
motor proteins and the cytoskeleton are arranged such that they slide over each other, pulling the cell into a different shape
What are the 3 components of the cytoskeleton?
- microtubules
- microfilaments (actin filaments)
- intermediate filaments
Microtubules
What are microtubules? Where are they found?
- long hollow tubes composed of repeating units of tubulin (dimer of πΌ-tubulin and π½-tubulin)
- most extend from the nucleus toward the centre of the cell, some found along the cell periphery
Microtubules
Do microtubules have polarity?
yes
Microtubules
What occurs at the plus and minus ends of microtubules?
- typically grow at (+) end
- typically shrink at (-) end
- microtubules shrink faster than they grow
Microtubules
What factors influence the rate of growth and shrinkage of microtubules? (4)
- concentration of tubulin
- whether or not GTP bound by π½-tubulin is hydrolyzed
- microtubule associated proteins (MAPs)
- temperature
Microtubules
How does tubulin concentration affect the rate of growth and shrinkage of microtubules?
- critical concentration for the plus end is lower than the critical concentration for the minus end
What is taxol and how does it function?
chemotherapy drug
- used to treat solid cancer tumours (ie. ovarian, bladder, lung, etc.)
mechanism of action
- targets microtubules β binds to π½-tubulin and prevents microtubule from disassembling (microtubule stabilizing agent)
- targets mitosis β blocks progression of mitosis (metaphase spindle is not properly formed)
Microtubules
What is the microtubule-organizing centre (MTOC)?
- located near the nucleus in most cells
- minus end of microtubules is typically at MTOC
- plus end of microtubules is typically extending out toward the cell membrane
Microtubules
How are motor proteins associated with microtubules?
microtubules act as tracks along which motor proteins kinesin and dynein move
- kinesin: moves towards plus end (towards cell membrane)
- dynein: moves towards minus end (towards nucleus)
Pigment Movement and Microtubules β Analysis of a Primary Data Paper
see notes
Microfilaments (Actin Filaments)
What are microfilaments?
polymers composed of the protein actin
- long strands of the globular protein π½-actin (G-actin)
- found in all eukaryotic cells
- mostly around the cell periphery
Microfilaments (Actin Filaments)
How do microfilaments form?
- G-actin polymerizes, forming filamentous actin (F-actin)
- similar to microtubules, growth of F-actin is spontaneous and has polarity (plus/minus ends)
Microfilaments (Actin Filaments)
How does microfilament movement arise? (2)
- actin polymerization (amoeboid movement) β important for cell movement, actin filament under the membrane gives cells its shape
- sliding filament model using myosin (more common) β microfilaments act as tracks that myosin moves along by pulling on microfilaments (important for intracellular transport β ie. myosin V)
Microfilaments (Actin Filaments)
What is actin treadmilling?
- actin monomers treadmill along F-actin toward the (-) end (similar to microtubules)
- growth on one end and shrinkage on the other end, results in treadmilling
Microfilaments (Actin Filaments)
What factors influence the rate of growth and shrinkage of microfilaments?
- concentration of G-actin (more monomers β more supplies for growth)
- capping proteins
Microfilaments (Actin Filaments)
How do capping proteins influence the rate of growth and shrinkage of microfilaments?
- increase length by stabilizing minus end
- prevents shrinkage on one end (no treadmilling)
Microfilaments (Actin Filaments)
What is myosin?
motor protein for actin filaments
- is an ATPase β transforms energy associated with ATP hydrolysis into mechanical energy
- consists of three distinct regions: head, tail, and neck (associated with light chains)
Microfilaments (Actin Filaments)
What are 3 main myosin types? What are they each involved with?
- myosin I: involved with cargo transport
- myosin V: involved with cargo transport
- myosin II: involved with muscle tissue
Microfilaments (Actin Filaments)
Describe the myosin gene family.
- found in all eukaryotic organisms, therefore likely first appeared around 1 billion years ago
- at least 30 classes of myosin expressed in eukaryotes, with multiple isoforms within each class
- myosin II has two classes and numerous isoforms
- numerous isoforms likely due to genome duplication events
Microfilaments (Actin Filaments)
Describe the directionality of myosin movement.
- most known types of myosin move towards plus end of actin filament
- exception: myosin VI moves towards minus end β functions include intracellular transport and endocytosis
Microfilaments (Actin Filaments) β Sliding Filament Model
What is the sliding filament model?
basic mechanisms of how actin and myosin interact with each other
- shared across myosin isoforms and biological processes from subcellular vesicular transport to muscle contraction
Microfilaments (Actin Filaments) β Sliding Filament Model
What are two important aspects to remember about proteins?
- proteins change shape when compounds bind to them
- changing shape can allow proteins to bind or unbind other compounds
Microfilaments (Actin Filaments) β Sliding Filament Model
What are the two basic processes that underlie the sliding filament model?
two processes form the crossβbridge cycle (which allows muscle contraction)
- cross-bridge: myosin binds to actin (chemical)
- power stroke: myosin bends (structural)
Microfilaments (Actin Filaments) β Sliding Filament Model
Describe the steps of the cross-bridge cycle.
- ATP binds, causing myosin to detach from actin
- detachment of myosin causes ATP to be hydrolyzed to ADP and Pi, which remain bound by myosin
- hydrolysis of ATP causes myosin to extend and attach to (+) end of actin (myosin forms cross-bridge with actin)
- release of phosphate results in conformational change and promotes power stroke (bending of myosin head)
- ADP is released
Microfilaments (Actin Filaments) β Sliding Filament Model
What is unitary displacement?
distance that myosin steps during each cross-bridge cycle
- step size depends on length of myosin neck
Microfilaments (Actin Filaments) β Sliding Filament Model
What is the unitary displacement for myosin monomers?
amount moved is variable
Microfilaments (Actin Filaments) β Sliding Filament Model
What is the unitary displacement for myosin dimers?
displacement depends on the periodicity of the actin filament
- myosin V dimer uses both monomers in tandem, with an average unitary displacement of 36 nm β βwalksβ along microfilament with ~36 nm steps on average (period of the helical actin filament)
Microfilaments (Actin Filaments) β Sliding Filament Model
What is the duty cycle?
proportion of time during each cross-bridge cycle that myosin is attached to actin
(time spent in cross-bridge aka attached to actin) / (time for full cross-bridge cycle)
Microfilaments (Actin Filaments) β Sliding Filament Model
What is the duty cycle of non-muscle myosin?
typically 0.5
- means that myosin is tightly bound to actin for only half of each cross-bridge cycle
- used for intracellular vesicle transport (ie. myosin V)
Microfilaments (Actin Filaments) β Sliding Filament Model
What is the duty cycle of muscle myosin?
typically 0.05
Microfilaments (Actin Filaments) β Sliding Filament Model
What is the unitary displacement and duty cycle of myosin V?
- unitary displacement: 36 nm
- duty cycle: 0.5
Microfilaments (Actin Filaments) β Sliding Filament Model
What is the unitary displacement and duty cycle of myosin in thick filaments?
- unitary displacement: 5-15 nm
- duty cycle: 0.05
