WEEK 5 (Cytoskeleton & Cell Motility) Flashcards
What is the most diverse class of filaments?
Keratin filaments
What is Epidermolysis bullosa simplex disease associated with?
Mutations in keratin genes
What is the Cytoskeleton?
A network of protein filaments that extends throughout the cytoplasm which gives a cell its shape, offers support and facilitates movement
Microtubules are formed of which protein?
Tubulin
Microfilaments are composed of the protein __________
actin
Cytoskeletal filaments are polymers of __________
Proteins
Microtubules of a dividing cell form the mitotic spindle (TRUE/FALSE)
TRUE
EXPLANATION: Microtubules of a dividing cell form the mitotic spindle required for chromosome segregation
The Cytoskeleton framework plays a role in positioning of the cell organelles (TRUE/FALSE)
TRUE
Explanation: The internal framework of a cell (the cytoskeleton) is responsible for positioning the various organelles.
Which protein filaments make up the cytoskeleton?
- Intermediate filaments
- Microtubules
- Actin filaments
Describe Intermediate Filaments
Ropelike fibers that are made of fibrous intermediate filament proteins
What are the properties of Intermediate Filaments?
- Forms NUCLEAR LAMINA meshwork beneath the inner nuclear membrane
- Provides MECHANICAL STRENGTH and distributes MECHANICAL STRESS in epithelial tissue by spanning cytoplasm
- VERY FLEXIBLE
- GREAT TENSILE STRENGTH
- DEFORM under stress but DO NOT RUPTURE
Describe Microtubules
Hollow cylinders made of the protein tubulin. They are long and straight and have one end attached to a single microtubule-organising centre called a CENTROSOME.
What are the properties of Microtubules?
- More RIGID than actin filaments or intermediate filaments
- Rupture when stretched
- Long and straight
- Hollow cylinders made of TUBULIN
Describe Actin Filaments/Microfilaments
Helical polymers of the protein ACTIN that are organised into a variety of linear bundles, two-dimensional networks and three-dimensional gels.
What are the properties of Actin Filaments?
- Flexible structures
- Dispersed throughout cell but highly concentrated in CORTEX (layer of cytoplasm just beneath the plasma membrane)
What are the different properties of Microtubules?
SUBUNITS INCORPORATED INTO POLYMERS: GTP-aB-tubulin heterodimer
PREFERENTIAL SITE OF INCORPORATION: + End (B-tubulin)
POLARITY: Yes
ENZYMATIC ACTIVITY: GTPase
MOTOR PROTEINS: Kinesins, Dyneins
MAJOR GROUP OF ASSOCIATED PROTEINS: MAPs
STRUCTURE: Stiff, hollow, inextensible tube
DIMENSIONS: 25nm outer diameter
DISTRIBUTION: All eukaryotes
PRIMARY FUNCTIONS: Support, intracellular transport, cell organisation
SUBCELLULAR DISTRIBUTION: Cytoplasm
What are the different properties of Intermediate Filaments?
SUBUNITS INCORPORATED INTO POLYMERS: -70 different proteins, likely incorporated as tetramers
PREFERENTIAL SITE OF INCORPORATION: Internal
POLARITY: No
ENZYMATIC ACTIVITY: None
MOTOR PROTEINS: None
MAJOR GROUP OF ASSOCIATED PROTEINS: Plakins
STRUCTURE: Tough, flexible, extensible filament
DIMENSIONS: 10-12nm diameter
DISTRIBUTION: Animals
PRIMARY FUNCTIONS: Structural support, mechanical strength
SUBCELLULAR DISTRIBUTION: Cytoplasm + nucleus
What are the different properties of Actin Filaments?
SUBUNITS INCORPORATED INTO POLYMERS: ATP-Actin monomers
PREFERENTIAL SITE OF INCORPORATION: + End (barbed)
POLARITY: Yes
ENZYMATIC ACTIVITY: ATPase
MOTOR PROTEINS: Myosins
MAJOR GROUP OF ASSOCIATED PROTEINS: Actin-binding proteins
STRUCTURE: Flexible, inextensible helical filament
DIMENSIONS: 8nm diameter
DISTRIBUTION: All eukaryotes
PRIMARY FUNCTIONS: Motility, contractility, intracellular transport
SUBCELLULAR DISTRIBUTION: Cytoplasm
What is the difference between the function of microtubules in epithelial and neurons and the dividing cell?
Microtubules of the epithelial and neurons = Support and organelle transport
Microtubules of the dividing cell = Form the mitotic spindle
What are the functions of the Intermediate Filaments?
Provide structural support for both the EPITHELIAL CELL and NEURON
What are the functions of Microfilaments?
- Support microvilli of epithelial cell
- Integral part of motile machinery involved in NEURONAL ELONGATION and CELL DIVISION
Describe how microtubules have a critical organising role in all eukaryotic cells
Microtubules are LONG and RELATIVELY STIFF hollow tubes of protein that can rapidly disassemble in one location and reassemble in another
Describe the Microtubules’s role in mitosis
When a cell enters mitosis, the cytoplasmic microtubules disassemble and then reassemble into an intricate structure called the MITOTIC SPINDLE
Why are Peroxisomes closely associated with Microtubules?
Microtubules are the tracks over which the peroxisomes are transported in mammalian cells
What is Tubulin?
A dimer composed of two very similar globular proteins called a-tubulin and B-tubulin
Where are Microtubules found?
- Cytoskeleton
- Mitotic spindle
- Centrioles
- Core of cilia and flagella
Where do microtubules function?
In diverse activities such as SUPPORT of the cell and movement of materials between the cell body and axon terminals of a neuron
Where does Tubulin polymerise from?
Nucleation sites on a centrosome
Describe the Centrosome
- Centrosome consists of a pair of centrioles surrounded by a matrix of proteins
- Centrosome matrix includes hundreds of RING-SHAPED STRUCTURES called Y-tubulin
- Each Y-tubulin ring complex serves as the NUCLEATION SITE for the growth of one microtubule
What is the difference between the minus and plus end of a microtubules?
- MINUS END = embedded in the centrosome having grown from a y-tubulin ring complex
- PLUS END = extends into the cytoplasm
- MINUS END = protected by organising centres from which the microtubules grow
- PLUS END = initially free but can be stabilised by binding to specific proteins
Describe how growing microtubules display dynamic instability
Each microtubules grows and shrinks independently of its neighbours -> Array of microtubules anchored in a centrosome is CONTINUALLY CHANGING as new microtubules grow and old microtubules shrink -> It might shrink partially then suddenly start growing again or might disappear completely
What happens when a growing microtubule disappears completely?
It gets replaced by a new microtubule that grows from the same y-tubulin ring complex
The selective stabilisation of microtubules can _________________ a cell
polarise
A newly formed microtubules will persist only if both its ends are protected from ____________________
depolymerisation
Describe the Growing microtubule
Tubulin dimers carrying GTP bind MORE TIGHTLY to one another than tubulin dimers carrying GDP -> Rapidly growing plus ends of microtubules, capped by NEWLY ADDED GTP-TUBULIN, there tend to keep growing -> Addition proceeds faster than GTP hydrolysis by the dimers
Describe the Shrinking microtubule
When microtubules growth is slow, the dimers in the GTP CAP hydrolyse their GTP to GDP before fresh dimers loaded with GTP have time to bind -> GTP cap is lost -> Since the GDP-carrying dimers are less tightly bound in the polymer, the PROTOFILAMENTS peel away from the plus end -> Dimers are released causing the microtubule to SHRINK
Describe the mechanism of Colchicine
A cell in mitosis is exposed to COLCHICINE which BINDS TIGHTLY to FREE TUBULIN DIMERS and prevents their POLYMERISATION into microtubules -> Mitotic spindle rapidly disappears -> Cell stalls in the middle of mitosis, unable to partition the chromosomes into two groups
Describe the mechanism of Taxol
Binds tightly to microtubules and prevents them from losing subunits -> New subunits can still be added, so the microtubules can grow but cannot compress -> Arrests dividing cells in mitosis
What can Taxol and Colchicine be used to treat?
Human cancers
What do microtubules in living cells depend on?
A large variety of accessory proteins that bind to them
Describe the transport of macromolecules in microtubules along a nerve cell axon
Microtubules guide the transport of organelles, vesicles and macromolecules in both directions along a NERVE CELL AXON -> All of the microtubules in the axon point in the same direction with their PLUS ENDS towards the axon terminal -> Oriented microtubules serve as TRACKS for the directional transport of materials synthesised in the cell body but required at the axon terminal -> Backward traffic includes worn-out mitochondria and materials ingested by the axon terminals
What is the difference between Kinesins and Dyneins?
Kinesins = generally move towards the plus end of a microtubule outward from the cell body
Dyneins = move towards the minus end towards the cell body
[Kinesins and cytoplasmic dynes are microtubules motor proteins that generally move in opposite directions along a microtubule]
What are Kinesins and Dyneins?
Dimers that have two globular ATP-binding heads and a single tail and are protein families of microtubules
Describe how motor proteins move along microtubules using their globular heads
Each dimer has TWO GLOBULAR HEADS at one end, which bind and hydrolyse ATP and interact with microtubules and a SINGLE TAIL at the other end which interacts with CARGO, either directly or indirectly through ADAPTOR PROTEINS
Motor proteins use the energy of _____ hydrolysis to move in one direction along the filament
ATP
Describe how Kinesin motor proteins travel
1) The two Kinesin heads use the energy of ATP BINDING and HYDROLYSIS to move in one direction along the filament
2) ATP hydrolysis and phosphate release by the rear motor head loosens its attachment to the MICROTUBULE
3) ADP release and ATP binding by the front motor head then cause a CONFORMATIONAL CHANGE that flips the rear motor had to the front
How does the motor protein determine the type of cargo that it can transport?
Heads of KINESIN and CYTOPLASMIC DYNEIN interact with microtubules in a STEREOSPECIFIC MANNER so that the motor protein will attach to a microtubule in only ONE DIRECTION -> Tail of a motor protein binds stably to a cell component -> This determines the type of cargo that the motor protein can transport
What is the function of motor proteins?
Motor proteins of a cell convert chemical energy into mechanical energy which is used to generate force (e.g when a muscle cell contracts)
What is transport to the minus end mediated by?
Cytoplasmic dynein which uses ADAPTOR PROTEINS to interact with its selected cargo
What are Kinesins?
Kinesins move along microtubule filaments and are powered by the hydrolysis of ATP. The active movement of kinesins supports several cellular functions including mitosis, meiosis & transport of cellular cargo
Describe the structure of Kinesin-1
- Two heavy chains that wrap around each other to form a SINGLE, COMMON STALK and two light chains associated with the GLOBULAR ENDS of the heavy chains
- Force-generating heads bind to the microtubule
- Tail binds to the cargo being transported
What does the human genome encode for Kinesin-1?
The human genome encodes three different heavy chains and four different light chains for Kinesin-1
What is Kinesin-1 essential for the transport of?
- Mitochondria
- Endoplasmic Reticulum
- Golgi-derived vesicles
- Messenger RNAs
What was the first microtubule-associated motor protein discovered?
Dynein
Name two functions of Cytoplasmic Dynein
- Force-generating agent in POSITIONING the spindle and moving chromosomes during mitosis
- As a MINUS-END directed MICROTUBULAR MOTOR with a role in positioning the CENTROSOME and GOLGI COMPLEX and moving organelles, vesicles and particles through the cytoplasm
Describe the structure of Cytoplasmic Dynein
- Two Dynein heavy chains
- A number of smaller intermediate and light chains at the base of the molecule
- Each dynein heavy chain contains a large, globular, force-generating head & a protruding stalk
How are both motor proteins attached to the vesicle membrane?
Both motor proteins are attached to the vesicle membrane by an INTERMEDIARY. KINESIN is attached by a variety of INTEGRAL and PERIPHERAL MEMBRANE PROTEINS and DYNEIN by a soluble protein complex called DYNACTIN.
What happens when a cell grows?
Kinesins attached to the outside of the ER membrane pull the ER outward along microtubules stretching it like a net -> CYTOPLASMIC DYNEINS attached to the GOLGI MEMBRANES pull the Golgi apparatus along microtubules in the opposite direction (inward towards the nucleus) -> REGIONAL DIFFERENCES in these internal membranes are created and maintained
How does a Cilium beat?
A cilium beats by performing a repetitive cycle of movements consisting of a POWER STROKE followed by a RECOVERY STROKE
How do flagella propel a cell through fluid?
Using repetitive wavelike motion
How are Microtubules in a cilium or flagellum arranged?
In a “9+2” array. The nine outer microtubules carry two rows of dynein molecules.
What is Ciliary Dynein?
An accessory protein that generates the bending motion of the core
What movement does Dynein cause in isolated doublet microtubules and in a normal flagellum?
IN ISOLATED DOUBLET MICROTUBULES = Dynein produces microtubule sliding
IN A NORMAL FLAGELLUM = Dynein causes microtubule bending
What is Kartagener’s syndrome?
Kartagener’s syndrome is a type of PRIMARY CILIARY DYSKINESIA that is also characterised by SITUS INVERSUS TOTALIS (mirror-image reversal of internal organs). It is caused by hereditary defects in CILIARY DYNEIN.
Mutations in genes -> Cillia are either IMMOTILE (cannot move) or DYSMOTILE (move wrong)
SYMPTOMS:
- Frequent respiratory infections
- Frequent sinus infections
- Frequent ear infections
- Chronic nasal congestion
Men with Kartagener’s syndrome:
- Infertile since sperm are NONMOTILE
- Increased susceptibility to bronchial infections because cilia that line their respiratory tract are PARALYSED -> Unable to clear bacteria and debris from lungs
Which superfamilies can motor proteins be grouped into?
- Kinesins
- Dyneins
- Myosins
What is the difference between kinesins, dyneins and myosins?
Kinesins & Dyneins move along microtubules
Myosins move along actin filaments
Do motor proteins use intermediate filament tracks?
NO
Cilia and Flagella contain ________________
Microtubules
What is found only in animal cells?
Intermediate Filaments
Which protein is used for the fluorescent labelling of cytoskeleton elements of the cell?
Green fluorescent protein
where in a eukaryotic cell can a microtubule not be found?
Nucleus
Explanation: Microtubules are hollow, rigid and tubular structures found in all eukaryotic cells. They are associated with the mitotic spindle and the core of cilia and flagella
Which type of macromolecules make up the wall of microtubules?
Globular proteins
Explanation: The walls of microtubules are composed of globular proteins arranged in longitudinal rows called ‘PROTOFILAMENTS’ aligned parallel with the long axis of microtubule
How many protofilaments are present in a single microtubule?
13
Explanation: Microtubules consist of 13 protofilaments aligned side by side in a circular pathway within the wall. Non-covalent interactions exist between adjacent protofilaments.
Alpha-tubulin subunits terminate the plus end of the microtubule (TRUE/FALSE)
FALSE
Explnation: The plus-end of the microtubule is terminated by a row of beta-tubulin subunits and the opposite, minus-end of the microtubule is terminated by a row of alpha-tubulin subunits
Which motor protein superfamily does not move along the microtubules?
Myosin
Explanation: Dyneins and Kinesis move along microtubules, Myosins move along microfilaments
Smallest micro tubular motor proteins are ________________
Kinesin
[Kinesin is a tetramer consisting of two light and two heavy chains]
Kinesins are plus end-directed micro tubular proteins (TRUE/FALSE)
TRUE
Explanation: Kinesins are plus end-directed micro tubular motor proteins. Plus end of a microtubule is where the tubulin molecules attach and detach and growth and shrinkage takes place
Which microtubule-associated motor protein is responsible for the movement of Cilia?
Dynein
How many cytoplasmic dyneins are present in eukaryotic cells?
2
Colcemid is a drug that acts on ___________
Tubulin
Explanation: Colcemid binds to tubulin subunits and prevents their use by the cell
Outer microtubules in a cilium are generated from _______________
Basal body
Explanation: The outer microtubules in cilia are generated from the microtubules called a basal body. The basal body resides at the base of the cilium and are identical to centrioles
Microtubules of cilia are stable (TRUE/FALSE)
TRUE
Cilia and Flagella are the same structure (TRUE/FALSE)
TRUE
Non-motile cilia are called ______________
Primary Cilium
The core of the cilium is called ___________
Axoneme
Intermediate filaments are branched structures (TRUE/FALSE)
FALSE
Explanation: Intermediate filaments are strong, flexible, unbranched structures found only in animal cells. They provide mechanical strength to cells.
