Cell Biology Chapter 5 Flashcards
Cytoskeleton
occupies large portion of cytosol and appears to link organelles to each other and to plasma membrane
The three elements/types of the Cytoskeleton
- Intermediate Filaments
- Microtubules,
3 Actin Filaments
Elements do not form mixed polymers
True or False?
True
Intermediate Filaments properties …
1(IFs) Are The Strongest, Most Stable Elements Of The Cytoskeleton
- Provides strength to the cell
- Typically stable polymers
- Wide variety enables cells to withstand many different types of forces
The IF Genes in Humans Are Classified Into Six Groups. Name them
Class I - VI
70 IF genes form 75 different IF proteins
- Alternative splicing
- Largely tissue specific
Characteristics of IF
- All IFs are only effective as a polymer
- Primary structure does not give tensile strength…essentially all strongs of amino acids have similar tensile strength in that form
- A central alpha-helical domain provides lots of tensile strength
- Stablized by hydrogen bonds
- Try to stretch it out H-bonds prevent it from collapsing
Intermediate subunits form diamers
IF Subunits Form Coiled-Coil Dimers. Thus Tertiary Structure is illustrated
Homologous or Heterologous Assembly
Two alpha-helices form a coiled-coil
VERY STRONG STRUCTURE
Maximizes surface contact between two polypeptide chains
AKA a leucine zipper motif
coiled diamers now form…
antiparallel staggered structures
An Anti-parallel staggered tetramer assembly
Assembly is spontaneous and does not require input of energy
Why Post-translational modifications of IFs? …..
modifications control the shape and function of intermediate filaments
Phosphorylation-dephosphorylation
Phosphorylation dissolves lamin intermediate filaments in order to degrade the nuclear envelope during cell division
types of Post-translational modificat
- Glycosylation
- Farnesylation
- Transglutamination of head and tail domains
Lamins
- Form a Cage Inside the Nucleus
- Found exclusively in the cell nucleus
- Three Lamin Genes in mammals undergo alternative splicing to produce 6 different Lamin subunits
- A, B1, B2, B3, C1 and C2
- Each cell contains at least one version of all three types (A,B,C)
Specialized long helical domain distinguishes them from cytoplasmic Ifs, ensuring that they do not co-polymerize with shorter IF
IFs Form Specialized Structures
Largest and most diverse group of IFs are Keratins In Epithelium
Keratins In Epithelium
- has Humans have 54 different genes
- Obligate heterodimers (TypeI and Type II subunits) dictate differential developmental and expression patterns
- Found in epithelial cell cytosol
- Essential components in cell adhesion complexes such as hemidesmosomes and desmosome
Desmin
Desmin is Essential for Muscle Function
- Desmin is a Type III IF that forms homodimers
Part of the contractile apparatus
- In Smooth muscle cells connects to actin filaments
- In Skeletal and Cardiac Muscle is concentrated in the Z-lines of the sarcomere
Which statement about intermediate filaments is false?
A) When they are phosphorylated, they completely unfold to primary structure.
B) They have a core helical structure
C) Some form a protective layer on the inside surface of the inner nuclear membrane.
D) Phosphorylation of some causes dissolution of the nuclear envelope.
E) They are found in most eukaryotic cells.
Points Earned: 1.0/1.0
Correct Answer(s): A
What property of intermediate filaments makes them so resistant to tensile force (pulling)?
A) They contain more disulfide bonds than most other proteins.
B) They form coiled-coil dimers, which resemble strands in a rope.
C) They form filamentous polymers like other elements of the cytoskeleton.
D) They attach to hemidesmosomes.
E) They are not dissolved by most detergents.
Points Earned: 1.0/1.0
Correct Answer(s): B
Can microtubules form in the absence of γTuRC?
) Yes, they can spontaneously polymerize in a test tube
Costameres as a Desmin-containing Complex
Costameres link the contractile appartus to plasma membrane-localized receptors.
Mutations in costamere proteins (like desmin) can result in different types of muscular dystrophies
Costameres are ESSENTIAL for proper muscle function
Microtubules Organize Movement Within the Cell
- Cells Can Sort and Direct its Contents
- Stable Tracks
- To a Destination
- Energy-Dependent
- Network Varies from Cell to Cell – No two cells are alike
MT cytoskeleton is a network
of “roads” for molecules “pass to and fro”
Microtubules – The Basics
Subunits called tubulin
Assembly requires a large complex of proteins and energy input
Microtubule-organizing Complex (MTOCs) called centrosomes can nucleate (initiate) the assembly of microtubules
- Not found in plant cells
- Found in most but not all animal cells.
MTOCs duplicate during S phase to form the two poles of the mitotic spindle during mitosis.
Which statement best describes an intermediate filament tetramer?
It consists of two overlapping, intermediate filament dimers arranged in a staggered orientation lacking structural polarity
Which observation best illustrates treadmilling by microtubules?
Which observation best illustrates treadmilling by microtubules?
The MTOC Contains
The Gamma Tubulin Ring Complex (γturc) That Nucleates MT Formation
cluster of centrosome proteins is called
Pericentriolar material
gamma (γ) tubulin
found in the Pericentriolar, its a protein organized into helical or ring shape. initiates the formation of microtubles
The Primary Building Block Of MTs
Is An Alpha-beta Tubulin Dimer
- α- and β-tubulin bind together to form stable dimer
- If purified α-β tubulin dimers bound to GTP are concentrated enough (critical concentration), they spontaneously form MTs
Which of the following exhibit treadmilling
A) Microtubules B) Microfilaments C) Intermediate filaments D) Answers A and B E) Answers A, B and C Points Earned: 1.0/1.0 Correct Answer(s): D
Three Properties of MT Assembly
1) MTs are only composed of two proteins: α and β tubulin.
2) Tubulin binds GTP.
α is married to its GTP (because it is hidden between the dimer), but β is not! β-bound GTP can be hydrolyzed to GDP and inorganic phosphate. The GDP can fall off and a new GTP can take its place.
3) Assembly is rapid, spontaneous and reversible
MTs are hollow “tubes” composed of 13 protofilaments
1) Most short polymers dissasemble and are unstable
2) Some reach a critical size of 6-12 dimers and begin to grow
Polymers of dimers sheet composed of 13 protofilaments folds into a tube
Three Important Differences Between in vitro and in vivo Microtubule Studies
1) MTs are seldom “alone” in cells and they do not all behave in the same way. MT-binding proteins regulate their stability
2) Every MT in a cell lies in its own unique environment – it is very heterogeneous as opposed to a test tube
3) Living cells don’t hold still and the internal state constantly changes so some variables are difficult to control in vivo.
The Growth And Shrinkage of MTs Is Called
Dynamic Instability
Some microtubules rapidly grow and shrink in cells = dynamic instability
Elongation is at the + end by GTP-bound
The growth of microtubules begins
The growth of microtubules begins at the gamma tubulin ring and continues as long as the plus end contains GTP-bound tubulin dimers.
Catastrophe
What happens when the supply of GTP-bound tubulin dimers runs out?
1) MT depolymerizes at the + end
OR
2) Capping proteins prevent depolymerization
Some MTs Exhibit Treadmilling
In cases where neither end of MT is stabilized, tubulin dimers are added to the + end and lost from the - end
Overall length of these MTs remains fairly constant, but the dimers are always in flux
Benefits Of Dynamic Instability
Allows cells to have
flexibility with trafficking during cell movement
ability to exert force by bonding with cargo molecules
and to explore the cytosol
MT-associated Proteins Regulate The Stability and Function of MTs
“MAPs” = capping proteins, rescue-associated proteins, and proteins that govern the motion
They can crosslink, bundle, or sever MTs also
Motor Protein = special type of MAP that transports organelles/vesicles
Dyneins and kinesins
Use ATP energy to accomplish this
The Two Most Common Motor Proteins That Bind To Microtubules
Dynein And Kinesis
How a microtubule motor protein moves along a microtubule.
Use ATP, release ADP
Dynesin to the negative end
Kinesin to the positive end
Dynein
will travel towards the (-) of an MT
Microtubule motor proteins transport pigment granules in a pigment cell
….like in a chameleon
Vesicles are Carried in Two Directions Along Microtubules
…
Specialized Microtubule-Based Structures Responsible for Motility in Some Cells
Cilia and Flagella
Cilia and Flagella Are Specialized Microtubule-Based Structures Responsible for Motility in Some Cells
Microtubles can be used to generate external forces as well as internal forces
Cell swimming
In stationary cells, used to move fluid over the cell surface
Cilia and Flagella share a basic structure
Axoneme
..
Sliding Dynein =
Whip Movement
Non motile cilium
In mammals, this forms on almost every cell.
It cannot “whip” like a normal cilium (lacks a central pair of MTs)
Plays a role in sensing external stimuli
Light
Odors
Actin Filaments Control….
the Movement of Cells. (Not just movement within cells)
Actin Filaments
Actin and its associated proteins greatly influence cell shape
Dominate in large-scale movements (like in muscle cells)
The Building Block of Actin Filaments Is
The Monomeric actin protein. There are 6 different isoforms. (α, beta, gamma, lambda etc)
Actin Filaments
Smallest diameter of cytoskeletal filaments 7nm “microfilament” Great tensile strength Structural polarity (+) end = barbed end (-) end = pointed end Often bound to myosin
The general structure of an actin filament
Figure 5-30 pg 172
ATP binding/hydrolysis regulate actin filament polymerization and disassembly
Actin monomers, like tubulin, adopt two different shapes and each is determined by ATP (not GTP) bound to the monomer plus a single divalent cation (Mg2+, Ca2+ etc.)
+ ATP = polymerization
Hydrolysis of ATP = depolymerization
(ATPADP +Pi)
Actin polymerization occurs in 3 stages. Draw & label
Figure 5-29 Pg. 172
Actin filaments have structural polarity
Actin filaments undergo treadmilling
Six Classes of Proteins Bind To Actin To Control Its Polymerization/Organization
MNCSDC
1 Monomer-binding proteins regulate actin polymerization eg. Proflin
#4 Severing Proteins #6 Cross-linking proteins organize actin filaments into bundles and networks
Break actin filaments in the middle of the filament…
#5 Depolymerizing Proteins Actin-Binding Motor Proteins Exert Force on Actin Filaments to Induce Cell Movement
Promote disassembly from ends of the actin filament
Actin depolymerizing factor (ADF) family
Cofilin
These motors provide ATP-mediated force
Potential energy converted to Kinetic energy
Bind-Move-Release-Shift cycle Like a Tug of War Line
More motors = More Force
Organization of primative body plans
Cell migration is essential for development
GI-tract cells are constantly replaced by cells migrating from the layer below
Intracellular trafficking
Cell Migration
Actin-binding motor proteins exert force on actin filaments to induce cell movement
Cell migration is a complex, dynamic reorganization of an entire cell
Migrating cells produce three characteristic forms of actin filaments: filopodia, lamellopodia, and contractile filaments
Three Characteristic Actin Formations
Lamellopodia, stress fibers, filopodia*
- be able to identify
Myosins
Are A Family Of Actin-binding Motor Proteins
Contractile cycle
Myosins move towards one end of the actin filaments
Myosin V crawls towards the - end,
All other myosins crawl towards the + end
Allows for movement of cell
Draw and label The contractile cycle of myosin.
..figure 5-37 pg 180
The contractile cycle of myosin.
Cortical = outer edge of the cytosol
When myosins pull on these actin filaments, cell, it squeezes itself forward
Combined with waves of lamellepodia and filopodia and integrin complexes that pull on the actin filaments, the cell rolls and squeezes itself forward.
Striated muscle contraction is a well-studied example of cell movement
..
Eukkaryotic cytoskeletal proteins arose from prokaryotic ancestors
Modern prokaryotic cells express a number of cytoskeletal proteins that are homologous to eukaryotic cytoskeletal proteins and behave similarly
Vimentin (IF)
FtsZ (MT)
MreB and ParM (actin)
Shared properties seem to include protection of DNA, compartmentalization and motility
What is the best explanation for why kinesin and dynein “walk” in opposite directions on microtubules?
Microtubules have structural polarity.
If cell biologists use the term “GTP cap” when discussing microtubules, why don’t they use the term “ATP cap” when discussing actin filaments?
Actin filaments do not undergo dynamic instability in cells.
Gelsolin is an actin filament severing protein. If you were to fluorescently label gelsolin in a migrating cell, where in the cell would it most likely be visible with a fluorescence microscope?
) In the rear of the cell
Which one of the following properties of actin filaments makes them different from other cytoskeletal proteins?
They bind ATP.
Intermediate filaments (IFs) are not used for intracellular transport of vesicles and organelles. Which statement best explains this?
) IFs have no structural polarity, and therefore cannot support directional movement
The nucleotide __________ is required to form polymers of actin and the nucleotide ____ is needed to form polymers of tubulin
ATP, GTP
Which of the following attributes of microtubules and microfilaments are also possessed by intermediate filaments?
A) They have structural polarity.
B) They require energy for polymerization.
C) They are involved with movement.
D) All of these attributes describe intermediate filaments.
E) None of these attributes describe intermediate filaments.
Points Earned: 1.0/1.0
Correct Answer(s): E
Keratin, neurofilaments and lamins are associated with which cytoskeletal element
) intermediate filaments
Axonemes and a 9+2 structure are found in
cilia and flagella
Dyneins and kinesins move towards the ______ and _____end of a microtubule, respectively
minus, plus
Filopodia and lamellopodia are associated with which cytoskeletal element?
microfilaments*
N/B not microTUBULES!!!
The microtubule organizing center is composed of
A) centrosomes B) centromeres C) chromosomes D) centrioles E) answers A and D Points Earned: 1.0/1.0 Correct Answer(s): E
What does it mean to say Keratins are obligate heterodimers?
It’s filament must contain type I & type II subunits.
Microtubules structure hierarchy
MTOC —> Centrosomes —-> centrioles —> pericentriolar —> gamma tubulins
Motor proteins
Drag, pull cargo along microtubules. Use microtubules as roads.
Draw & label Dynein & Kinesin
..
