Lecture 21: Cytoskeleton II Flashcards
List the basic properties of cilia and flagella
• Both are hair-like structures that project from cell surface • both are used for locomotion • distinction between cilia and flagella is blurred, because both are made of the same components
Compare and contrast cilia and flagella
cilia are typically shorter and more numerous
- cilia are like oars – cell movement is perpendicular to direction of cilia themselves
- flagella are longer, rarely more than two
- flagella tend to be aligned in the direction of movement
Cilia move single celled organisms, or material around in multicellular organisms. These are lung epithelial cells.
Describe the structure of cilia and flagella
- both are made of microtubules surrounded by plasma membrane
- core is 9 + 2 array of MTs called the axoneme; misnomer
- 9 peripheral doublet MTs with 2 central single MTs; 9 x 1.8 x2 (there’s not complete 2 MT in each doublet)
- all MTs have + ends at tip, - ends at base
- At the base of the flagellum, there is a MTOC called the basal body (looks exactly like a centriole)
Describe the structure of the axoneme
• each MT doublet in the axoneme has: • 1 complete MT (A -13 protofilaments) and • 1 incomplete MT (B- with 10 or 11 protofilaments)- clamps on to the side of the complete MT
• A-tubule is linked to B-tubule of
adjacent doublet by nexin
• ciliary dynein arms and radial
spokes are attached to A tubules
ciliary dynein
arms and radial
spokes are attached to A tubules
nexin,
s an elastic protein
binds doublet together but connects each doublet with one beside it
axoneme
core is 9 + 2 array of MTs
• 9 peripheral doublet MTs with 2
central single MTs
Describe how dynein and nexin move the axoneme
MT doublets slide past each other. Dynein arms ‘walk’ along
tubulin wall of the adjacent doublet.
1. Dyneins attach one microtubule as ‘cargo’
2. ATP hydrolysis allows dynein to ‘walk’ along neighboring microtubule
3. Nexin proteins linking the two doublets create a bend
4. Dynein releases
5. The cycle repeats
. Describe the hypothesized mechanism in which the “central pair” control the beating of the
axoneme
Hypothesis: activity of dynein arms is
regulated by central MT pair & radial spokes
• central pair rotates as beating proceeds
• rotation promotes sequential contacts with
each radial spoke, making the spoke signal
to the dynein arm on the adjacent A MT
Alternation of this sliding causes the cilia or
flagella to bend
The sliding on one side of axoneme alternates
with sliding on other
• cilium first bends one way, then other
• activity of dynein arms alternates from one
side to other
List the properties and functions of primary cilia
Primary cilia are on some cells and differ
from other cilia in that they lack the
central pair of microtubules; ( 9+0)
- They do not have a locomotory function
- There is only one per cell
• They are a type of antenna, picking up chemical signals (they are covered in cell surface receptors), and they carry other signaling molecules up and down the microtubules, into the cell
• They can act as mechanoreceptors
(bending send signal), sensing flow outside
cell
What does primary cilia not having a locomotion function suggest?
suggests the central pair of MT are key to locomotion
ex- of reverse genetics (if you delete something and then look at its funtcion and see how imp it is)
What does primary cilia not having a locomotion function suggest?
suggests the central pair of MT are key to locomotion
ex- of reverse genetics (if you delete something and then look at its funtcion and see how imp it is)
List the properties of intermediate filaments
Intermediate in size (10 nm
diameter) between MT (25 nm)
and microfilaments (8 nm)
Strongest tensile strength of all cytoskeleton fibers (some have been found to tolerate being stretched to 3x normal length)
Made of fibrous proteins that intertwine with each other to make dense, rope-like fibers
Especially abundant in cells that endure physical stresses such as muscle cells, neurons, and epithelial cells
Connected to other components of cytoskeleton by cross-linking proteins, such as plectin
In which types of organisms have IF been found?
intermediate filaments have only
been observed in animal cells. All other
lineages, including prokaryotes, have MTlike and actin-like proteins.
Do IF have polarity?
Note: unlike MTs and microfilaments, these have no
polarity and there are no motor proteins that use
them for tracts.
IF proteins are a heterogeneous group
The primary amino acid sequence
of these can be quite different. The
all however exhibit similar
structural properties.
Describe how intermediate filaments assemble
Despite differences in type of protein, all IFs share
similar structure
Each has conserved central, α-helical domain
Differences arise due to globular domains of variable size & sequence that flank the helical core.
Two monomers wrap around each other in coiled-coils to form dimers with both subunits aligned parallel to each other
Dimers bind to each other to form tetramers. Dimers
are aligned in opposite orientations & are staggered: tetramers are NOT POLAR (no plus or minus end)
Tetramers aggregate in a staggered manner to form 60 nm filament, which aggregates to form IFs.
IFs continuously assemble and disassemble (like MTs), but subunits can insert in the middle, not the ends of the filaments. No ATP or GTP needed.
Assembly/disassembly controlled by phosphorylation
(e.g. MPF phosphorylates lamins in mitosis)
. List the functions of intermediate filaments
Main function is providing mechanical
support because they are highly
resistant to tensile forces
Hair, fingernails are made of keratin IFs
skin is mainly a mat of keratin Ifs
Nuclear lamins form network lining
inside of nuclear envelope.
Neurons are filled with neurofilaments
IFs are useful in cancer diagnosis
. How may intermediate filaments identify the origins of metastatic cancer cells?
Many cell types have specific IF profile.
Keratins have subtypes that are unique to
different epithelial cells (bladder, skin, etc)
or even different subsets of one cell type
(like basal epidermal cells). This is useful in
detection of the origin of cells in a tumor,
especially cells that have metastasized
Explain how Intermediate filaments help hold cells together
In epithelia, keratin intermediate filaments form junctions that hold cells together (desmosomes), or attach cells to matrix (hemidesmosomes).
The intermediate filaments loop into the plaques and spread out into the cytoplasm. This links two cells together structurally
Epithelial tissues
line the outer surfaces of organs and blood vessels throughout the body, as well as the inner surfaces of cavities in many internal organs.
List the functions of actin filaments
forms a band just beneath the plasma membrane
that provides mechanical strength to the cell
• links transmembrane proteins (e.g. cell surface
receptors) to cytoplasmic proteins
• anchors the centrosomes at opposite poles of the
cell during mitosis pinches dividing animal cells
apart during cytokinesis
• generate locomotion in cells such as white blood
cells and the amoeba
• interact with myosin filaments in skeletal muscle
fibers to provide the force of muscular contraction
microfilaments
Thinnest of the 3 cytoskeletal fibers (8 nm)
Composed of actin -
actin
monomers of the protein actin polymerize to form long, thin
and flexible fibers
Actin needs ATP to become part of the
growing filament; ATP is hydrolyzed once
actin is part of the microfilament
Each actin subunit is
made of 4 subdomains
“pointed” end (-)
“barbed” end (+)
Note that all of the actin subunits are made of the same actin (i.e. it’s a homopolymer).
How is the polarity of actin filaments defined?
Because of the shape of actin subunits, one end of a
micro-filament appears pointed, and the other
appears barbed.
– Pointed end = minus end
– Barbed end = plus end
What is actin assembly/disassembly dependent on?
Actin assembly/disassembly in vitro depends upon concentration of actin monomers. Filament
assembly leads to drop in ATP-actin. Actin subunits are then preferentially added to plus end
and removed from the minus end (steady state). Microfilament cytoskeleton is organized by
controlling equilibrium between assembly and disassembly of microfilaments.
Describe how microfilaments assemble
Step 1: Add a preformed actin filament to a
solution of actin subunits and ATP.
Yellow beads = filament
Red beads = actin subunits added
Step 2: increased concentration of ATP-actin,
actin is added at both ends.
Step 3: Drop in ATP-actin, addition of
monomers at plus end (higher affinity for ATPactin).
Step 4: subunits continually added to plus
end, and now being lost from minus end.
Step 5: + end still gains and – end loses ATPactin. In a process called ‘treadmilling’…. The
filament will ‘move’ to the right
Myosins
are the motor proteins of microfilaments
are a large family of
motor proteins that move along actin
filaments, while hydrolyzing ATP.
All myosins share a characteristic
motor head for binding actin and
hydrolyzing ATP.
The myosin tail is divergent.
Myosins can be divided into two groups: Conventional (type II) myosin Unconventional myosins (I, V, VI, VII, XV) All move to the + end of microfillaments, except myosin VI (there’s always an exception!)
Myosin II
found in muscle cells- most common type includes two heavy chains, each with a globular motor domain that includes a binding site for ATP and a domain that interacts with actin
Myosin V
V has two heavy chains like
myosin II, but myosin V has a longer
neck region and a cargo binding domain
