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BIOL 266 FINAL NOTES Concordia University
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BIOL 266 FINAL NOTES 2023-2024 ConcordiaUniversity
,BIOL 266 FINAL NOTES
UNIT 9.0 – THE CYTOSKELETON
Two functions of the cytoskeleton:
1. Provides structural framework for the cell, serving as a scaffold that determines cell shape
and the general organisation of the cytoplasm.
2. Responsible for the movements of the entire cells and for the internal transport of
organelles and other structures (such as mitotic chromosomes) through the cytoplasm.
The cytoskeleton is composed of 3 principal types of protein filaments:
1. Intermediate filaments
2. Microtubules
3. Actin filaments
These 3 types of protein filaments are held together and linked to subcellular organelles
and the plasma membrane by a variety of accessory proteins.
Cytoskeletal Components
Intermediate Filaments
- In contrast to actin filaments and microtubules, intermediate filaments are not directly
involved in cell movement.
- Main function: enables cells to withstand the mechanical stress that occurs when cells are
stretched. (Every time you open and close your hand, cells are changing shape and there is
stress on them. IFs are the support so the cells don’t just tear apart)
, - 4 classes of IFs – 3 in the cytoplasm, 1 in
the nucleus. Between all four classes there
are nearly 50 different types of IFs.
Appearance in the Cell:
IFs form a network throughout the cytoplasm,
surrounding the nucleus and extending out to
the
cell periphery, where they are anchored to the plasma membrane. They are also
indirectly connected to neighboring cells through a cellular structure called the
desmosome.
Assembly of intermediate filaments:
- Each monomer has an extended, central, α-helical domain, and
unstructured carboxy and amino-terminal domains.
- Two monomers wrap around each other in parallel using their α-
helical domains to form a coiled-coil dimer
- Two dimers associate in a staggered and anti-parallel fashion to form a
tetramer. This means that a mature IF will not have polarity (one end
of the IF resembles the other). This is in contrast to microtubules and
microfilaments that do have polarity.
- Eight tetramers then laterally associate and are added to the growing IF.
Unlike microfilaments and microtubules, there is no nucleation
involved and IFs build into existing IFs. Thus, the IF network is not very
dynamic.
- Mature cytoplasmic IFs have a rope-like structure.
Nuclear Lamina
Intermediate filaments underlying the inner face of
the nuclear envelope forms the nuclear lamina, a
fibrous network that:
1. Supports the nuclear membrane
2. Provides attachment sites for the
chromatin (de-condensed DNA)
The nuclear lamina differs from the cytoplasmic IFs in structure since it forms a meshwork as opposed
to a rope-like structure.
, The nuclear lamina disassembles with each cell division when the nuclear envelope breaks down. This is
regulated by phosphorylation of the lamina to cause disassembly, and de-phosphorylation to allow for
its reformation.
IFs and Human Disease
Mutations in nuclear lamin proteins are associated with progeria (premature aging in children) leading to
death at a very young age. Mechanism is unknown – may involve impaired cell division, increased cell
death, inability to repair tissue.
Microtubules
Microtubules are rigid, hollow rods approximately 25 nm in diameter. They are dynamic structures that
continually undergo assembly and disassembly.
Main functions of Microtubules:
1. The separation of chromosomes during mitosis
2. The intracellular transport of membrane-bound vesicles and organelles (organelles can use
microtubule tracks, they attach through a motor protein, one end bound to the organelle,
other end bound to microtubule and then it walks)
3. Cell movements
- MTs are composed of a single type of globular protein, called
tubulin. Tubulin is a heterodimer consisting of 2 closely
related proteins, α-tubulin and β-tubulin.
- These dimers stack together via α-tubulin - β-tubulin interactions
(non-covalent) to form a protofilament. This gives the filament
polarity: α-tubulin is exposed at one end (the minus end) and
β- tubulin is exposed at the opposite end (the plus end).
- The plus end is considered the growing end. This polarity plays a
role is determining the direction of movement along MTs.
- 13 such protofilaments arrange in a tube structure to give
the microtubule.
- Growth and retraction happen at the plus end, the minus end is
going to be anchored to a region that organizes the microtubules
MTs are highly dynamic. The dynamic nature of MTs is dictated by the ability of GTP to bind to tubulin.
Β- tubulin binds GTP and this form of the dimer is incorporated into the MT. Upon GTP hydrolysis, the
interaction with an incoming dimer is weakened. Thus, if the concentration of GTP-tublin is high, the
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