What is the function of the cytoskeleton?
- Maintaining cell shape
- Forming cell adhesion
- Assisting in cellular movement
What is the 3 dimensional system of tubular and filamentous structures that makes up the cytoskeleton?
- Microtubules
- Microfilaments
- Intermediate filaments
Structure of microtubules?
- Composed of two different tubulin monomers (alpha and beta tubulin)
- Microtubules will self assemble in vitro when these two monomers bond together to form a dimer
- Then the next step is called nucleation. It is when several of these dimers bind together, forming a hollow ring called the microtubule seed
- The dimers align so all of the alpha tubule tails are on one side called the minus end, and all of the beta tubule heads are on the other side called the plus end
- They are rigid and are the thickest filament
Function of microtubules?
- They form the axoneme of the cilium and flagellum
- They form the components of the centrioles
- They also form the spindle fibres of cell division
- Associated with intracellular movement in the axons of nerve cells
Structure of microfilaments?
- Also known as Actin filaments
- Found in the cytoplasm
- Largely composed of the protein actin
- Many actin molecules will join together to form actin polymers
- Actin polymers will twist around each other to form an actin filament
- They are found in most animal cells and in some plant cells
- They are concentrated in networks or bundles just below the plasma membrane
- They are dynamic (can lengthen-actin polymerisation and shorten-actin depolymerisation very frequently)
Function of microfilaments?
- Mainly involved in the gross movement of the cell
- Intestinal microvilli are produced by the microfilaments
Structure of intermediate filaments?
- Heterogeneous in composition (Any given cell will have more than one type
One type of filament usually predominates) - Size is in between actin filaments and microtubules
- This is a separate system of rope-like filaments found in most cells
- Non-alpha-helical (globular) domain at the N and C-termini which surrounds the alpha-helical rod domain.
- Basic building block is parallel dimer.
- Dimer is formed through interaction of the rod domain to form a coiled coil
Five main types of intermediate filaments?
- Type I & II- keratins divided into acidic and basic types
- Type III - vimentin-like proteins, found in cells of mesodermal origin
- Type IV - neurofilaments, found in nerve axons
- Type V - nuclear lamins - forming the nuclear lamina
Explain the role of centrioles in cell division and centrosome formation.
- Prior to cell division the centrioles replicate and move to the opposite poles of the cell
- Most cells’ centrioles duplicate by the growth of new daughter centriole at right angles to the ‘old’ centriole
- One new centriole and one old centriole form areas known as centrosomes
Describe the microtubular structure of centrioles in the animal cell.
- Usually found as two cylindrical structures at right angles to one another,
- Typically one pair per cell during interphase
- Composed of 9 sets of triplets arranged as a cylinder - The protein that forms this is called Tubulin)
- Called a 9x3 structure (9+0 structure) of microtubules. There are 9 tubule fibres around the outside of the centriole, and each of them has 3 proteins in it.
Centrioles?
- Characteristically found in all animal cells and ciliated plant cells
- Usually found in the cytoplasm in an area near the nucleus
- Often associated with the Golgi apparatus
- This area is called the Microtubule Organizing Centre
Show the origin of the microtubular spindle fibres from y-tubulin rings in the centrosome.
- Microtubules originate from y-tubulin rings in the centrosomal matrix (Distal appendages) not from the centrioles
- Each y-tubulin ring serves as the nucleation site (starting point) for one microtubule
- An aster of microtubules (fibres) extends to form the visible spindle fibres of cell division
- Some spindle fibres bind to the chromosomes at the centromere
Explain the significance of the intercellular junctions noting the three main types.
- Most cells in tissues are attached to other cells
- The cells in tissues are separated by an intercellular space of between 20-30nm wide, but in some areas of specialised intercellular junction more direct contact is maintained between adjacent parts of the two membranes
Three main types:
- Adhering (Anchoring) Junctions
- Tight Junctions
- Gap Junctions
Describe the structure and function of the adhering junctions.
Structure:
- The intercellular space is normal, but filled with transmembrane proteins called cadherins
- Adhering: The cadherin attachment is via linker proteins to actin microfilaments in the cytoplasm.
- Desmosomes: disc-like plaques in cytoplasm
Function:
- Principal mechanical interlinks between cells
- Strong junctions that tightly bind adjacent epithelial cells, such as those found in outer layer of mammalian skin
- Serve as a bridge connecting the actin cytoskeleton of neighbouring cells through direct interaction
Recall the clinical condition of pemphigus vulgaris which involves the desmosomes.
- Pemphigus vulgaris is an autoimmune, intraepithelial, blistering disease affecting the skin and mucous membranes.
- Autoantibodies target proteins of the desmosome leading to disruption of cell adhesion.
- Adjacent layers of the skin can pull apart and allow abnormal movements of fluid within the skin, resulting in blisters and other tissue damage.
Describe the structure and function of tight junctions.
Structure:
- These appear as a fusion of the plasma membranes of two cells
- Form a belt-like structure of branching sealing strands
- Network of strands are thought to be transmembrane proteins in the adjoining membranes forming interconnections
Function:
- To seal off the intercellular space, forming almost impermeable barrier
- Also have a role in the maintenance of concentration gradients between the exterior lumen and the intercellular space
Describe the structure and function of the gap junctions.
Structure:
- Hexagonal array of proteins in each membrane
- Each array is aligned to an array in the opposing face
- This results in the formation of a hydrophilic channel with a pore
- The transmembrane proteins are called connexins
Function:
- Associated with the intercalated discs in cardiac muscle where they facilitate the transmission of electrical impulses between cardiac muscle cells.
- Involved in the interchange of small inorganic ions and molecules between adjacent cells.
What are the two types of adhering junctions?
Desmosomes and adhering junctions
Cadherins?
Cadherins form links to cytoplasmic structures
Link membrane to cytoskeleton
3 types:
E-cadherin – epithelial
N-cadherin – neuronal
P-cadherin – placental
What are the two types of desmosomes?
- Spot desmosomes (Macula adherens), which are like spot rivets. These connect cells to each other.
- Hemidesmosomes connect the basal surface of epithelial cells to the underlying basal lamina. The transmembrane linker proteins here are different - integrins.
While testing an experimental drip on a vertebrate cell, you notice that the cell cortex becomes more fluid and the cell loses its ability to move. Despite this, you determine that cell strength is maintained. Based on these observations, you correctly conclude that the drug most directly affects?
Actin filaments
What is the microtubule arrangement of a flagellum or cilium?
9 x 3
