Lec 21: Cytoskeleton Flashcards
Roles of a cytoskeleton:
supports cell shape and structure, organizes cytoplasm, allows movement, allows intracellular transport, allows cell division
Components of the cytoskeleton
intermediate filaments: provide cell’s mechanical strength
microtubules: determine the position of membrane enclosed organelles and intracellular transport
actin filaments: determine the shape of the cell’s surface and necessary for whole locomotion
Intermediate Filaments
very strong, main function is to allow cells to withstand mechanical stresses.
Formed from long rope-like strands, with the ends on each are identical (lack polarity)
Found in the cytoplasm and nucleus (nuclear lamina)
Especially common in cells that are subject to mechanical stress (ie long neurons, muscle cells, epithelial cells
Four classes of intermediate filaments
1) keratin filaments (epithelial cells)
2) vimentin and vimentin-related filaments (connective tissue)
3) neurofilaments (nerve cells)
4) nuclear lamins (strengthen nuclear envelope)
They rupture under stress: they have great tensile strength and deform under stress, but do not rupture. They help maintain the integrity of epithelial cells
Microtubules
Help organize all eukaryotic cells. They form long, stiff tubes that create a system of tracks within the cells along which vesicles, organelles, and other macromolecules can be transported. They also position membrane-enclosed organelles within the cell.
They are not permanent structures
Form the mitotic spindle, cilia, flagella
Structure of microtubules
Hollow tubes with structurally distinct ends–they are chemically and functionally distinct.
Hollow structure gives a good strength to weight ratio
Formed from monomers; starts with a heterodimer that makes a subunit. Always grows so that alpha and beta alternate, alsays has an alpha end (minus end) and a beta end (plus end). This polarity is important for intracellular transport
centrosome
most prominent microtubule organizing center, sits near the nucleus in animal cells. During mitosis, it duplicates to form the two poles of the mitotic spindle
microtubules grow out of an organizing center
centrioles
cyclindrical array of microtubules usually found in pairs at the center of a centrosome in animal cells. Also found at the base of cilia and flagella, where they are called basal bodies
Dynamic instability of microtubules
They independently grow outward from their organizing center, and then can suddenly start shrinking back, or growing out again. Allows for rapid remodeling
When bound to GTP they can constantly add more. But the GTP gets hydrolyzed to GDP from the back end. Once hydrolyzation catches up, the microtubules starts to break down
the microtubules don’t have sensors: they don’t know where to go, so it will grow randomly until it finds its target (capping protein), which stabilizes it
microtubule associated proteins
accessory proteins that bind to microtubules; they stabilize them, link them to other cell structures, or transport things along them
Microtubules would polymerize and grow longer. (They would not have permanent GTP caps)
GTP
Actin Filaments
Essential for maintaining cell shape and for movements that involve the cell’s outer membranes. Their arrangement in a cell depends on the type of actin binding proteins present.
They are thin and flexible, and has a structural polarity with distinct ends
Microtubules and intermediate filaments don’t branch, actin does
Myosin
actin-binding protein, present in muscle cells, a type of motor protein
It remains about the same length, although the subunits that comprise it are replaced (it gains monomers at the plus end at the same time as it loses monomers at the minus end, so it won’t have any dramatic changes in length
Muscle contraction
Actin filaments slide against myosin filaments during muscle contraction. Calcium release from sarcoplasmic reticulum allows myosin to engage actin filaments. The calcium release is triggered by neural impulses
the lamellipodia at the leading edge of a crawling cell. It contains actin but not myosin
troponin, which moves the tropomyosin that otherwise blocks the interactions of actin and myosin.
When a muscle cells is stimulated to contract, Ca2+ binds to troponin, which moves the tropomyosin that otherwise blocks the interaction of actin and myosin. In the absences of Ca2+, tropomyosin binds to actin monomers, preventing their interaction with myosin.
intermediate filament construction
How intermediate filaments help support tissue integrity
skin epithelial cells
How is the dynamic instability of microtubules suppressed?
by binding to a protein
How do microtubules help nerve cells
the rate of diffusion is very slow for big proteins, so it can carry them along a track from one end of a long cell to another. This does consume energy
Mircotubule tracks for protein transport
Diffusion takes too long, so transport is needed
Actin and cell movement
How do actin filaments grow?
It depends on ATP to grow
A protein that can bind to two actin filaments
How does actin allow a unicellular organism to move
Anatomy of a muscle cell
Different types of intercellular anchors
cadherins
adherins junctions
intermediate filaments are connected this way as well.
The filaments don’t pass through the cell membrane, they bind to proteins, and the proteins bind together.
How the intermediate filaments, microtubules, actin filaments, and cadherins give cells structure and hold them together
