microfilaments and intermediate filaments presented by: leslie hargis
TRANSCRIPT
Microfilaments and Intermediate Filaments
Presented by: Leslie Hargis
Cytoskeleton- Cytoplasmic system of fibers, critical to cell motility
3 Types: – Microfilaments (Actin Filaments)
– Intermediate Filaments
– Microtubules
Microfilaments and Intermediate Filaments Both are usually attached to plasma
membrane proteins They form a skeleton that helps support
the plasma membrane
Actin Filaments
Thinner, shorter and more flexible than microtubules
Contains G-actin, and F-actin Actin- most abundant intracellular
protein in most Eukaryotic cells Comprises 10% by weight of total cell
protein in muscle cells, 1-5% in non-muscle cells
Actin polymerization requires K+, Mg 2+, ATP, and Calcium
Here is an animation of the sliding of a single myosin molecule along an actin filament
www.sci.sdsu.edu/movies/actin_myosin_gif.html
G-actin and F-actin
G-actin: exists as a globular monomer F-actin: is a helical filamentous polymer of G-
actin subunits all oriented in the same direction
Microfilaments in a cell are constantly shrinking or growing in length
Bundles and meshworks of microfilaments are forming and dissolving continuously
Dynamics of Actin Assembly
Nucleation- G-actin clumps into short, unstable oligomers, 3-4 subunits long, and acts as a stable seed or nucleus.
Elongation phase- The nucleus rapidly increases in length by the addition of actin monomers to both of its ends.
Dynamics of Actin Assembly
Steady-State – The ends of actin filaments are in a steady
state with monomeric G-actin. – After their incorporation into a filament,
subunits slowly hydrolyze ATP and become stable F-actin.
Copyright (c) by W. H. Freeman and Company
Nucleation, Elongation, and the Steady-State
Figure 18-11
Dynamics of Actin Assembly
All subunits in an actin filament point toward the same direction of the filament– Exhibits polarity-actin subunit exposed to the
surrounding solution is the (-) end– The cleft that has contact with the neighboring
actin subunit that is not exposed is the (+) end– Actin filaments grow faster at the (+) end than the
(-) end
F-actin has structural and functional polarity
Actin Filaments participate in a variety of cell functions: Anchorage and movement of
membrane proteins- – filaments are distributed in 3-dimensional
networks throughout the cell – used as anchors with in specialized cell
junctions
Actin Filaments participate in a variety of cell functions: Formation of the structural core of
microvilli– On epithelial cells, help maintain shape of
the cell surface
Actin Filaments participate in a variety of cell functions: Locomotion of the cells
– Achieved by the force exerted by actin filaments by polymerization at their growing ends
– Used in many migrating cells, particularly on transformed cells of invasive tumors
– Cells extend processes from their surface by pushing the plasma membrane ahead of the growing actin filaments
Essential in cytoplasmic streaming
Functions and structure of intermediate filaments distinguish them from other cytoskeletal fibers
Intermediate Filaments (IF)
Found in most animals but not in plants and fungi
Smaller than microtubules but larger than microfilaments
Subunits are -helical rods that assemble into ropelike filaments
Unlike microfilaments, IF’s don’t contribute to cell motility
Intermediate Filaments (IF)
Provides mechanical support for the plasma membrane where it comes in contact with other cells or with the extracellular matrix
Extremely stable- even after extraction with solutions containing detergents and high concentrations of salts
IF’s are broken down into 4 groups: Lamins- found in the nucleus Keratins (cytokeratins)- in the epithelia
– Acidic or basic– “hard” epithelial tissues- nails, hair, wool
IF’s are broken down into 4 groups: Type III IF proteins (Vimentin)- most
abundant type– In leukocytes, blood vessel endothelial
cells, mesenchymal cells Neurofilaments- neuronal axons
– Extend from the cell body into the ends of axons and dendrites
– Provides structual support
A purified neurofilament
Intermediate Filament Assembly
Assembled from a pair of helical monomers that twist around each other to form coiled-coil dimers
Then 2 coiled-coil dimers twist around each other to make a tetramer of 2 coiled-coil dimers
This forms the non-polarized unit of the IF’s (unlike microfilaments that are polarized)
Diseases caused by defects in the IF
Epidermolysis bullosa simplex– Blisters form due to lack of normal bundles of
keratin filaments
Alzheimer’s disease– Caused by changes in the neurofilaments with in
brain
Alcoholic liver cirrhosis– Accumulation of keratin filaments forming
inclusions called mallory bodies in liver
Any Questions?