Microfilaments and Intermediate Filaments Presented by: Leslie Hargis.

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?