Ch 17 Intermediate Filaments

What does the cytoskeleton allow eukaryotic cells to do?

organize internal components, adopt various shapes, move, and mechanically interact with the environment and other cells

In which type of cells is the cytoskeleton most prominent?

eukaryotic cells

Name some processes that would not occur without the cytoskeleton.

• wound healing

• muscle contraction

• sperm movement toward the egg

Day to day, what is one major role of the cytoskeleton in all cells?

organizing the interior of the cell

How does the cytoskeleton influence organelles?

it controls their location and organizes their specialized functions

What does the cytoskeleton provide for intracellular transport?

the machinery that moves materials between organelles

What major event in cell biology relies heavily on the cytoskeleton?

collecting and distributing duplicated chromosomes during cell division

What final process in cell division requires cytoskeletal action?

cleaving one cell into two daughter cells (cytokinesis)

What are the main types of cytoskeletal filaments?

• intermediate filaments

• microtubules

• actin filaments

What forms intermediate filaments?

a family of fibrous proteins that entwine together

What protein subunits form microtubules?

globular tubulin subunits

What protein subunits assemble to form actin filaments?

globular actin subunits

What is the main function of intermediate filaments?

to enable cells to withstand mechanical stress such as twisting or deformation

Which cytoskeletal filaments are the toughest and most durable?

intermediate filaments

Where in animal cells are intermediate filaments found?

in the cytoplasm of most animal cells

What is the typical arrangement of intermediate filaments in the cytoplasm?

they form a network surrounding the nucleus and extending to the cell periphery

At what cell structures are intermediate filaments often anchored?

desmosomes (cell-cell junctions)

Where are intermediate filaments found inside the nucleus?

in the nuclear lamina

What is the nuclear lamina?

a meshwork of intermediate filaments that underlies and reinforces the nuclear envelope

What are intermediate filament proteins composed of?

a central elongated rod domain with unstructured domains at both ends

What structural feature of the rod domain allows dimer formation?

an extended alpha-helical region that forms a coiled-coil between two proteins

What structure is formed by two coiled-coil dimers running in opposite directions?

a staggered antiparallel tetramer

What are the soluble subunits of intermediate filaments?

dimers and tetramers

What type of bonding holds intermediate filament subunits together?

noncovalent bonding

What gives intermediate filaments their great tensile strength?

many overlapping lateral interactions along the length of the proteins

Why do intermediate filaments lack polarity?

because paired dimers assemble in opposite orientations

• making both ends of the tetramer identical

Why do different intermediate filament proteins form filaments of similar diameter?

because their central rod domains are similar in size and amino acid sequence

How do intermediate filaments help epithelial and muscle cells?

they distribute locally applied forces to prevent tearing from mechanical shear

In what types of cells are intermediate filaments especially prominent?

cells subject to mechanical stress

• axons

• muscle cells

• epithelial cells

What are the major classes of intermediate filaments?

• keratin filaments

• vimentin-related filaments

• neurofilaments

• nuclear lamins

Which intermediate filaments are found in epithelial cells?

keratin filaments

Where are vimentin and vimentin-related filaments found?

• connective tissue cells

• muscle cells

• glial cells

Where are neurofilaments found?

along axons of nerve cells

Where are nuclear lamins located?

in the nucleus

• forms the nuclear lamina

Which intermediate filament class is the most diverse?

keratin filaments

How are keratin filaments arranged in epithelial cells?

they span the cell from one side to the other

What is the function of neurofilaments?

they provide strength and stability to neuronal axons

The intermediate filaments in epithelial cells extend across the

cytoplasm of cells connected to each other in a sheet

• cells would rupture when stretched if there were no intermediate filaments

What is the function of the nuclear lamina?

to line and strengthen the inner surface of the nuclear envelope

What happens to the nuclear lamina during mitosis?

it disassembles when the nuclear envelope breaks down and reassembles in daughter cells

What controls lamina disassembly and reassembly?

phosphorylation and dephosphorylation of lamins

What effect does phosphorylation of lamins have?

protein kinases phosphorylate lamins → lamin-lamin interactions weaken → filaments fall apart

What effect does dephosphorylation of lamins have?

protein phosphatases remove phosphate groups → lamins reassemble at the end of mitosis

Defects in which protein class are linked to progeria?

nuclear lamins

How could lamin defects affect gene expression?

because lamins bind chromatin, defects may alter chromosome positioning and change gene expression patterns

The cytoplasmic skeleton is connected across the

nuclear envelope to the nuclear lamina or chromosome through sets of linker proteins

The nuclear lamina lines the

inner face of the nuclear envelope