bio cytoskeleton

What are the three major cytoskeletal filament systems?

Intermediate filaments, actin filaments, and microtubules.

What is the main structural role of the cytoskeleton?

Provides mechanical strength, structural support, and organization while enabling dynamic cellular processes.

How do cytoskeletal filaments assemble?

Protein subunits associate through reversible, non-covalent interactions.

What are the main properties of intermediate filaments?

Flexible, high tensile strength, and resistant to mechanical stress.

What is the basic subunit of intermediate filaments?

An antiparallel tetramer composed of two parallel coiled-coil dimers.

How do intermediate filament tetramers assemble into filaments?

Eight tetramers associate laterally, then groups associate end-to-end into rope-like filaments.

Why are intermediate filaments nonpolar?

Their antiparallel tetramer subunits are symmetric, producing identical filament ends.

What is keratin?

A cytoplasmic intermediate filament protein found in epithelial cells.

What is the function of keratin filaments in epithelial tissue?

They distribute mechanical stress and help maintain skin structure.

What is epidermolysis bullosa simplex?

A genetic disease involving mutant keratin that weakens skin and causes blistering under stress.

What are nuclear lamins?

Intermediate filament proteins that form a supportive network beneath the inner nuclear envelope.

What is the nuclear lamina's function?

Provides strength and structural support to the nuclear membrane and nucleus.

What disease is associated with defective nuclear lamin proteins?

Progeria, a premature-aging disease associated with abnormal nuclear shape.

What is the basic subunit of an actin filament?

A single actin protein monomer.

How is an actin filament structurally organized?

Two strands of actin subunits twist around each other in a helical arrangement.

What is the basic subunit of a microtubule?

An alpha-beta tubulin heterodimer.

What is a heterodimer?

A complex consisting of two different protein subunits.

How is a microtubule structurally organized?

Thirteen protofilaments associate laterally to form a hollow cylinder.

What are protofilaments?

Linear chains of alpha-beta tubulin heterodimers that form the microtubule wall.

Which cytoskeletal filaments are polar?

Actin filaments and microtubules.

Why are actin filaments and microtubules polar?

Their asymmetric subunits assemble in a consistent orientation, creating distinct ends.

Why are plus and minus ends named this way?

They reflect different growth dynamics, not electrical charges.

Which end of actin filaments generally grows faster?

The plus end.

Which end of microtubules generally grows faster?

The plus end.

Why is filament polarity important for intracellular transport?

It allows motor proteins to move cargo directionally along cytoskeletal tracks.

What are the two major microtubule motor protein families?

Kinesin and dynein.

dynein

• we focus on cytoplasmic dynein

• walk to mt minus ends

Kinesins

• walk toward the microtubule plus end

• many families

What are the head and tail domains of motor proteins responsible for?

Heads bind and walk along filaments; tails connect motors to cargo.

How does ATP drive motor protein movement?

ATP binding and hydrolysis cause conformational changes that drive stepping along filaments.

What motor proteins move along actin filaments?

Myosins.

What is a gliding filament assay?

An experiment that anchors motor proteins to a surface and observes their movement of filaments using ATP.

How does myosin II promote muscle contraction?

It slides oppositely oriented actin filaments together, bringing Z disks closer.

What is polymerization?

The assembly of individual protein subunits into a polymer.

What does it mean that cytoskeletal filaments self-assemble?

Purified subunits can form filaments in vitro without additional factors.

What is actin nucleation?

Formation of a stable cluster of three actin monomers that initiates filament growth.

Why is nucleation the slow step of actin polymerization?

Three monomers must randomly form a stable configuration.

What is critical concentration?

The monomer concentration threshold needed to support filament polymerization.

What is elongation during actin polymerization?

Rapid addition of actin monomers to both ends of an established filament.

What is steady state during actin polymerization?

The stage when subunit addition equals subunit loss, stabilizing total polymer mass.

What is the lag phase of actin polymerization?

The initial slow phase during which stable nuclei form.

How does adding preformed actin filaments affect polymerization?

It eliminates the lag phase by providing seeds for elongation.

Why does the actin plus end grow faster than the minus end?

Monomers associate more strongly at the plus end, giving a higher on-rate and lower off-rate.

What nucleotide is bound to actin monomers before filament incorporation?

ATP.

What happens to actin-bound ATP after incorporation into a filament?

ATP is hydrolyzed to ADP.

What is the ATP cap of an actin filament?

A region near the growing end enriched in newly incorporated ATP-actin subunits.

Why is ATP-actin more stable in a filament than ADP-actin?

ATP-actin binds more tightly to neighboring subunits.

Why is ADP-actin more likely to dissociate?

ADP-actin has reduced affinity for neighboring subunits.

All intermediate filaments are

flexible, but also high tensile strength

intermediate subunit

tetramer

intermediate tetramer is arranged in an

antiparallel fashion

how intermediate subunits form

monomer associates with another to form a dimer

dimer is colied-coil

dimers form antiparallel tetramers (subunit = tetramer)

one unit intermediate filament

8 tetramers

cytoplasmic intermediate filament

keratin (in epithelial cells)

nuclear intermediate filament

nuclear lamins (in all animal cells)

keratin filament

• distribute stress when skin is stretched

• in cytoplasm

• anchored to plasma membrane

• filaments linked to sites of connection w neighboring cells

purpose of anchoring filament networks togeter

couples then as one unit, gives shape and structure to a whole sheet of cells

result of epidermis bullosa simplex

weak keratin, skin can’t withstand stress, vulnerable to blistering and injury

nuclear lamins

form network called nuclear lamina

progeria

premature aging disease, caused by defects in nuclear lamin proteins

actin and microtubules are present

in all eukaryotes

actin filaments are highly __

dynamic

actin subunits assemble into

2-stranded helix

microtubule subunit

dimer, technically heterodimer of a and B

microtubule polymer

made of 13 individual filaments, protofilaments, of alternating a-B

seed experiment where one end grows faster

applies to microtubules and actin

why would you want filaments to have polarity?

can serve as tracks within the cell for motor proteins to walk on

walking behvior of motor proteins

ATP hydrolyzed to ADP, conformational change in head domain

• affects if head binds or releases and swings forward

NO KNOWN MOTORS

on intermediate filament

gliding filament assay setup

motors stuck to slide, but filaments are passed and move

myosin II

• dimer (two heads, coiled-coil tail)

• slide actin, forming contractile structures in cells

bipolar filament

heads point outwards in different directions

muscle contraction

• myosin heads walk toward actin + ends at the Z-disks

• Z-disks come closer together

• contractile unit shortens