Cell Bio Exam 3

cytoskeleton

organized networks of polymers within cells that provide mechanical strength to cell

T or F: cytoskeletons are static

F: the cytoskeleton is dynamic

three types of cytoskeletons in eukaryotic cells

1. intermediate filaments

2. microtubules

3. actin filaments

What type of interactions do the cytoskeletons form through

non covalent protein-protein interactions

Where are intermediate filaments exist

in muscle cells

why are intermediate filaments called intermediate

their diameter is between actin (thin) and myosin (thick)

characteristics of intermediate filaments

flexible and high tensile strength (can withstand stress)

subunit of intermediate filaments

staggered antiparallel tetramer of two coiled dimers

What ends stick out of sides of the intermediate tetramer subunit

N termini ends 

How do tetramer subunits associate to each other

8 tetramers associate with lateral protein to protein interactions

T or F: lateral interactions between tetramer subunits are stron

True: lateral association allows a multitude of protein to protein interactions along the subunits

T or F: intermediate filaments only associate laterally

False: subunits associate laterally and groups of 8 associate end to end 

two types of intermediate filaments

1. cytoplasmic (keratin)

2. nuclear (nuclear lamins)

T or F: cytoplasmic intermediate filaments are in all eukaryotic cells

False: they are in most, not all

T or F: nuclear lamins are in all eukaryotic cells

True: these intermediate filaments are in all animal cells

Keratin function

distributes stress when skin is stressed in the cytoplasm

where does keratin anchor in the cell

the plasma membrane

desmosomes

points of connection with neighboring cells

desmosomes function

couples cells to create sheets of cells (tissues)

Epidermis Bullosa Simplex cause and effect

• mutant keratin is the cause

• layers of cells rupture and makes skin highly vulnerable to mechanical injury because skin can’t withstand stress

Nuclear lamins function

support and strengthen nuclear membrane

T or F: nuclear lamins are found around nuclear pores

False: there is a gap in the nuclear lamins where there is a nuclear pore

Progeria cause and effect

• premature aging caused by defects in nuclear lamin

• irregularly shaped nucleus caused by defects

• irregularly shaped nucleus makes cell more prone to cell death

T or F: Actin filaments are present in all cells

True

T or F: actin filaments are less dynamic than intermediate filaments

False: they are much more dynamic

Subunit of actin

a monomer 

polymer of actin

monomers assemble into two stranded helix

T or F: Microtubules are present in all animal cells

True

T or F: Microtubules are dynamic

True

subunit of Microtubules

dimer or alpha and beta tubulin

T or F: alpha and beta tubulin proteins are the same

False: they are different proteins but very similar properties

What type of interactions form the microtubule dimer

non covalent protein protein interactions

T or F: the Microtubule dimer is highly dynamic

False: the dimer is tightly bound and permanent

protofilament

long string a heterodimers (microtubules)

structure of microtubules

lateral interactions between protofilaments forms hollow cylinder of 13 protofilaments

T or F: intermediate filaments are polar

False: they are nonpolar and symmetric

T or F: microtubules are asymmetric

True: they are asymmetric and polar due to the heterodimers

T or F: actin is nonpolar

False: actin is asymmetric and polar due to the monomers having a flat end and cleft end

Which filaments have a plus and minus end

actin and microtubules

describe the seed experiment

• minus end is slow growing

• plus end is fast growing 

T or F: if IFs went through the seed experiment, the plus end would grow faster than the minus

False: IFs have no plus or minus end, so the filament would have equal growth on both sides

which filaments act as tracks in cells for directed transport of vesicles and organelles

Actin and Microtubules because they are asymmetric

motor protein function

walk along tracks and deliver vesicles and organelles

T or F: intermediate filaments have directed transport

False: they are symmetric so directed transport is not possible

Motor proteins associated with microtubules

Dynein and and kinesin

where does the head domain of the motor protein bind to

the microtubule

where does the tail domain of the motor protein bind to

the cargo

T or F: motor proteins associated with microtubules are tetramers

False: they are dimers with two heads and and two tails

T or F: Dynein walks to the plus end of the microtubule

False: it walks to the minus end

T or F: Kinesin walks to the plus end of the microtubule

True

What drives the movement of motor proteins

ATP binding and hydrolysis in the binding pocket in the head of the motor protein

T or F: there is one binding pocket in each motor protein to drive its movement

False: there are two binding pockets. However, the hydrolysis of 1 ATP at a time drives one step of the motor protein

Motor protein(s) associated with actin

Myosin

What end of actin do Myosin proteins walk towards

the plus end

What drives the movement of Myosins

ATP binding and hydrolysis

How do motors slide filaments

• motor remains in one place and begins to walk (anchored somewhere in the cell)

• motor walks toward plus end

• minus end is pushed forward (leading end)

gliding filament assay

• motor tails attach to the slide

• filaments and ATP are added on top of the motor proteins

• filaments are pushed around on the slide 

Myosin 2

• motors assemble and form bipolar filaments on actin

• drives muscle contraction (sarcomeres) 

Sarcomere

• contractile unit in muscle

• contracts when myosin heads slide filaments together (Z discs are moved towards the middle)

• relaxes when myosin heads stop moving (Z discs farther a part)

stages of actin polymerization

1. nucleation 

2. elongation

3. steady state 

Nucleation

• formation of stable nucleus

• slow step

• aka lag phase

elongation

• monomer addition to the ends of actin

• fast step

• aka growth phase

steady state

• rate if addition equals rate of loss

• aka equilibrium phase

T or F: during the steady state, the actin filaments mass increases and decreases

False: the mass remains the same

actin nucleus

3 subunits of actin (forms trimer)

T or F: before the actin trimer forms, the dimers made are strong and do not fall apart

False: all monomer complexes made before the nucleus are weak and fall apart

Critical concentration

• concentration of monomers for subunits to form nucleus 

• is concentration is below, the subunits will not form a nucleus randomly

T or F: if the reaction of actin polymerization is seeded, the lag phase is eliminated

True: the nucleus is already made, there is no slow step

T of F: actin monomers only bind the plus end

False: they bind to the plus and minus end of actin

Binding properties of actin at plus end vs minus end

• at plus end, monomers bind tightly- more polymerization

• at minus end, monomers bind loosely- less polymerization

Plus or minus actin end: low ON, high OFF

minus end

Plus or minus actin end: high ON, low OFF

plus end

T or F: Actin binds and hydrolyzes GTP

False: ATP

What is the conformational change that occurs when actin binds ATP

the cleft creates a binding site for ATP so another monomer can be added

ATP cap

• on plus end of actin that facilitates polymerization

• actin monomers bind tightly here

T or F: both ends of the actin polymer have ATP, the minus end has less

False: the minus end of actin has ADP (monomers are less likely to associate to ADP)

T or F: actin requires ATP to polymerize

False: polymerization still occurs at minus end with ADP 

T or F: an actin filament would not polymerize if it had two minus ends

False: it would still polymerize, but it wouldn’t get as big

Tredmiling

• ATP adds to plus and dissociates from minus end of actin

• filament length stays the same (this occurs in steady state)

• subunits shift through filament like a tredmil

T or F: a filament with only ATP will still treadmill

False: a filament with only ATP or only ADP will not treadmill because the ends are equal

T or F: a filament with only ATP or ADP will not reach steady state

False: it will

actin binding proteins

control actin assembly and organization. categories include:

1. nucleating proteins

2. promote filament disassembly

3. organize filaments’

4. motor proteins

Nucleating proteins

enhance filament formation Ex. ARP complex and formin

severing proteins

cuts actin

monomer sequestering protein

binds monomers and prevents them from adding to filaments (decreases number of free actin monomers)

capping proteins

blocks plus end of actin

cross linking protein

creates actin networks in the cell cortex

bundling protein

creates parallel bundles of actin Ex. Filopodia

what drives cell migration

actin polymerization at leading edge of cell

lamellipodium

• sheet like

• actin creates branched network

filopodium

• spiky protrusions

• composed of straight actin

actin cortex

• under plasma membrane

• couples front and back of cell so cell moves as a unit

Lamellipodia protrusion causes…

actin cortex to tense and rear of cell to contract

focal contacts

• between cell and substrate (like feet)

• forms at front of cell and disassembles at rear

T or F: actin filaments continue to grow towards the rear of the cell

False: old filaments disassemble as they get farther away from the front. The plus ends get capped, keeping polymerization concentrated at the front

ARP Complex

• creates branched actin networks

• binds to side of filaments and nucleates new branch

T or F: the ARP complex remains bound to minus end of the new filament

True: the minus end is capped by the ARP complex so there’s no depolymerization there

Formins

• ring structure

• stabilizes initial actin trimer

• formin remains at the plus end and helps the  monomers add 

• nucleates linearly

Bacteria in relation to actin and cell mobility

• bacteria hijacks polymerization machinery

• protrusion pushes its way into other cells and spreads

T or F: microtubule dimer binds and hydrolyzes ATP

false: it binds GTP