Chapter 17: The Cytoskeleton Part II

Question 1

Actin

Answer 1

• actin filament -> polymers of actin
• mnost abundant protein in nearly all cell types
  -maintain cell shape and movements that involve cell outer membtane
• actin filaments interact with actin-binding proteins that enable filaments to serve various functions
• has movement driven with motor proteins (like microtubules!!)

Question 2

cytoskeleton filaments

Answer 2

• constructed from smaller protein subunits
• smaller soluble subunits & large filamentous polymer
  -dynamic and adaptable
• formation of protein filaments from smaller protein subunits allows regulated filament assembly and dissassembly to reshape to cytoskeleton

Question 3

Actin filaments

Answer 3

• thin, flexible protein with structurally distinct ends
• two stranded helix with twists, twists of identical globular monomers, they “face” same direction
  -lateral interaction prevent separation
• shorter than microtubules
• grow by addition of monomers on either end (plus side faster)
• similar to microtubules: such as polarity with plus and minus end

Question 4

Actin polymerization

Answer 4

• actin monomers in cytosol carry ATP
• ATP hydrolyzed to ADP soon after assembly into growing filament
• ADP molecules remain trapped within actin filament, unable to exchange with ATP until actin monomer carries them dissociates from filament

Question 5

Actin “treadmilling”

Answer 5

• rate of monomer addition = rate of monomer loss (remains one length)
• causes move through of actin from plus end to minus end
• if actin monomers added to plus end faster than ATP is hydrolyzed, then plus end grows
• if ATP hydrolyzed faster than monomers added, the minus end loses (ADP-actin destabilizes, loose structure)

Question 6

Microtuble dynamic instability

Answer 6

• switching between polymerization and depolymerization
• undergo rapid remodeling
• crucial to function (division)
  -stems from intrinsic capacity of tubulin dimers to hydrolyze GTP

Question 7

Growing microtubule

Answer 7

• grow since bound to GTP
• GTP-tubulin combo hydrolyzes to GDP after dimer is added
• rapid polymerization, tubulin dimers bind faster than GTP hydrolyze
• this makes end completely GTP-tubulin dimers, creates GTP cap(packs efficiently and grows)

Question 8

shrinking microtubule

Answer 8

• hydroylzed GTP to GDP + phosphate
• protofilaments composed of GDP tubulin pack loosely, favoring dissassembly
• once depolymerization starts, it continues
• dissasssembled GDP tubulin in cytoplasm (exchange bound GDP for GTP so they can be added to growing microtubule

Question 9

microtubule building-

Answer 9

• microtubules grow when bound to GTP, but when GTP is hydrolyzed to GDP microtubules shrink

Question 10

nucleation

Answer 10

• occurs in actin and microtubules
• the process by which atoms or molecules come together to form a new phase or structure.
• rate limiting step in polymerization
  -helical polymer stabilized by multiple contacts between adjacent subunits
• (in actin, two actin molecules bind realtively weakly to each other, third actin creates trimer to make entire group more stable)
• further monomer addition can take place for trimer, which therefore acts as nucleus for polymerization
• slow assembly from lag phase(can be reduced or abolished)

Question 11

Lag phase

Answer 11

• seen during polymerization
• when cells are adjusting to a new environment before starting exponential growth
• reduced or abolished entirely by adding premade nuclei, such as fragments of already polymerized microtubules or actin filaments

Question 12

Course of polymerization

Answer 12

• assembly of protein into a long helical polymer such as a cytoskeletal filament or bacterial flagellum
  -lag phase(lower polymers and low time)
• growth phase (increasing polymers over time)
• equilibrium phase (no net change, higher polymers and time)

Question 13

Lag phase

Answer 13

corresponds to time taken for nucleation

Question 14

growth phase

Answer 14

occurs as monomers add to exposed ends of growing filament, causing filament elongation

Question 15

equilibrium phase

Answer 15

steady phase, reached when growth of polymer due to monomer addition precisely balances the shrinkage of the polymer due to disassembly back to monomers

Question 16

Nucleatin of Actin

Answer 16

• involves ARP complex, Formins
• makes different types of actin filaments
• typically occurs in response to extracellular signals and is catalyzed by actin related proteins(ARP complex)nucleating at minus end, allowing rapid elongation at plus end

Question 17

Formin

Answer 17

• proteins
• actin elongation mediated by formins
• forma dimeric complex that can nucleate new actin filaments
• nucleates growth of straight, unbranched filaments that form parallel bundles and remain associated with rapidly growing plus end as it elongates
• maintains binding to one of the two actin subunits at the plus ends
• formins connect indirectly to plasma membrane

Question 18

ARP complex

Answer 18

• actin related proteins
• contrasts to formins
• nucleates and remains bound to minus end and form branched webs of actin

Question 19

Actin Cell Movement

Answer 19

• depends on polymerization and depolymerization of actin in response to signals
• actin nucleatin influences the type of protrusions formed at leading and trailing edge (involved cell crawling)

Question 20

Cell crawling

Answer 20

• cell sends out protrusions at its front/leading edge
• thes protrusions adhere to the surface over which cell is crawling
• rest of cell drags itself forward by traction on points of anchorage

Question 21

ARP promotion

Answer 21

• promotes formation of branched actin filaments in lamellipodia (filaments produce side branches; web undergoes addition at leading edge and disassembly further back, pushes lamillipodium forward)

Question 22

Actin-binding proteins

Answer 22

• bind to actin monomers in cytoplasm (regulate when/where actin filaments form and grow; ARP and formins promote actin polymeriztion)
• controlled by extracelular signal molecules (act through cell surface receptor proteins, activate signal pathways; coverage on group relaed monomer GTPases)

Question 23

Cell motility

Answer 23

-cell polymerization here involes activation of different Rho members at cell opposite ends that give motility, allows movement in particular direction
involves cell signaling(changes in actin cytoskeleton)
- have receptor proteins that act on intracellular targets (Rac, Rho)

Question 24

Rac and Rho

Answer 24

• GTP binding proteins involved in cell motility
• molecular swtiches: control intracellular processes by cylcing between active GTP-bound state and inactive GDP bound state

Question 25

Rho protein family

Answer 25

- alter organization of actin filaments - one triggers bundling of actin filaments and activation of formin proteins (promotes filopodia formation) - one activates nucleation promoting factors, stimulate ARP complexes that iduce lamellipodia formation - one (MAJOR) bundles actin filaments with motor proteins to form contractile fibers

Question 26

Actin motor proteins

Answer 26

thymosin and myosin

Question 27

thymosin

Answer 27

bind and prevent addition to actin filaments until needed (contribute to regulation)

Question 28

myosin

Answer 28

in family or motor proteins that bind to and hydrolyze ATP; provides energy for actin filament to move forward toward plus end)

Question 29

various actin binding protein functions

Answer 29

actin bundling that holds parallel bundles together, cross-links together to support membrane, and severing proteins to create shorter lengths

Question 30

Muscle contraction

Answer 30

- triggered by sudden rise in cytosolic Ca2+ (released to ion channels in sarcoplasmic reticulum membrane; often relays messages from cell exterior to cell interior - small, bipolar myosin II filament can slide two actin filaments of opposite orientation past each other(generates strong contractile force!) - myosin II head group walks toward the plus end of the actin filament with which it interacts; multiple myosin molecules required to generate movement

Question 31

muscle myosin in myosin II subfamily

Answer 31

-muscle contraction depends on these interacting filaments of actin and myosin - has dimer proteins: two globular ATPase heads at one end and single coiled coil tail at other - clusters of myosin II bind through tail to form bipolar myosin filament(heads project outwards)

Question 32

structures of muscle contraction

Answer 32

- skeletal muscle fibers formed by fusion of many separate cells(bulk made of myofibrils)

Question 33

myofibrils

Answer 33

- contractile elements of muscle cell, actin filaments and myosin II filaments arranged in organized structure call sacromeres! - myosin II in middle, actin on outer edges extending inward

Question 34

sacromere

Answer 34

- consits of ordered array of parallel and partly overlapping thin(actin, light bands) and thick filaments(myosin, dark bands) - attachment of each thin filament occurs at Z-disc/line (no myosin)

Question 35

Detailed description of rise in Ca2+ for muscle contraction

Answer 35

- activates molecular switch of accesory proteins - includes tropomyosin and toponin to control skeletal muscle contraction - in resting muscle troponin I pulls tropomyosin out of the helix therby blocking myosin binding site on actin - binding of Ca2+ to troponin C forces conformational change diriving the tropomyosin complex deep with the actin helix exposing the myosin binding site on actin - single passed rapidly

Question 36

Mutant keratin

Answer 36

- skin more prone to blistering - rare human genetic disease, epidermolysis bullosa simplex - keratin genes intefere with formation of keratin filaments in epidermis

Question 37

Anti-mitotic drugs

Answer 37

- can control caner that target mitotic spindle - some drugs destabilize microtubules by interfering with polymerization of tubulin dimers - cause inappropriate proliferation tumor cells to die - at expense of death of healthy tissue as well

Question 38

Ciliary dyenin

Answer 38

- left-right asymmetry caused by mutations in these

Question 39

neutrophils

Answer 39

- migrate out of blood into infected tissues when small molecules released by bacteria (engulf them when found)