Exam 3

Question 1

Actin Monomer and Structure/Polarity

Answer 1

Globular Actin Monomer (G-Actin); made of two lobes separated by a nucleotide binding cleft (ADP or ATP) - Cleft opening is at minus end and closed end is plus end —> POLAR

Question 2

Actin Filament Formation

Answer 2

Multiple G-actin monomers bind in the same direction (one’s plus ends interact with another’s minus ends) to form the helical, polar F-actin (Cleft is exposed at one end and not exposed at the other)

Question 3

Inducing F-Actin Formation

Answer 3

Add Mg2+ –> Associates with Nucleotide bound in Cleft

Question 4

3 Steps for F-Actin Formation

Answer 4

1. Nucleation - involves trimer assembly as a seed for growth; slow step because actin dimers are unstable
2. Elongation - subunits add to the trimer; growth
3. Steady State - rate of elongation = rate of subunit loss; occurs when G-actin is at critical concentration

Question 5

Critical Concentration for Actin

Answer 5

If G-Actin exists below critical concentration, no polymerization will be occurring yet

If G-actin exists above critical concentration, polymerization will occur
Always trends toward making [G-actin] = [Critical]

Question 6

Rate of Polymerization at each end of F-Actin and why?

Answer 6

Critical Concentration at (+) end is lower and thus growth occurs much faster = 0.12 um

Critical Concentration at (-) end is higher and thus growth occurs much slower = 0.6 um

This is due to polymerizing actin monomers containing ATP. As these monomers exist in the filament, said ATP is hydrolyzed and the affinity of the actin monomers for each other decreases. Because it will be typical that subunits will “move down” F-actin towards minus end, the affinity for a free G-actin for the (-) end (ADP-containing monomer) will be lower than for the (+) end (ATP-containing monomer)

Question 7

Real Cell Critical Concentration for F-Actin Formation and what occurs when this value is reached?

Answer 7

0.2 um; Actin Filaments will exist in the Steady State. (+) ends are adding G-actin and (-) ends are releasing monomers such that there is no net growth or shrinkage – known as Treadmilling!!

Question 8

Reality is G-actin does not exactly exist at critical concentration, in fact a concentration much higher…what then prevents a great extent of F-actin formation?

Answer 8

Actin-Binding Proteins and Actin Sequestering Proteins

Question 9

Examples of ABPs and ASPs that prevent spontaneous filamentation

Answer 9

Thymosin Beta-4: binds to free ATP-Actin to prevent polymerization

Profilin: promotes ADP to ATP exchange in cleft while remaining bound to (+) end; promotes polymerization while preventing nucleation

Cofilin: sequesters considerable segment of ADP-Actin from F-actin; promotes depolymerization by “creating more (-) ends”

Question 10

What exists to catalyze nucleation? 3 Classes

Answer 10

Actin Nucleating Proteins (3 General Classes)
1. Formin Family - attaches to (+) end and uses its FH2 domain to preform multiple (+) end dimerizations, leading to long unbranched filament formation

2. Tandem Actin Monomer-Binding Proteins: caps (-) end to promote nucleation and (+) [open] end growth
3. ARP2/3 Complex: binds to existing F-actin to create new branch and stays bound to said new branch’s (-) end; MUST pair with WASP protein to get function/nucleation

Question 11

Chemical Interference with Actin Cytoskeleton: 3 Compounds

Answer 11

Phalloidin: binds along actin filaments to stabilize and prevent depolymerization

Cytochalasin: caps (+) ends to prevent elongation and eventually cause complete depolymerization of said F-actin

Latrunculin: Binds actin monomers to gatekeep them out of polymerizing

Question 12

Microtubule Subunit and Structure/Polarity

Answer 12

Alpha- Beta- Tubulin Heterodimer; alpha subunit will bind GTP and never hydrolyze it while beta subunit will hydrolyze it

Question 13

Microtubule Formation

Answer 13

Multiple heterodimers align to form linear structures - protofilaments.
13 protofilaments laterally interact (in parallel fashion) to create a cylindrical shaped structure with a (+)/beta end and (-)/alpha end

Question 14

3 Steps of Microtubule Formation

Answer 14

1. Nucleation - involves several heterodimers associated to create seed for growth (slow step)
2. Elongation - more heterodimers add to the seed; growth
3. Steady State - rate of elongation = rate of subunit loss; occurs when free/available tubulin is at critical concentration

Question 15

In-Vitro versus In-Vivo Microtubule Assembly

Answer 15

In Vitro: Subunit Addition or Loss is observed at both ends

In Vivo: (-) end is capped/anchored in MT Organizing Center/Centrosome to help with initial nucleation – only (+) end growth is observed

Question 16

Free Microtubule Dynamics - Treadmilling

Answer 16

Undergoes treadmilling can experience addition from both ends

GTP bound heterodimers will add and create a GTP cap; as these subunits “progress down” the microtubule, the beta subunit undergoes hydrolysis and “leaves” the GTP cap

If [free tubulin] > [critical], rapid addition and thus rapid GTP cap growth occurs
If [free tubulin] ≈ [critical], there is a stochastic chance that GTP cap could be lost if addition is too slow; this loss is destabilizing

Question 17

Behavior of Microtubule Growth about Critical Concentration

Answer 17

If [free tubulin] > [critical], rapid addition and thus rapid GTP cap growth occurs

If [free tubulin] ≈ [critical], there is a stochastic chance that GTP cap could be lost if addition is too slow

Loss of GTP cap is destabilizing as it causes a conformational change that induces curving and weakens lateral protofilament interactions

Question 18

Free Microtubule Dynamics - Dynamic Instability

Answer 18

Based on rates of addition, microtubules randomly alternate between two growth states due to nucleotide conversions at the (+) end

Sudden Growth to Shrinkage: Catastrophe
Sudden Shrinkage to Growth: Rescue

When in the steady state, these effects cooccur such to keep total amount of polymerized heterodimers constant

Question 19

Where are microtubules typically anchored? What is the purpose?

Answer 19

The Microtubule Organizing Center/Centrosome

Contains two centrioles surrounded by pericetriolar material, inclusive of the gamma-tubulin ring complex (γ-TURC)

γ-TURC is a nucleation site where the (-) ends anchor; treadmilling is no longer possible but dynamic instability at the (+) ends is

Question 20

Chemical Interference with Microtubule Cytoskeleton: 3 Compounds

Answer 20

Taxol: Binds along filaments to stabilize polymer and prevent catastrophe

Colchicine: Caps filament ends to prevent addition/polymerization, causing formation of a GDP cap [destabilizing, promoting depolymerization]

Nocodazole: Binds to tubulin subunits to prevent them from adding, causing inevitable formation of the GDP cap and subsequent depolymerization/catastrophe

Question 21

Actin Capping Proteins

Answer 21

Stabilizes F-actin by binding to ends to prevent addition/loss of subunits

CapZ for (+) ends
Tropomodulin for (-) ends

Question 22

Actin Stabilizing Proteins

Answer 22

Stabilizes F-actin by binding length-wise to stabilize filament

Tropomyosin

Question 23

Actin Severing/Depolymerizing Proteins

Answer 23

Does not have a consistent manner of binding but always leads to disassembly

Cofilin: severs filaments to create more (-) ends, more sites for depolymerization
Gelsolin: breaks filaments and binds to (+) ends

Question 24

Actin Cross-Linking Proteins (2 Results)

Answer 24

Produces different assemblies of F-Actin based on built-in spacer domain
Typically has two binding domains to bind multiple filaments and create the assembly

With Flexible, Longer Spacer Domains, we see Gel-Like Networks
Caused by Filamin, Spectrin, ARP2/3

With Rigid, Shorter Spacer Domains, we see Aligned Bundles
Caused by Fimbrin, Villin, Fascin, and Alpha-Actinin [slightly longer]

Question 25

Membrane-Actin Linker Proteins

Answer 25

Create connections between Actin Cytoskeleton and Plasma Membrane, usually Peripheral Membrane Proteins that serve as the linkage between cytoskeleton and integral membrane protein ERM Family (Ezrin, Radixin, Moesin) Spectrin Dystrophin

Question 26

Spectrin-Based Cytoskeleton (Example)

Answer 26

Spectrin binds to Actin Filaments and Ankyrin Ankyrin binds to Band 3 (Integral Membrane Protein)

Question 27

What happens consequence to mutations in Membrane-Actin Linker Proteins?

Answer 27

Mutations in genes encoding any part of Spectrin-Based Cytoskeleton leads to Hereditary Spherocytic Anemia or conditions where RBC rupture easily Mutations in Dystrophins lead to Duchenne Muscular Dystrophies (progressive weakening of skeletal muscle)

Question 28

Microtubule Tubulin Oligamer Binding Proteins

Answer 28

Sequester heterodimers or oligamers to prevent rebinding/rescue and thus promote catastrophe/depolymerization Stathmin

Question 29

Microtubule Nucleating Proteins

Answer 29

Assist with Nucleation at Centrosome γ-TURC

Question 30

Microtubule End Binding Proteins

Answer 30

Regulate end dynamics and linkages to other cellular structures EB1: (+) tip - specifically for linkages to organelles CLIP-170: (+) tip - specifically for interactions between the end and chromosomes or membrane

Question 31

Microtubule Severing Protein

Answer 31

Break MTs to create more (-) ends and promote catastrophe/depolymerization Katanin

Question 32

Microtubule Depolymerizing Proteins

Answer 32

Promote (+) end depolymerization via binding protofilaments and promoting a destabilizing curling conformation Depolymerizes GTP-Tubulin; IS NOT destabilizing by hydrolysis Kinesin 13 + Related Subset

Question 33

Polymer Binding Microtubule Associated Proteins (4 Functions)

Answer 33

i. Stabilizes by binding to sides ii. Enhance assembly by stabilizing multi-dimer nuclei iii. Organize MTs into Bundles iv. Mediate interactions between MTs and Proteins, other Filaments, or Actin Typically have two domains 1. Microtubule Binding: binds several dimers at once 2. Projection: stems off of Microtubule Binding Domain to interact with other structures Plectin: used to link different types of filaments Tau and MAP2: MT organization in neuronal axons and dendrites (MAP2 provides more spacing than Tau)

Question 34

Loss of Tau PB-MAP Function

Answer 34

Results from rapid phosphorylation of Tau and its consequent sequestration into neurofibrillary tangles; can no longer stabilize microtubules Associated with Alzheimer's Disease and Taupathies

Question 35

Actin Motor Protein

Answer 35

Myosin -- mechanochemical enzyme that uses ATP hydrolysis for conformational change/movement All except Myosin VI undergo (+) end movement

Question 36

Composition of Actin Motor Protein

Answer 36

Myosins create complexes using heavy chains and light chains Heavy chains contain Head (for actin binding and ATPase activities) Neck (binds Calmodulin: binds Ca2+ and/or the light chain) and Tail (determines what the myosin binds, whether it's a monomer/dimer, it polymerizes to create thick filaments domains, and/or whether it binds to other proteins/membranes) Light Chains are used for structural and regulatory function

Question 37

Studying Actin Motor Protein Movement

Answer 37

Microscopy of Filament Binding Assays - Myosin sticks to glass with bound tails and free heads - Label Actin with Phalloidin (stabilizing) - Observe binding between Myosins and Actin - Add ATP, Myosins now push Actin Filaments in a certain direction

Question 38

Mechanism of Actin Motor Protein Movement (5 Stages)

Answer 38

Myosin goes through a Cross-Bridge Cycle 1) Rigor State: Due to lack of ATP, myosin is tightly bound to actin -- stiffness/rigor mortis 2) ATP Binding: ATP binds, ATP Cleft Closes and Actin Binding Cleft Opens -- weakens interaction between myosin and filament 3) ATP Hydrolysis: When myosin releases from actin, ATP hydrolysis occurs, causing head to undergo conformational change analogous to forward movement. Rebinds actin after this 4) Inorganic Phosphate Release: Pi release induces a 2nd conformation change (aka the Power Stroke) to exert force on actin, causing movement. Myosin now readopts Rigor State 5) ADP Release: simple a nucleotide release; remains in rigor state (potential to repeat process now)

Question 39

Myosin I Description

Answer 39

Exists as monomer that either has an Actin-Binding site to help filaments move past each other OR has membrane-binding sites and is involved with movement of vesicles and organelles

Question 40

Myosin II Description

Answer 40

Well known isoform is involved with muscle contraction Exists as dimer with 2 heavy chains and 2 coiled alpha-helices in tail region which creates a bipolar filament/polymer organization for itself [head region on outsides, tails on interior]

Question 41

Myosin V Description

Answer 41

Exists as a dimer that undergoes processive movement (head-over-head movement along F-actin) Also thought to be involved in vesicle/organelle transport

Question 42

Mutations in Myosin II and Myosin VII genes leads to what?

Answer 42

Myosin II: bleeding problems, hearing loss, kidney disease, cataracts Myosin VII: deafness and blindness in humans

Question 43

Disappearance of Myosin VI leads to what?

Answer 43

Potential Deafness (observed in mice)

Question 44

Microtubule Motor Proteins

Answer 44

Kinesins and Dyneins - both mechanochemical ATPases

Question 45

3 Major Classes of Kinesins

Answer 45

1. Kin-Ns (Conventional): motor domain at N-terminus -- (+) end movement 2. Kin-Cs: motor domain at C-terminus -- (-) end movement 3. Kin-Is: motor domain is internal; used for protofilament peeling ex) Kin 13

Question 46

Kinesin Movement Mechanism and Purpose

Answer 46

Can study using same setup as Myosin Binding Assay Undergoes Processive Movement to travel long distances without disassociating 1) Forward head binds beta-tubulin and releases its ADP 2) Allows said forward head to bind ATP, causing conformational change that swings rear head around 3) Previously forward head hydrolyzes its ATP; New forward head binds beta-tubulin and releases its ADP (REPEAT) Has light chains that bind to tail domains to mediate interactions with membrane vesicles and organelles that it will be transporting

Question 47

Structural Description of Dynein + Movement Pattern

Answer 47

Heavier MW-wise compared to Kinesins; KNOWN FOR (-) END MOVEMENT Mainly concerned with Cytoplasmic Dyneins - Dynactin Binding Domain Tail - 2 ATPase Domains - 2 Microtubule Binding Domains Power Stroke for movement produced by a change in the angle between the DBDT (stem) and MBD (stalk)

Question 48

What is Dynactin Complex?

Answer 48

The thing that mediates the attachment of Dyneins to its vesicles and organelles it is transporting using an inherent ARP1 (Actin-Related Protein) that binds Spectrin + Ankyrin to create its own membrane connection

Question 49

What is caused by defects in Kinesins, Dyneins, and Dynactin?

Answer 49

Kinesin defects lead to Charcot-Marie Tooth Disease or Kidney-Related Problems Dynein defects lead to Chronic Respiratory Tract Infections Dynactin defects lead to ALS

Question 50

Considering Cell Locomotion?

Answer 50

Consider Fibroblasts

Question 51

Fibroblast Structure and Behaviors: Lamellipodia

Answer 51

Thin sheets contained in a branched network; protrudes cell to give a little force to encourage movement (driven by ATP hydrolysis) Membrane is known to exhibit some resistance to the protrusion of lamellipodia and thus pushes back Depolymerization at negative ends is occurring, so treadmilling is observed

Question 52

Fibroblast Structure and Behaviors: Filopodia

Answer 52

Thin projections or spikes arranged in parallel bundles; protrudes cell to give a little force to encourage movement (driven by ATP hydrolysis) Membrane is known to exhibit some resistance to the protrusion of filopodia and thus pushes back; said to experience "retrograde flow," being pushed by membrane towards where its (-) end resides; into the cell Depolymerization at negative ends is occurring, so treadmilling is observed

Question 53

Fibroblast Structure and Behaviors: Stress Fibers

Answer 53

Supports movement encouraged by Filopodia and Lamellipodia, located behind the other fibroblast structures to exist as a contractile unit Made of long bundles of anti-parallel actin filaments with Myosin II inbetween each layer (both heads try to move to (+) end; contraction!) and orthogonally attached alpha-actinin (spacer protein) End of fiber contains Integrin-Containing Focal Adhesions for binding to extracellular substrates/matrix

Question 54

Formation of Lamellipodia

Answer 54

Uses Rac-GTP (Rho Family GTPase) for activation This activates WAVE (analogous to WASP) which activates the ARP2/3 Complex --> Polymerizes Branched Actin (Lamellipodia)

Question 55

Formation of Filopodia

Answer 55

Uses Cdc-42 (Rho Family GTPase) for activation This activates i) Par6 to generate polarity ii) Formin iii) WASP which activates ARP2/3 Therefore, ii and iii lead to the formation of Parallel Actin Polymerization and thus Filopodia formation

Question 56

Regulation of Stress Fiber Activity

Answer 56

Contractile Behavior is regulated by Phosphorylation - Myosin Light Chain Kinase phosphorylates; activates; contraction - Myosin Light Chain Phosphatase dephosphorylates; deactivates; unbound fibers

Question 57

Regulation of Myosin Light Chain Phosphorylation Activity

Answer 57

Uses Rho-GTP (Rho Family GTPase) This activates Rho Kinase which interacts with Myosin LC Phosphatase to determine how much Myosin LC is phosphorylated --> regulates contraction and expansion

Question 58

Formation of Stress Fibers

Answer 58

Uses Rho-GTP (Rho Family GTPase) This activates Formin that polymerizes the Actin to form Stress Fibers

Question 59

Mechanism for Endocytosis + Endosome Trafficking in Interphase

Answer 59

Actin is typically used for transport near membrane and then transfers cargo to a microtubule-based network Actin is known to potentially polymerize on only one side of an endosome and encourage "rocketing" Can also polymerize on membranes to mark certain structures for segregation

Question 60

General Role of Microtubule Cytoskeleton

Answer 60

Used for Organelle Positioning and Membrane Transport Most especially, the ER and Golgi localization is consequence of Microtubule Motor Proteins - Dynein used to keep Golgi close to Centrosome - Both Dynein and Kinesin used to spread ER all around the cell

Question 61

General Overview of the Cell Cycle

Answer 61

1. G1 (Gap 1) - period between completion of previous mitosis and next DNA replication 2. S - DNA/Chromosome replication 3. G2 (Gap 2) - period between DNA replication and initiation of mitosis 4. M (Mitosis) - includes mitosis (nuclear division) and cytokinesis (cytoplasmic division) Characterized by Chromosome Condensation, Mitotic Spindle Assembly, and Formation of Contractile Ring 1-3 are all considered Interphase

Question 62

Importance of S/G2 (4 Things)

Answer 62

(Replication) 2 Identical Sister Chromatids result from Replication --> In long linear form (Cohesion) "Glued" together length-wise by cohesins (Condensation) Linear form is supercoiled and compacted by condensins (ATPases) and topoisomerases; left attached at centromere - The kinetochores assemble on each side of said centromere At some point, centrosome is duplicated but the 2 remain closely associated

Question 63

First Step of M Phase (What signifies its beginning + 3 Things)

Answer 63

Prophase begins when condensed chromosomes are seen in microscopy i) Interphase microtubules gone, centrosomes disassociate ii) Each centrosome approaches opposite sides of nucleus to form aster array of microtubules (more arrays and more dynamic than interphase microtubules) iii) Beginning of Mitotic Spindle Formation (proper organization of centrosomes and chromosomes)

Question 64

Second Step of M Phase (What signifies its beginning + what does it enable?)

Answer 64

Prometaphase Starts when the Nuclear Envelope begins to breakdown as cytoplasmic dynein binds to nuclear envelope and pulls membrane towards (-) ends at centrosomes - also includes phosphorylation of nuclear lamin to promote breakdown Now spindle fiber microtubules can enter nuclear region

Question 65

Types of Spindle Fiber Microtubules in 2nd Step of Mitosis and Beyond

Answer 65

Prometaphase 1) Kinetochore MTs: connect chromasomal kinetochore to spindle pole at centrosome 2) Polar MTs: stem from both poles and overlap each other in antiparallel fashion; connected by Kin-Ns 3) Astral MTs: grow towards plasma membrane and interact with cell cortex

Question 66

Organization/Forces of the 3 Types of Spindle Fiber Microtubules in 2nd Step of M Phase

Answer 66

Prometaphase For kinetochore MTs, a single kinetochore is captured and the accompanying chromosome is pushed towards to the opposite spindle pole where it can be more easily captured by the second microtubule For polar MTs, 2 of them are cross-linked by Kin-N Kinesin 5 --> pushes MTs back to their source and thus the centrosomes apart Also cross-linked by Kin-C Kinesin 14 --> pushes MTs toward opposite pole and thus brings the centrosomes closer to each other For astral MTs, there exist membrane-bound dynein motors --> pull MTs and thus centrosomes toward membrane

Question 67

Can Spindle Fiber Organization be achieved without Centrosomes? What is this process driven by?

Answer 67

Yes, MTs will be assembled about chromatin; it is thought that RanGTP will be concentrated near cells and create a "gradient," attracting importins like in nuclear transport and then the consequent binding of RanGTP releases cargo (which is throught to be MT nucleating and stabilizing proteins)

Question 68

How do Kinetochore MTs bind to Kinetochores in the proper orientation? What do they have to ensure this?

Answer 68

Encounter and attachment is stochastic Kinetochores contain a Kin-N (CENP-E), a dynein, a Kin-I (Kin 13), and the Ndc80 complex If MT is bound incorrectly, CENP-E will walk chromosome to (+) end The MT is held in place by Ndc80, which leaves enough room for dynamic growth and shrinkage without disconnecting from MT - Becomes bi-oriented when both spindle poles are connected properly

Question 69

What type of movement occurs after chromosomes become bi-oriented?

Answer 69

Congression (Oscillatory movement that decreases in distance covered OT) until they reach a still state at the equator between the two spindle poles Assisted by (de)polymerization of the (+) ends on each side -- one grows while the other shrinks --> MVMT TO MIDDLE

Question 70

2 Characteristic Kinetochore MT Motions

Answer 70

1) Poleward MT Flux There are length changes at both the (+) and (-) ends (+) end: grows with Kinesin 7 and shrinks with Kinesin 13 (-) end: only see shrinkage with Kinesin 13 Therefore, treadmilling is possible!! 2) Polar Ejection Force Kinesin 4 (Kin-N) is located at the top of the chromosome arms and can attach to polar MTs, bringing to equator

Question 71

3rd Step of M Phase (What characterizes it and when does that happen?)

Answer 71

Metaphase Characterized by all chromosomes becoming aligned in a single plane halfway between spindle poles (named the Metaphase Plate) -- held in tension state This occurs when all (+) end polymerization, (-) end depolymerization, and chromokinesin promote movement equilibriates

Question 72

4th Step of M Phase (What must be checked?)

Answer 72

Anaphase Must pass Spindle Assembly Checkpoint before anything can continue Kinetochores must be properly attached to MTs and under proper tension; if not, SAC signals are produced that inhibit Anaphase Promoting Complex

Question 73

4th Step of M Phase (What happens after passage of checkpoint?)

Answer 73

Anaphase's 2 Simultaneous Movements Movements are dependent on activation of Anaphase Promoting Complex by M-Cdk and Cdc20 -- this allow polyubiquitination of securin protein that inhibits a separase - This separase degrades the cohesins and allows chromosome separation Anaphase A: Kinetochores shorten using Kinesin 13, bringing chromosomes towards poles (chromosomes remain attached using CENP-E and Ndc80) Anaphase B: Polar MTs elongate while Kinesin 5 walks to force spindle poles further apart; also includes dynein motors pulling astral MTs toward membrane

Question 74

5th Step of M Phase (What signifies it? What happen?)

Answer 74

Telophase Chromosomes arrive at poles, Kinetochore MTs disappear, Nuclear Lamins dephosphorylate to reform Nuclear envelope begins to reform; chromatin decondenses

Question 75

6th and Final Step of M Phase (What happens?)

Answer 75

Cytokinesis Contractile Ring forms that draws plasma membrane in to create cleavage furrow that closes until it meets midbody (a spindle remnant) When this midbody disappears, the cell divides completely and the division is complete!

Question 76

How does cleavage furrow end up in the proper orientation/alignment in the 6th and Final Step of M Phase?

Answer 76

Cytokinesis Info comes in the form of signals from astral spindles and central spindle to direct contractile ring precursors (called NODES) to the center of the cell, position is marked by the first arrivals using RhoA

Question 77

Formation of cleavage furrow in the 6th and Final Step of M Phase? What is the final step that does the separation?

Answer 77

Cytokinesis The already existent nodes accumulate formin (for nucleation and elongation) and myosin II (ring organization and contraction of filaments) This closes to midbody and eventually proteins accumulate (most importantly ESCRT) and membrane abscission occurs!

Question 78

Intermediate Filaments - Simile and Structure Description

Answer 78

Act like a strong rope for a cell Important for cell-cell attachments and mechanical integrity/support Does not bind any nucleotides Nonpolar, symmetrical structure Able to withstand stretching forces/tension

Question 79

Intermediate Filaments - Structure/Assembly + How do Units Add?

Answer 79

Basic building block is alpha-helical coiled rod that creates parallel dimer These assemble into antiparallel tetramers, creating symmetrical and nonpolar structure The tetramers assemble end to end to create the protofilaments, which group laterally to create protofibrils (sheet-like) Protofibrils then further assemble into intermediate filaments Tetramers substitute in structure (pop in and out) -- overall less dynamic

Question 80

Types of Intermediate Filaments

Answer 80

1) Keratins: epithelial cells, hair, nails, skin 2) Vimentin: fibroblasts, endothelial cells, white blood cells, embryonic cells 3) Desmin: muscle cells 4) Neurofilaments: strength to axons and determining axon diameter 5) Nuclear Lamins: nuclear structure/organization

Question 81

Intermediate Filament Relationship to Extracellular Structures

Answer 81

IFs run between desmosomes (cell-cell interactions), hemi-desmosomes (cell-extracellular matrix), and nuclei (for strength and rigidity)

Question 82

intermediate Filament Final Structure?

Answer 82

Network or Bundle, typically cross-linked with other structures using Intermediate Filament Binding Proteins

Question 83

Septin Structure Description

Answer 83

Binds and hydrolyzes GTP Known to colocalize with F-actin in stress fibers and contractile ring Forms nonpolar heterohexamers or heterooctamcers that can become filaments, rings, or cages

Question 84

Septin Functions (4)

Answer 84

Cytokinesis, Scaffolding, Diffusion Barrier (Compartmentalization), Organellle/Vesicle Trafficking and Dynamics