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145 Cards in this Set
- Front
- Back
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how do actin and tubulin organize structure?
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self association
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filament proteins are high conserved. what is the sequence similarity b/w yeast and human filaments?
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75%
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how many actin isoforms? how many isoforms of alpha and beta tubulin?
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at least 3; 6 or more
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another term for actin
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microfilament
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actin formed in vitro
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F-actin
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what method was used to determine structure of actin and actin filaments
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X ray crystallography
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what are 2 ways AF can be organized
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tightly anchored bundles, or networks (2D and 3D)
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AF diameter?
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5-9nm
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3 isoforms of actin in vertebrates?
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alpha, beta, and gamma-actin
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define cytoskeleton
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cytoplasmic system of filaments/fibers
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which filament has largest diameter
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MT
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filament assembly is:
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dynamic and highly regulated
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AF function:
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shape's cell surface
cell locomotion |
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MT function:
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positions mmb-enclosed organelles
directs IC transport |
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IF function:
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provide mechanical strength and resistance to shear stress
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filament fns in neuron:
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AF - line cortex of axon, just beneath plasma mmb
MT - bundles found in neurites for axonal tranport IF - NF are specialized IF that provide structural support for axon |
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what is neurite?
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buds of neurons developing into axons and dendrites
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what is fn of 4 subdomains on monomeric actin?
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facilitates interaction with other actin subunits and as well between strands
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which protein binds to ATP?
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G-actin
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Which protein binds to GTP?
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alpha and beta tubulin
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MT diameter
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24nm
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how many protofilaments are needed to make a MT
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13
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which portion of the tubulin heterodimer does not hydrolyze GTP?
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alpha tubulin
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living cell filaments maintain a dynamic equlibrium b/w:
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monomeric and polymeric states
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what is faster end of growing polymer?
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plus end
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in order for a monomer to polymerize with existing polymer, what state must the nucleotide be in?
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T form; i.e. ATP or GTP
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which rate is NOT porportional to [monomer]?
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koff
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what is unit for kon?
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per mole per second
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what if unit for koff
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per second
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critical concentration (Cc)
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concentration of monomers in equilibrium with filaments i.e. rate of subunit addition equals rate of subunit loss
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eqn for Cc
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Cc = [monomer] = koff/on = 1/K
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which is higher Cc(T) or Cc(D)?
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Cc(D) > Cc(T)
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When does treadmilling occur?
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Cc(T) < [monomer] < Cc(D)
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what is steady-state treadmilling?
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polymer maintains constant length though there is a net flux of subunits through the polymer; plus end grows while minus end shrinks
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at what concentration does dynamic instability occur?
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uniform monomer concentration
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catastrophe
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rapid shrinking
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rescue
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rapid growth
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treadmilling more likely occurs in:
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AF
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dynamic instability more likely occurs in:
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MT
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AF structure
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two-stranded helical polymers of protein actinl 2 protofilaments forming right handed hleix
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where is actin most concentrated?
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cell cortex underlying plasma mmb
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whats stronger: AF or MT?
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MT
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large-scale cell polarity
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cell being able to differentiate between top and bottom; made possible by cytoskeleton
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how is MT oriented in epithelial cell?
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positive end towards basal end, negative end towards apical end
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protofilament
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long linear string of subunits joined end to end
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protofilament bind:
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noncovalently; laterallly
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IF can tolerate twisting more than MT because:
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IF have strong lateral contacts than do MTs
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nucleus
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an aggregation of subunits with strong contacts
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filament nucleation
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process of initial nucelus assemblyl
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how is alpha and beta tubulin bound
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noncovalent bond
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do lateral bonds in MT occur b/w like or opposite tubulins?
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like tubulin; i.e. alpha-alpha and beta-beta
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what are the 2 bonds involved in filaments
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longitudinal and lateral
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how is tubulin heterodimer oriented?
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alpha is down, beta is up
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which protein filament is flexible?
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AF
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which end associates faster? which end dissociates faster?
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plus end adds faster when [subunit] > Cc, and dissociates faster when [subunit] < Cc
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what are alternate names for AF plus and minus ends?
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plus end - barbed end
minus end - pointed end |
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What happens at ΔG < 0? When ΔG > 0?
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filament elongation; filament dissociation
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free energy is greater when subunit released from D-form or T-form polymer?
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D-form polymer, which is why Cc of D-polymer is greater than Cc(T)
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is subunit more likely to dissociate from polymer in D or T state?
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D form
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When does ATP/GTP cap form?
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high [subunit]; subunit addition is higher than subunit hydrolysis
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how can lag phase be eliminated?
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adding premade nuclei
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dynamic instability
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sudden conversion b/w period of slow growth and period of rapid disassembly in AF and MT; occurs at uniform [subunit]
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treadmilling
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maintaining constant length by addition of protein subunits on 1 side and loss of subunits on other
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MT depolymerize faster in GTP or GDP tubulin?
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MT depolarize faster in GDP tubulin (100x faster than GTP tubulin)
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GDP tubulin causes what structure in protofilaments?
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curved rod
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Which filament undergoes greater length fluctuations?
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MT
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what do Kt and Kd represent?
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Kt = plus end
Kd = minus end |
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why does T end have lower Cc than D end?
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b/c subunit addition is easier on T end, so it takes lower concentration for addition to halt
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what is [actin monomer] inside cell?
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200micromoles
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where does AF nucleation take place?
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cell cortex
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Arp2/3 complex mimicks?
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barbed (+ end) of Af
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filament branching occurs via what structure? what is the angle of branching?
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Filament branching occurs with Arp2/3 complex binding to side of preexisting AF, forming 70 angle
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Arp2/3 complex composed of?
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Arp2,3 and ARPC1-5
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what must Arp2 and 3 be loaded with to initiate assembly of branched AFs?
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ATP
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how can Arp2/3 complex increase nucleation?
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binding to pre-existing filament rapidly activates nucleation
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ARP complex nucleates AF growth from which end?
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minus end, allowing rapid elongation of plus end
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what protein promotes nucleation which also facilitates Arp2/3 mediated branching?
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VCA domain containing protein (part of WASP family)
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What are 3 sequences of VCA domain arranged in tandem?
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1) actin-binding verprolin-homology (or V) domain
2) a conserved connected (or C) region that interacts with with both Arp2/3 complex and monomeric actin 3) An acidic (or A) region that binds the Arp2/3 complex |
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how does actin bind to formins?
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FH2 domains
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What 3 accessory proteins promote nucleation?
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formins, Arp2/3 complex, and spire
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profilin fn?
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promotes filament assembly by binding to actin monomers and making them available at the + end of growing polymer.
only actin-binding protein that allows ATP exchange for ADP |
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Thymosin beta4 fn?
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binds and sequesters pool of actin monomers so they aren't available for polymerization
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gelsolin and cofilin fn?
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control filament size by breaking them into shorter size by causing filaments to twist
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CapZ and Tropomodulin fn?
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proteins that stabilize filaments by capping ends
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Calponin-homology domain (CH-domain) superfamily?
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proteins that organize filaments into bundles and networks
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what proteins allows formation of actin bundles and networks?
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crosslinking proteins; contains at least 2 actin binding domain
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what AF associated protein breaks filaments and caps the + end?
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gelsolin
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Cofilin prefers to bind with:
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ADP-containing actin filaments
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2 classes of crosslinking proteins?
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bundling proteins
gel-forming proteins |
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bundling protein e.g?
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fimbrin, alpha-actinin, villin
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gel-forming protein e.g.?
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spectrin, filamin
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which bundling protein allows tight packing of parallel AF?
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fimbrin
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what bundling proteins are used in microvilli?
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fimbrin and villin
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spectrin structure? Where can it be found?
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2 alpha and beta chains; helps maintain shape of RBC, concentrated just beneath plasma mmb
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filamin fn?
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facilitates cell movement; clamps 2 actin filaments at roughly right angles
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Why are cancers cell unable to mobilize?
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lack filamin protein
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CapZ and tropomodulin bind to which end of AF?
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CapZ - plus end
tropomodulin - minus end |
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in formin where do monomeric actins bind?
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each subunit for formin dimer has actin binding site
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how does profilin bind to actin monomer?
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binds to face of monomer opposite of ATP binding cleft
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where is profilin found?
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near cytosolic face; binds to protein rich in Pro domains
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stathmin?
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binds to 2 tubulin dimers and prevents them from binding to MT; increases chance of catastrophe
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tropomyosin fn?
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binds to 7 adjacent protofilaments of actin preventing them from interacting with other proteins
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when will tropomodulin bind?
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only if AF are attached to tropomyosin
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what structure can filamin form?
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actin gels that can have lamellipodia
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What is purpose of gamma-tubulin?
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involved in nucleation of microtubules
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what's in gamma-TuRC?
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gamma tubulin and associated protein
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MTOC?
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specific location inside cell where MT nucleation initiates
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centrosome?
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a major MTOC in animal cells
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which variation of tubulin is found in lowest quantities?
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gamma-tubulin
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where is centrosome found?
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in the cytoplasm near the nucleus
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how many gamma-TuRCs does a centrosome have?
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+50 gamma-TuRCS found on centrosome matrix
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what do centrioles become?
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centrioles, found in MTOC, become basal bodies of cilia and flagella in motile cells
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centriole fn?
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organize centrosome matrix
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where are IFs found?
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in cells subjected to mechanical stress i.e. not found in all cells
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where does MT nucleation occur?
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In MTOC, meaning in cytoplasm near nucleus
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katanin fn?
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severes MT from MTOC
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can gamma-TuRC act as cap?
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yes, for minus end of MT
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what are 2 known MTOCs?
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centrosome (animal) and spindle polar bodies (yeast)
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another name for centrosome matrix?
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pericentriolar material (PCM)
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how are actin-severing protein activated?
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high cytosolic Ca2+ levels, as they are members of gelsolin superfamily
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what regulates nucleating activity of ARP complex?
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IC signalling and components of cytosolic face of plasma mmb
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what crosslinking structures do ARP complex and formin promote?
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ARP complex promotes formation of gel branched networks, whereas formin promotes formation of unbranched actin bundles
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how does formin differ from ARP complex and gamma-tubulin in terms of nucleation?
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former 2 stay bound at minus end, whereas formin moves up as it elongates the AF
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ARP complex and gamma tubulin prevent what at minus end?
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subunit addition or loss
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What are MAPs?
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Microtubule Associated proteins; control (dis)assembly of MTs
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What are the 2 groups of MAPs?
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Those that stabilize or destabilize the filament
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e.g. of stabilizing MAPs?
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MAP1,2,4 and Tau
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Which MAP ensures tighter bundling of MTs: Tau or MAP2?
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Tau
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What is the most abundant MAP? where is it found?
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MAP4, found in neuronal as well as non-neuronal tissue
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What is the result of MTs with hyperphosphorylated Tau?
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accumulates as interneuronal tangles of:
paired helical filaments twisted ribbons straight filaments neurofibrillary tangles |
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What diseases result in hyperphosphorylated Tau?
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Alzheimer's, Down Syndrome, and several forms of Dementia
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what does mutation in presenilin cause?
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increase in synthesis of amyloid-beta (A-beta) peptide; misfolded A-beta peptide in AD initates Tau aggregation
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What mutations have been identified in AD?
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mutations in amyloid precursor protein (APP) and presenilins
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What is EB1?
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capping protein in yeast binding to + end of MTs
facilitates anchoring to plasma mmb through interactions with Kar9 |
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XMAP215 fn?
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+TIP stabillizer; decreases frequency of catastrophic events
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catastrophin fn?
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+TIP destabilizer;increases frequency of catastrophic events
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What are +TIPs?
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plus-end tracking proteins bind to + end of MTs and are positive regulators of MT growth
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What is a "seam" in MT?
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a weak point in MT due to helical nature of tube
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what is FtsZ?
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tubulin homolog in bacteria that mediates cell division
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What is MreB?
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actin homolog in bacteria that forms filaments and maintains cell length; gives bacteria rod shape
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typically, 1 tau should be present for every __ tubulin subunits?
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4
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does ARP complex always remain at negative end of AF?
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No, it may dissociate
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what regulates activity of MAPs?
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protein kinases
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centrosome creates an _____ array of MT.
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astral
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crescentin fn?
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an IF that pushes on bacterial cell wall giving healthy bow
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