Exam 3: lecture 7-cytoskeleton structure Flashcards
What are the 3 types of the cytoskeleton?
- microfilament
- intermediate filament
- microtubule
(in order of size: small->large)
What is the connection between bacteria and
eukaryotic cytoskeletal proteins?
bacteria have HOMOLOGUES for eucaryotic cytoskeletal proteins
a) staphylococcus aureus: FtsZ(microtubule-like)
b) e.coli: MreB (actin-like)
c) caulobacter crescentus: crescentin (intermediate filament-like)
Technique for visualizing the cytoskeleton: fluorescence microscopy on fixed specimens
fluorescent bind directly
antibodies indirectly label
once dye, cytoskeletal proteins will glow.
ex. fibroblast
technique for visualizing the cytoskeleton: live cell fluorescent
fluorescent versions of the proteins are made and put into living cells.
-microscopy/video/cameras are used to view
ex.tubulin
technique for visualizing the cytoskeleton: computer-enhanced digital video
high-res images from a vid/camera attached to a microscope are computer processes to increase contrast and remove background features that obscure the image
technique for visualizing the cytoskeleton: electron microscopy
revolve individual filaments prepared by thin section, quick-freeze deep-etch
microtubules and its functions
the largest of the cytoskeletal component
25nm
functions:
1. move chromosomes around
2.flagella movement
3.”highways” for vesicles
microtubules: cytoplasmic
pervade the cytosol
functions:
1. maintaining axons
2. formation of mitotic and meiotic spindles
3. maintaining/altering cell shape
4. placement & movement of vesicles
microtubules: axonemal
include organized & stable microtubules found in
-cilia
-flagella
-basal bodies to which cilia + flagella attach
axenome: the central shaft of cillium/ flagellum is highly ordered bundle of MTs
what is the order of structure of microtubules?
tubulin dimers (heterodimers)->oilgomers-> protofilament-> sheets of protofilaments-> closing microtubules-> elongating microtubules
the kinetics of microtubules in vito
- lag phase (nucleation): dimers-> oilomers-> protofilaments
- elongation phase: growing microtubules with - & + ends
- plateau phase (equilibrium): microtubules with subunits coming on and off (lose and gain the same amount- the size stays the same)
in a cell w/ MTOC (microtubule organizing center)
-anchors the - end= only shrinkage happens at the + end
how do new dimers fall off the + end of microtubules?
dimer consists of alpha & beta (tubulin) is bound with GTP
to add: the chain will stay on until the gtp is hydrolyzed
to remove:
1. gtp on B-tubulin hydrolyzes (break off pi) to GDP
2. hydrolysis weakens bonds in polymer
(gtp–>GDP)
- rapid depolymerization occurs
- GDP is exchanged for Gtp
the dynamic instability model of microtubules
a) growing in length
GTP cap is present at +end to allow polymerization (high tubulin concentration)
b) shrinking in length
GDP cap is at +end to allow rapid depolymerization (low tubulin concentration)
Dynamic instability of microtubules: frequency
-plus ends experience larger changes even though both have dynamic instability
Frequency of catastrophe (how often is shrinkage)
- end is shorter in length
frequency of rescue (how often is stabilize)
- end is longer in length (more stable)
what regulates microtubule stability in cells?
microtubule-binding proteins
-some use ATP to drive vesicles/ organelle transport/ to generate sliding forces between MTs
-others regulate MT structure
Microtubules: what are microtubules -stabilizing/ bundling proteins? + examples
-bind at regular intervals along a microtube wall
Tau: form tight bundles in axons
MAP2: promotes the formation of looser bundles in dendrites (fingerlike)
how does MAP2 + Tau promote bundling?
one region binds to the MT wall
another part of the protein extends at right angles for interaction
the length of the extended arm control the spacing
how are MTs stabilized?
by +- TIP proteins (tubulin interacting proteins)
capture and protect the growing + ends
-decrease the likelihood that MTS will undergo catastrophic subunit loss
How are MTs destabilized? + examples
by microtubule-destabilizing/ severing proteins
-stathmin/OP 18: bind to tubulin heterodimers and PREVENT polymerization
-catastrophins: acts at the end to PROMOTE the peeling of subunits from the + ends (MCAK)
-katnins: SEVER MTs
What is “The Mesh”
new organelle
a network of microtubules connectors that stabilizes individual kinetochore fibers of the mitotic spindle (during meiosis/ mitosis) where chromosome attaches
-each mesh holds 2 MTs tgt bc clathrin is in its trimeric state
How are kinetochore stabilized in the MEsh
uses clathrin complexes with TACC3 (Transforming Acidic Coiled-Coil) to stabilize fibers where chromosomes attach
- recruitment of TACC3 and ch-TOG to MTs: regulated by Aurora-A-Kinase-mediated phosphorylation on Ser558
- Clathrin recruitment: clathrin binds to TACC3 or TACC3/ch-TOG sub-complexes on MTs
- complex accumulation: clathrin binds more than one TACC3 on adjacent MTs-> locking the complex= bridges between adjacent parallel
