UNIT 3 EXAM – study guide I
Chapter 13 – Cytoskeletal systems
1. Compare and contrast microtubules, microfilaments and intermediate filaments;
I – Acidic cytokeratins II – Basic cytokeratins III – Vimentin,
Desmin, GFA protein IV – Neurofilament protein
V – Nuclear lamins VI - Nestin
8 – 12 nm
of animal cell
∙ Cell motility
∙ Maintenance of animal
∙ Maintenance of animal cell
∙ Formation of
of nerve cell
∙ Keeping muscle fibers in
GTP or ATP
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Drugs affecting microtubules (MT)
Binds tubulin monomers,
Aggregates tubulin heterodimers
Drugs affecting Microfilaments (MF)
Prevents monomer addition to plus ends
Sequesters actin monomers
Binds and stabilizes assembled
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2. How does ftsZ support the theory of endosymbiosis?
ftsZ proteins have been found to be produced by certain organelles in in some eukaryotes, such as chloroplasts and mitochondria, and localizes to sites where these organelles divide. The overall structure of ftsZ protein in eukaryotes are strikingly similar to the counterparts in bacterial cells and even bind to the same phosphonucleotides.
3. Explain the following terms:
This is the process in the formation of microtubules where oligomers serve as nuclei from which new microtubules can grow.
This is the growing of the microtubule by the addition of subunits at either end
This is the simultaneous assembly and disassembly at either the plus or minus ends of microtubules. If you want to learn more check out Why is pareto efficiency important?
∙ GTP/ATP hydrolysis
This is the process by which energy stored in ATP/GTP is released.
Microtubules form primarily by addition of GTP-bound beta-tubulin at the (+) end, but after being added, the beta-tubulin molecules later hydrolyze their GTP, leaving them GDP-bound.
4. Explain the following terms:
This is the event in which when microtubule switches from growth to shrinkage, the microtubules disappear completely Don't forget about the age old question of How do you tell if an allocation is pareto efficient?
This is the event in which when microtubule switches from growth to shrinkage, the microtubules abruptly switch back to growth phase.
∙ GTP cap:
This stabilizes the plus end of growing microtubules.
5. What is the dynamic instability model of microtubules
∙ This is the growth of microtubules in which there are periods of slow growth and rapid shrinkage.
6. Explain the functions of these parts of the cell
This serves as a site at which MT assembly is initiated and acts as an anchor for one end of the microtubules.
A centrosome is an MTOC near the nucleus of a cell containing centrioles from which spindle fibers develop in cell division.
Centrioles are organelles which help cell division in animal cells by the forming of spindle fibers which separate the chromosomes during cell division If you want to learn more check out What is the configuration of an electron?
∙ Basal bodies:
These are MT associated structures found at the base of the cilia
∙ Gamma tubulin:
These nucleate the assembly of new microtubules away from the centrosome
7. Describe the role of the following proteins
∙ MAPs (microtubule associated proteins)
MAPs bind at regular intervals along the wall of a microtubule, to increase MT stability and affects density of MT bundles
∙ +TIPS ( plus end tubulin interacting proteins)
These capture and protect the growing plus ends of microtubules, stabilizing them.
These bind to tubulin heterodimers preventing them from polymerizing
These act at the ends of microtubules once they have been polymerized to promote the peeling of subunits at the end.
Katanins sever microtubules
8. What is tau and how does it link to disease?
Tau is a MAP that causes microtubules to form tight bundles in axons. Large amounts of hyperphosphorylated tau protein cause neurofibrillary tangles which can lead to alzheimer’s disease, pick’s disease and several types of palsy. If you want to learn more check out What is the meaning of respiratory epithelium in the respiratory system?
9. Structures of microfilaments
These are contractile bundles of actin which stretch from the trailing end to the front end of the cell found in cells which do not move well.
These lie immediately beneath the plasma and crosslinked into a gel. They are found in rapidly moving cells.
These are thin, sheet-like membrane protrusions found at the leading edge (front) of motile cells. The actin in lamellipodia are typically less well organized than in filopodia.
These are thin, actin-rich plasma membrane protrusions that function as antennae for cells to probe their environment. Actin in filopodia are typically more well organized than in lamellipodia.
10.What is the relationship between actin fibers and myosin S1?
The polarity of actin fibers can be observed by decorating them with myosin S1 (globular heads of myosin II cleaved by proteases).
11.Describe the competition between thymosin β4 and profilin in actin polymerization
∙ Thymosin β4 and profilin both compete for binding to G-actin monomers. They are involved in the dynamic turnover and restructuring of actin filaments.
12.Describe the role of the following proteins in actin polymerization
These bind to ADP-G-actin and F-actin and are thought to increase the rate of turnover of ADP-actin at the minus ends of microfilaments. They also sever filaments creating new plus ends as they do.
Capping proteins (CapZ and
Capping proteins prevent either further addition or loss of subunits thereby stabilizing them.
CapZ binds to the plus end of actin filaments and prevents further addition of subunits.
Tropomodulins bind to the minus end of actin filaments and prevents further loss of subunits.
Gelsolin severs actin MFs and cap their newly exposed plus ends, thereby preventing further polymerization
Mediates the bundling of actin filaments into tightly organized arrays.Very prominent in focal contacts and focal adhesions.
Fascin keeps actin within the core of a filopodium tightly bundled, contributing to the spike-like appearance of such protrusions
Cross-linking protein important in the formation of filopodia
Villins also binds tightly together , adjacent MFs
MFs in the actin bundle are connected to the plasma membrane by lateral proteins consisting of Myosin I and calmodulin.
Slides MFs in muscle
MFs in the actin bundle are connected to
the plasma membrane by lateral proteins consisting of Myosin I and calmodulin.
Spectrin connects microfilaments to each other, to proteins with the plasma membrane and to the network of intermediate filaments beneath the terminal web.
This helps branches to form nucleating new branches on the sides of existing filaments.
Formins are required to assemble certain unbranched F-actin structures
These promote the spatially regulated actin polymerization required for efficient chemotaxis in response to attractive and repulsive guidance cues
Act as splices to join two MFs where they intersect
Classifications of actin proteins
a. Regulate polymerization
∙ Thymosin beta4
b. Cap actin filaments
∙ CapZ (+ ends)
∙ Tropomodulin (- ends)
c. Crosslink actin filaments
d. Sever actin filaments
e. Bundle actin filaments
∙ Myosin I
f. Link actin to membranes
∙ Band 4.1
g. Promote actin branching and growth
∙ Arp 2/3 complex
13.What is the role of Rho family GTPases and growth factors for regulating MF formation?
Rho GTPases are regulators of actin cytoskeleton . They perform functions ranging from formation of protrusions to assembly and disassembly of the cytokinetic furrow to the regulation of endo and exocytosis. They are stimulated by;
guanine-nucleotide exchange factors (GEFs): Foster exchange of a bound GDP for GTP.
GTPase activating proteins (GAPs): Stimulate Rho GTPases to hydrolyze their bound GTP thereby inactivating them.
Guanine-nucleotide dissociation inhibitors (GDIs): Sequester or isolate inactive Rho GTPases in the cytosol
14.How can immunofluorescence aid in the identification of cancer cell types based on IF?
Intermediate filament typing is used to determine the type of IF present in cancer cells as tumor cells are known to retain the IF proteins characteristic of the origin.
15.Explain the dynamic nature of IF in the nuclear lamina
IFs are not static structures. The nuclear lamina contains proteins called lamins which become phosphorylated and breakdown as part of the nuclear envelope disassembly in mitosis.
16.What are plakins?
Plakins are specific linker proteins which are responsible for the mechanical integration of intermediate filaments, microfilaments and microtubules.