1. What are cytosolic proteins and what is their function?
Cytosolic proteins don‟t have a transfer sequence since they just stay in the
The cytosol is a fluid matrix surrounding organelles and contains water, ions,
sugars, ATP, free ribosomes and many proteins
2. What is the targeting sequence for proteins destined to stay in cytosol?
The targeting sequence for proteins destined to stay in the cytosol is nothing; there is
no targeting sequence for these proteins.
Proteins are translated in the cytosol and stay there by “default”
3. What are some types of cytosolic proteins that can be found?
Examples of two types of cytosolic proteins that can be found are: Rabs and SNARES,
coat proteins like the clathrin and cops, there is also signaling proteins – kinases,
phosphatases, GTPases, glycolysis enzymes organelle proteins “in transit”, and
chaperone proteins, soluble receptors for transport organelle proteins (example
Hsp70), and lastly cytoskeleton
4. What is the cytoskeleton made up of?
The cytoskeleton is made up of 3 components:
c. Intermediate filaments
The cytoskeleton is made of an extensive protein networks and cytosolic proteins, and
are made on free ribosomes with no targeting sequence
5. What is the function of the cytoskeleton?
The function of the cytoskeleton is for:
a. Dynamic scaffold – structure and support
b. Spatial positioning of organelles
c. Intracellular transport
d. Cell contraction/Cell motility/Phagocytosis
e. Machinery for cell division
Figure 9.1 – An overview of the structure and
(a) an epithelial cell, (b) a nerve cell, and (c) a dividing
cell. The microtubules of the epithelial and nerve cells
function primarily in support and organelle transport,
whereas the microtubules of the dividing cell form the
mitotic spindle required for chromosome segregation.
Intermediate filaments provide structural support for
both the epithelial cell and nerve cell. Microfilaments
support the microvilli of the epithelial cell and are an
integral part of the motile machinery involved in nerve
cell elongation and cell division
Figure 9.2 – An example of the role of microtubules
in transporting organelles. The peroxisomes of this cell
are closely associated with microtubules of the
cytoskeleton. Peroxisomes appear green because they
contain a peroxisomal protein fused to the green
fluorescent protein. Microtubules appear red because
they are stained with a fluorescently labeled antibody.
Figure 9.11 – Localization of
microtubules of a flattened, cultured
anti-tubulin antibodies. Microtubules
are seen to extend from a perinuclear
region of the cell in a radial array.
Individual microtubules can be followed
and are seen to curve gradually as they
conform to the shape of the cell 6. What are microtubules (MTs)?
Microtubules are composed of tubulin heterodimers each with an a- and b-tubulin
subunits which polymerize to make MTs
Heterodimers arrange into protofilaments
Microtubules are tertiary structures from what can be seen in x-ray crystallography
Figure 9.9 – The structure of microtubules. Diagram of a
longitudinal section of a microtubule shown in the B-lattice,
which is the structure thought to occur in the cell. The wall
consists of 13 profilaments composed of aB-tubulin
heterodimers stacked in a head-to-tail arrangement. Adjacent
profilaments are not aligned in register but are staggered
about 1nm so that the tubulin molecules form a helical array
around the circumference of the microtubule. The helix is
interrupted at one site where a and B subunits make lateral
contacts. This produces a “seam” that runs the length of the
7. What is the function of microtubules (MTs)?
Tubulin heterodimers – B-tubulin binds GTP to allow polymerization and the GTP
slowly converts to GDP after incorporation into the MT wall
This polymerizes into a protofilament which is a 13 protofilaments arranged around
a hallow core and forms the microtubule (MT)
MTs have polarity thus with a plus and minus end
The ends structurally alittle different the negative end is embedded in the
centrosome which is the MT-organizing centre (MTOC) where
This starts with gamma-tubulin and slightly different structure than the a and B
The positive ends extend towards the plasma membrane
MTs polymerize and depolymerize at the positive end called the growth and
Figure 9.9 – The structure of
microtubules. EM of a cross section
through a microtubule of a juniperus
root tip cell revealing 13 subunits
arranged within the wall of the tubule.
Figure 9.13 – Axonal transport. EMof a
portion of axon from a cultured nerve cell,
showing the numerous parallel microtubules
that function as tracks for axonal transport
Figure 9.18 – The centrosome. Fluorescence
micrograph of cultured mammalian cell
showing the centrosome at the center of an
extensive microtubular network
Figure 9.8 – The study of the
cytoskeleton using FRAP.
Micrographs from a FRAP
experiment performed on interphase
cells expressing GFP tubulin. Two
side-by-side cells were bleached in
the boxed region with a laser, and the
cells were then imaged over time. 8. What does Figure 9.19 Microtubule nucleation at the centrosome signify?
In picture (a) Fluorescent micrograph of a cultured fibroblast that had been exposed
to colcemid to bring about the disassembly of the cell‟s microtubules and was then
allowed to recover for 30 minutes before treatment with fluorescent anti-tubulin
antibodies. The bright starlike structure marks the centrosome together with newly
assembled microtubules that have begun to grow outward in all directions.
(b) Schematic diagram of the growth of microtubules showing the addition of subunits
at the plus end of the polymer away from the centrosome.
Thus basically in the experiment the biologists washed out the drug quickly to see if
the microtubules polymerize and to where they came from: thus they removed the
drug and watched MTs reassemble – assembled at the MTOC/centrosome
9. What else are MTs capable of doing?
MTs rapidly turnover, that is that they polymerize and depolymerize from end called
“dynamic instability” which insists that MTs are constantly growing and shrinking
The half-life of average MT is 10 minutes and MTs stability can be increased (some are
Also the binding of structural MAPs (MT-associated proteins) to the walls of MTs, eg.
MAP2, tau can prevent MTs from disassembling and depolymerizing
Figure 9.10 – Microtubule-associated proteins (MAPs).
Schematic diagram of a brain MAP2 molecule bound to the
figure contains three tubulin-binding sites connected by short
segments of the polypeptide change. The binding sites are
spaced at a sufficient distance to allow the MAP2 molecule to
attach to three separate tubulin subunits in the wall of the
microtubule. The tails of the MAP molecules project outward,
allowing them to interact with other cellular components.
- MT wall
- important componentom falling apart and is a very
- Stable MTs are frequently used to transport
organelles and function as “highways”
10. What are microtubule motors?