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Lecture 9

BIOB10H3 Lecture Notes - Lecture 9: Immunoglobulin Light Chain, Golgi Apparatus, Inner Membrane

Biological Sciences
Course Code
Tanya Da Sylva

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Lecture 9: Cytosolic Proteins and the Cytoskeleton
Fluid matrix surrounding organelles
Ions, sugar, ATP
Free ribosomes, amino acids
Many proteins
Cytosolic Proteins
How are proteins targeted to cytosol?
They arent: targeting sequence
= no targeting sequence
-proteins translated in cytosol
and stay there by default
Some examples:
Clathrin and COP proteins :
proteins outside of the vesicles
Signalling proteins: bring
proteins through the cell
Glycolysis enzymes
Organelle proteins during
Chaperone proteins
What is the cytoskeleton?
Extensive protein network
Made on free ribosomes in the
cytoplasm, no targeting sequence
Three main components:
1. Microtubules
2. Actin
3. Intermediate Filaments
Cytoskeleton -Functions
Scaffold structural support and cell
Internal framework organize
organelles within a cell
Movement –directs cellular locomotion
and movement of materials within the
Machinery for cell division
1)Structure and Support
2)Intracellular Transport
3)Contractility and Motility
4)Spatial Organization
-Peroxisomes associated with
-Red-tubulin incorporated in micro

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Cytoskeleton is not static
Cytoskeleton components
1) Microtubules
Hollow, cylindrical
Set of globular proteins arranged in rows : profilaments
Contain 13 proto filaments
Composed of tubulin heterodimers
a- and b- tubulin subunits
Subunits polymerize to make microtubules
B- tubulin binds GTP to allow polymerization
Polymerize into proto filament
-13 proto filaments arrange around a hollow core
and forms the microtubule = entire rod like
Interactions are mainly non-covalent
4nm= heterodimer = 2 balls . Protofilament = strings of those alpha beta

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–alpha beta. 13 of those protofilaments=microtubules.
MT have polarity
A plus (+, alpha end) and minus (-, beta end) end
Structurally different
(-) end embedded in centrosome
(+) end extends towards
Growth from the positive end, heterodimers add on to the positive end and
also fall off the positive end = shrinkage
MTs polymerize/depolymerize at the (+) end therefore it has direction.
1) Microtubules Assembly
Microtubules assemble in two stages
Nucleationsmall portion of the microtubule is initially formed
Followed by elongation
In vitro nucleation is a slow phase, in a cell (in vivo) nucleation occurs rapidly in
special structures: Microtubule organizing centres (MTCOs) : provide an area in
the cell specialized to perform nucleation
Best studied MTOC in animal cells is the centrosome
Two barrel shaped centrioles surrounded by pericentriolar material (PCM)
Pair of centrioles at right angles to each other
Centrosomes Experiment
1)Cells treated with colcemid–disassembly of microtubules
2)Remove drug, fix and image cells (fluorescent tubulin)
3) Within 30 minutes labeled filaments radiating out of centrosome-
microtubules dont make contact with centrioles but terminate in PCM:
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