BIOL 1090 Lecture Notes - Lecture 24: Tubulin, Microtubule, Kinesin

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April 3, 2017 – L12: Cytoskeleton and Motors
Cytoskeleton
A dynamic network of protein filaments that forms the cellular scaffolding as well as
transport system for organelles and vesicles
3 major elements:
oMicrotubules: hollow, unbranched tubules of tubulin outer, diameter: 25nm
oIntermediate filaments: fibrous, various proteins, diameter: 10-12nm
oMicrofilaments: solid, thin, branched, actin polymers, diameter: 8nm
Functions of the cytoskeleton
Microtubules (MT)
Largest cytoskeletal element (25 nm diameter)
Polymer of proteins alpha-tubulin and beta-tubulin
Alpha/beta heterodimers form ong protofilaments
13 protofilaments form longitubdinal array hollw
cylinder
Heterodimers aligned in same direction
Head to tail structural polarity
MTs have fast-growing “plus end” and slow-growing
“minus end”
oStructural polarity is important for MT growth/shrinkage and direction of
movement of material along MT
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Dynamic Assembly and Disassmebly
In vivo, this leads to rapid turnover of most MTs within cell (half-life is minutes)
oDynamic instability
Shrinkage can occur very rapidly at the ‘plus’ end
oCatastrophe
MicroTubule-Organizing Centre (MTOC) = central site of MT assembly
Centrosome is a major MTOC in animal cells
Formation of MTs is regulated/controlled by Microtubule-Associatd Proteins (MAPs)
oProteins that bind MTs
oModulate assembly, function
oMediate interactions w/other cellular structures ex. Vesicles/organelles
oBumps at the surface of microtubules are MAPs
2 Classes of MAPs
1. Motor MAPs
a. 2 main types: kinesin and dynein – power
intracellular transport
b. Use ATP to generate force
c. Can move material along MT “track”
d. Can generate sliding force b/w MTs
2. Non-motor MAPs
a. Control MT organization in cytosol ex. Tau
protein in neurons
i. Defective TAU protein causes neurofibrillary tangles, which is a cause of
Alzheimer’s
Microfilaments
Smallest cytoskeletal element ~8nm
Polymer of actin
Polypeptide = 42kDa, binds ATP
oIndividual molecules = G-actin (globular)
oPolymerized filament = F-actin
Polar (has plus and minus ends)
Several well-characterized functions
oMaintenance of cell shape
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Document Summary

April 3, 2017 l12: cytoskeleton and motors. Cytoskeleton: a dynamic network of protein filaments that forms the cellular scaffolding as well as transport system for organelles and vesicles. 3 major elements: microtubules: hollow, unbranched tubules of tubulin outer, diameter: 25nm, intermediate filaments: fibrous, various proteins, diameter: 10-12nm, microfilaments: solid, thin, branched, actin polymers, diameter: 8nm. Polymer of proteins alpha-tubulin and beta-tubulin: alpha/beta heterodimers form ong protofilaments. 13 protofilaments form longitubdinal array hollw cylinder: heterodimers aligned in same direction, head to tail structural polarity, mts have fast-growing plus end and slow-growing. Minus end : structural polarity is important for mt growth/shrinkage and direction of movement of material along mt. In vivo, this leads to rapid turnover of most mts within cell (half-life is minutes: dynamic instability. Shrinkage can occur very rapidly at the plus" end: catastrophe, microtubule-organizing centre (mtoc) = central site of mt assembly, centrosome is a major mtoc in animal cells.

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