BIO 211 Lecture Notes - Lecture 6: Chromatin, Cytoskeleton, Fibroblast
30 May 2018
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Prior to Week 6 Class 1 – pp. 925-943
Know
• What tubulin is
o Protein that makes up microtubules
o Heterodimer formed from alpha and beta tubulin (globular proteins) that are
tightly, noncovalently bound
o Alpha tubulin
▪ GTP bound to this is physically trapped at dimer interface and never
hydrolyzed or exchanged
o Beta tubulin
▪ Can be either GTP or GDP bound
▪ Exchangeable within tubulin dimer
• What the MTOC, γ-TuRC, centrosomes and centrioles are
o Gamma-tubulin
▪ Involved in nucleating microtubule growth
o MTOC
▪ Microtubule organizing center
▪ Intracellular location from where microtubules are nucleated
▪ Gamma-tubulin rich
o Gamma-tubulin ring complex (γ-TuRC)
▪ Two accessory proteins in the complex bind directly to γ-tubulin and other
proteins that crate a spiral ring off γ-tubulin molecules
• Creates a microtubule template
o Centrosome
▪ Single well defined MTOC
▪ Located near nucleus
▪ Microtubules are nucleated at their minus ends and plus ends point
outward and grow/shrink
▪ Essential in mitosis, move to opposite sides and form 2 poles of mitotic
spindle
o Centrioles
▪ Embedded in centrosome
▪ Pair of cylindrical structures arranged at right angles in L-shaped
configuration
▪ Cylindrical array of short modified microtubules arrange in a barrel shape
▪ With accessory proteins, they organize the pericentriolar material (where
microtubule nucleation takes place)
• What stathmin, kinesin-13, katanin, MAPs, XMAP215, +TIPs, and filament bundling and
cross-linking proteins are and what they do
o MAPS
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find more resources at oneclass.com
▪ Microtubule associated proteins
▪ Proteins that bind to microtubules
▪ Can stabilize microtubules against disassembly
▪ Can mediate microtubule interaction with other cell components
▪ One domain binds to microtubule surface, one projects outward
▪ Target of protein kinases, phosphorylation controls activity & localization
o Stathmin
▪ Binds to two tubulin heterodimers and prevents their addition to ends of
microtubules
▪ Decreases concentration of tubulin subunits available for polymerization
▪ Phosphorylation inhibits, allows tubulin to be available for polymerization
o Kinesin-13
▪ Catastrophe factor
▪ Bind to microtubule ends, pries protofilaments apart, induces catastrophe
o Katanin
▪ Made up of 2 subunits
• Smaller hydrolyzes ATP and does the severing
• Larger directs katanin to centrosome
▪ Releases microtubules from attachment to mtoc
▪ Overall promotes depolymerization
o XMAP215
▪ Binds free tubulin subunits and delivers them to plus end
▪ Promotes polymerization, induces rescue
▪ Inhibited by phosphorylation during mitosis
o +TIPs
▪ Plus end tracking proteins
▪ Accumulate at active ends during growth and dissociate during shrinkage
o Filament bundling and cross-linking proteins
▪ Link them to each other
▪ I.e. MAP2 (longer) and tau (shorter)
• What motor proteins move along microtubules, and how they differ from each other and
from myosin
o Kinesin
▪ Similar to myosin II because it has 2 heavy chains per active motor
• form 2 globular head motor domains held together by coiled coil
▪ most walk towards plus end of microtubule
• kinesin 13 has central motor domain, doesn’t walk
• one family walks opposite, towards minus end
o Dynein
▪ Family of minus-end directed microtubule motors
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▪ 1,2, or 3 heavy chains
• Cytoplasmic dyneins
o Homodimers of 2 heavy chains
o Used for organelle and mRNA tracking, positing
centrosome and nucleus, and construction of microtubule
spindle
• Axonemal dyneins (ciliary dyneins)
o Monomers, heterodimers, and heterotrimers
o Specialized, rapid and efficient sliding movements of
microtubules that drive beating of cilia and flagella
• What the axoneme, primary cilia, and basal bodies are
o Axoneme
▪ Core of cilia and flagella
▪ Bending it produces motion
o Primary cilia
▪ Shorter nonmotile part of cilia and flagella
o Basal body
▪ Plasma membrane associated structure that creates motile and nonmotile
cilia during interphase
▪ Has a centriole at its core
Understand
• How tubulin assembles to form microtubules, and the difference between D and T-form
polymers
o Alpha/beta tubulin heterodimers are stacked head to tail and folded into a tube to
make a protofilament
o 13 parallel protofilaments make up a microtubule
o Longitudinal axis
▪ Top of one beta-tubulin molecule interfaces with bottom of a alpha-tubulin
▪ High binding energy
o Lateral axis
▪ Between monomers of same type (alpha-alpha and beta-beta)
o The lateral and longitudinal have slight staggers that gives a helical microtubule
lattice
▪ The loss and addition of units only happens at ends
o Multiple contacts make them stiff and straight
o Form with all protofilaments pointing the same way, creates polarity (alpha
exposed at minus, beta exposed at plus end)
o D form
▪ GDP bound form
o T form
find more resources at oneclass.com
find more resources at oneclass.com
