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Section 1

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Biology 2382B
Sashko Damjanovski

SECTION 1 How is Cellular MovementAchieved? • Cellular trafficking of organelles • Cell migration Cytoskeleton • Intricate network of protein filaments that extend throughout the cytoplasm • Microtubules – 25 nm • Microfilaments – 7-9 nm • Intermediate filaments – 10 nm • Functions often require energy • Cytoskeleton roles o o Organelle/protein o Mitosis/cytokinesis o Cell migration trafficking o Muscle contraction o Extravasation o Cilia/flagella o Cell adhesion Microtubules: Structure • Microtubules are repeated structures of tubulin, which are monomers • Polymer of a and β tubulin • Monomers 55kDa each • aβ dimer (110kDa) is basic “subunit” o Dimers can polymerize by adding to itself, end-to-end, to result in a long structure or chain called a protofilament o Is mostly what is found in the cell (exist as a dimer) o Each dimer is 8 nm wide • Have polarity o One side is different from the other o One side is always a (- end) and the other side is always β (+ end) • Roughly 13 protofilaments come together to form a microtubule • Hollow tubes • Roughly 25 nm in diameter, up to 100s of mm long The organization of tubulin subunits in a microtubule – the dimers are aligned end to end into protofilaments, which pack side by side to form the wall of the microtubule. The protofilaments are slightly staggered so that a-tubulin in one protofilament is in contact with a-tubulin in the neighboring protofilaments, except at the seam, where an a-subunit contacts a β-subunit. The microtubule displays a structural polarity in that subunits are added preferentially at the end where β-subunits are exposed. Dimer-Tubulin Subunit • Dimer is very stable o When microtubules fall apart, they fall apart to the dimer stage (not monomer stage) because of its stability • a binds permanently to GTP (nonexchangeable), β can hydrolyze GTP so can be bound to GTP or GDP (exchangeable) • As the polymer (protofilament) grows, β’s GTP is hydrolyzed Arrangement of MT Protofilaments • Protofilaments come together to form a barrel structure (microtubule) • Singlets (most common) – 13 protofilaments forming a single tube of 25 nm diameter o Dynamic (can be polymerized or depolymerized) o Not stable (grow and shrink all the time) • Doublets and triplets (stable) can also be formed – made of 13 protofilament tube with one or two 10 protofilament tubes attached o Do not readily polymerize or depolymerize, therefore stable • Doublet –Aring (13 protofilaments) + B ring (10 protofilaments) • Triplets have an additional C ring with another additional 10 protofilaments Cytoplasmic vs.Axonemal Microtubules • Microtubules organize the interior of the cell • 2 “types” – cytoplasmic and axonemal (cilia, etc.) o Cytoplasmic – microtubules in cytoplasm (microtubules in the axon of nerve cells are still in the cytoplasm, therefore they are still cytoplasmic) o Axonemal – special structures always found in cilia or flagella Microtubule Organization • The MTOC functions to nucleate the assembly of microtubules • Dynamic process • Minus (-) ends of microtubules are associated with MTOC • Centrosome o The centrosome is the major (but not only) microtubule-organizing center (MTOC) in animal cells o Centrosomes not found in plants o Centrosomes contain centrioles • Centrioles o Stable o Play a role in microtubule growth, but don’t directly contact microtubules • Around the centrioles are many proteins – these proteins help polymerize and organize microtubules (g-tubulin, augmin complex) o These are in the pericentriolar matrix Centriole Details • Two centrioles made of triplet microtubules • Singlet (A) – 13 protofilaments • Doublet (A, B) – 13, 10, 10, protofilaments • Proteins in pericentriolar matrix (like (g-TuRC) polymerize growth of the microtubule, help microtubules grow • g-tubulin forms g-tubulin ring complexes (g- TuRC) Microtubule Organization (Continued) • Minus (-) ends are found in the MTOC • Plus (+) ends grow away from the MTOC; is away from the center of the cell and tends to face the plasma membrane • Microtubules in the (-) end cannot change because they are capped, while the (+) end grows and shrinks • Dendrites are an exception o Have microtubules in them that are antiparallel (that is, the (-) end points away from the cell body instead) o This process is poorly understood, in terms of why it happens and what it means In an interphase cell (c), the MTOC is called a centrosome; in a mitotic cell (d), the two MTOCs are called spindle poles; in a neuron (e), microtubules in both axons and dendrites are assembled from an MTOC in the cell body and then released from it; the microtubules that make up the shaft of a cilium or flagellum (f) are assembled from an MTOC known as a basal body g-Tubulin Nucleates Polymerization • g-tubulin ring complex (gTuRC) (many proteins) provides nucleating sites for microtubules • g-tubulin starts off the process of nucleation in microtubules o Doesn’t do it alone, does it with a group of proteins o Together, they form the g-TuRC • g-tubulin tends to start at the (-) end, causes nucleation, and allows things to be added at the (+) end o Can observe growth at the (+) end o This process is still poorly understood • Because g-TuRC is at the (-) end, it is essentially capping the (-) end o Once g-TuRC is there, the (-) end can’t grow or shrink, but the (+) still can Microtubule Formation • MTs assembly and disassembly is important to their function. Subunit critical concentration (Cc) and temperature important (microtubules disassemble when chilled to 4°C) • The graph illustrates polymerization and the concept of critical concentrations • Mass (y-axis) = amount of aβ dimers • Graph illustrates an increasing amount of aβ dimers being made in the cell – steadily increases • Tubulin has the ability to polymerize o After it reaches a certain critical concentration, aβ dimers stick together to make a protofilament • Once you get above a certain concentration in the cytoplasm, you’ve reached a critical concentration o Once it’s above the critical concentration, the amount of dimers in the cell stays the same o All other dimers being made are now going to make microtubules • Above the critical concentration, tubulin starts to make microtubules (polymerization of microtubules); below the critical concentration, depolymerization of microtubules occurs • Cells can regulate microtubules by regulating the amount of dimers present • Another useful tool is the use of temperature o At 4°C, microtubules depolymerize – cells can be obser
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