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Lecture 15

Lecture 15 - Chapter 16 Cytoskeleton - March 7.docx

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York University
BIOL 2021
Patricia Lakin- Thomas

March 7, 2013 Lecture 15: Chapter 16: Part 2 – Cytoskeleton Polymerization  Panel 16-2  Actin and microtubulin: plus and minus 1. Rapid addition at plus, slow addition at minus  T form hydrolyzes to the D form after addition 1. ADP/GDP form (“D form” has lower affinity for neighbors) 2. T form likes to add, D’s like to leave  Can’t leave unless you’re at an end! 3. High monomer concentration – T-form adds rapidly 4. Low monomer concentration – D-form can leave 5. Intermediate: Treadmilling – Panel 16-2 heading  Add at one end, leave the other, filament stays the same length – often occurs in actin  With tubulun (microtubules) at intermediate concentration, we have dynamic instability  Adds on, grows and then hydrolyses, causing a collapse – rapid shrinking from the plus end (When all GTP goes to GDP)  Dynamic 1. Requires a constant supply of energy, ATP/GTP hydrolysis in filament  ADP/GDP  Exchange D for T in monomer Regulation of Cytoskeleton Nucleation  Microtubule Nucleation 1. Gamma tubulin ring complexes (Fig 16.29)  Contains gamma tubulin and other proteins, accessory proteins  Starts at minus end, leaves free end plus  Gamma tubulin gives a nice binding site for the alpha and beta monomers 2. Mictrotubule Organizing Centre  Where microtubules start  Animals – centrosome (figure 16.30a), with microtubules (Fig 16.30b) [note the plus end is on the periphery]  Contains gamma tubulin ring complexes and centrioles  Fungi and plants do NOT have centrioles, but use GTRC on nuclear envelope  Actin Nucleation 1. ARP – Actin related protein 2. ARP complex nucleation actin filaments start from minus end (Figure 16.34b) 3. Signals cause activation of proteins that activate ARP 4. We find these complex out in the periphery of the cell – cell cortex (right below plasma membrane)  Here is where actin webs are being made (Fig 16.34c)  Branching the ARP proteins, bind to existing filament and bind at 70 5. Formins – nucleate actin filaments, straight unbranched bundles – stress fibers  Other process regulation 1. Actin polymerization (Panel 16-3)  Profilin compete with thymisin for binding to the subunits, speeds up elongation (accessory protein)  Thymosin binds free subunits, preventing polymerization  Can be activated – Profilin at PM (lots of actin filaments)
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