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Lecture 21.doc

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Biology (Sci)
BIOL 201
Gary Brouhard

Lecture 21 Controlling Actin Review: Plus-end actin binds to the end with an on-rate constant (multiplying concentration to the constant) Balance of two numbers gives a critical concentration (below - plus end will shrink) ATP hydrolysis cycle that happens at the same time, Actin polymerizes in ATP state. Phosphate eventually dissociates to be left with ADP-actin. The C-c is higher than C+c means that actin filaments will depart from minus end but adding to the plus end. The net length of filament not changing - treadmilling, but filament translocates in space. Should be able to calculate conc. at which this process occurs. ADP-actin - actin has to regenerate into ATP state before it can repolymerize. The number of proteins that the cells have created allows them to modify the underlying properties of the actin network. -Formins sit at plus end and control the rate of which the actin filament grows. Other formins gate the polymerization and slow down the filament elongation. -Ability to cut: cofilin: severs the actin filament into pieces; to make depolarization happen faster, you fragment the filament into large pieces -stop actin from growing: capping protein so that no new actin monomers are able to add to it. -generation of branched networks: Arp2/3 complex -need to arrange the filaments according to one another in space: alpha-actinin You need 2 actin binding domains to cross link the actin filaments together. -fimbrin: single gene product; has these two actin-binding domains; has single repeat of these actin binding domains; location: microvili, filopodia, focal adhesions -single protein has these actin filament-binding domains able to dimerize alpha- actinin actin aren't as close together as they are in fimbrin; location: stress fibres, filopodia, muscle Z line -Formation of cell cortex - tetramer, alpha and beta spectrin, they dimerize within itself and able to cross-link short fragment Spectrin forms a hexagonal network beneath the plasma membrane; location: cell cortex -filamin single protein; add rigidity by cross-linking; location: leading edge, stress fibres, filopodia A pathogenic bacterium hijacks the actin machinery of human cells. A branched network good for migration and involved in pathology of a food-born pathogen called Listeria monocytogenes. It hijacks the actin machinery and uses it to propel itself around inside the cell, building. It has comet tails (actin filament networks which are identical to the actin filament found at the leading edge of the cell). It punctures the cell and goes to infect the other cells. The bacteria have evolved to hijack the actin cytoskeleton in order to properly propel themselves to other cells. Its comet tails are nucleated by the Arp2/3 complex. It’s pretty deadly in the immunocompromised and the pregnant. Arp2/3 (actin related protein) acts as a nucleation factor for actin. It is difficult for actin to create a nucleus, but once it's formed, it's good to go. Arp 2 and Arp 3 are too far apart (1) but this WASp domain induces a conformational change in the protein (3) and brings them together and creates a daughter filament growing on the side at a 70 degree angle. 70 degrees is the optimal angle to be pushing out from. Lamellipodia must overcome two challenges: 1. Running out of actin; keep enough actin for polymerization to keep moving forward. Growth rate of actin is concentration dependent. If you don't have s
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