1. APBs involved in regulating actin dynamics (Cont.) 2. How high rates of actin polymerization are maintained at the protruding edge? Thymosin  -4 Profilin Arp2/3 Capping Severing Lodish 5th Ed. Chapter 19, p786-791

2. The combined effect of capping, severing and depolymerizing F- actin is important for sustaining rapid protrusion From: “Control of actin dynamics” Carlier 1998

3. Mullins 1998

4. FACT: In a steadily migrating cell lamellipodium width is ~ constant, - due to balance between assembly at the front and disassembly further back Treadmilling occurs within the whole population of F-actin

5. FSM movies Check “Movies” link on HuskyCT to see the ones I showed in class.

6. Alberts Fig. 16.58 Models of actin filament growth during protrusion: Treadmilling versus nucleation-release Vic Small Tim Mitchison

7. Alberts Fig. 16.56 Photobleaching fluorescent actin in a fibroblast The fluorescent mark moves backward with respect to the front cell edge (and with respect to the substratum). Slow moving cell Y-L Wang et al., 1985. JCB 101:597-602 Experiment #1 Conclusion: 1. Actin meshwork is flowing backward 2 New actin is polymerized at the edge

8. Photoactivation of fluorescence in a moving keratocyte The fluorescent mark moves backward with respect to the front cell edge (but not with respect to the substratum). Fast moving cell Theriot and Mitchison, 1991 Nature 352: 352-131 Fig. 18-19b