1. Cofilin is an actin binding protein that severs actin filaments. Katanin is an ATPase that binds to microtubules and severs them. In class, we discussed how acute activation of cofilin inside cells triggers a burst of actin polymerization off the newly created filament (+) ends. In contrast, activating katanin in cells typically results in fast microtubule depolymerization from newly created microtubule (+) ends. Why do actin filament (+) ends grow following cofilin mediated severing but newly generated microtubule (+) ends shrink following katanin mediated severing? The correct answer requires a clear discussion of critical concentrations and the role of nucleotide hydrolysis in polymer dynamics.

2. Pure actin and pure tubulin can be induced to polymerize in vitro. In both cases, there is a long lag phase that precedes fast polymerization. 2a) What is the physiological significance of the long phase prior to assembly? 2b) What are the key factors thought to control the length of the lag phase for actin and for microtubules? 2c) Describe the basic biochemical and/or structural mechanism underlying the activities of the factors described in question 2b. In other words, how do these factors control the length of the lag phase.

3. All biology students are asked to distinguish prokaryotes from eukaryotes. The following lines are from wikibooks.org:

“Cytoskeleton[edit] Prokaryotes have none of this sort. The Eukaryotes have complex array of cytoskeleton which composed of microtubules, intermediate filaments and actin filaments. This protein-developed cytoskeleton reinforces the cell’s outer structure to help with the rigidity of cell and intracellular transport.”

Please assess the validity of the statement that prokaryotes do not have a cytoskeleton. If they do not have a cytoskeleton, then just simply state that. If they do have a cytoskeleton then compare their cytoskeletal elements to eukaryotic actin, tubulin, and intermediate filaments. Are they of the same structure? Do they have the same basic turnover dynamics? Or did they evolve a completely different cytoskeleton than eukaryotes?