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The Role of the Formin Protein Family in Membrane Dynamics

Abstract
Using molecular genetics, and high end imaging techniques, I assessed the function of the formin protein family in the moss Physcomitrella patens. Formins are proteins that can nucleate and elongate actin filaments. P. patens has 9 formins, divided over three classes. I found that a class II formin (For2A) is essential for polarized growth and specifically binds to the phosphoinositide PI(3,5)P2. Additionally, I show that this formin polymerizes actin filaments in vivo. I demonstrated that binding PI(3,5)P2 is essential for formin function. My work also shows that one of the class I formins (For1F) is involved in exocytosis and likely is a part of the exocyst tethering complex, directly linking exocytosis to the actin cytoskeleton in plants. For1F is an essential gene, but its deletion can be rescued by overexpression of For1D, another class I formin, suggesting that class I formins are involved in exocytosis. Class I formins associate with actin filaments, but their interaction with actin differs from class II formin interaction with actin. Drug treatments show that their dynamics are dependent on both microtubules and actin filaments. This is in contrast to class II formins that do localize to endocytic sites and whose dynamics are only dependent on actin filaments. An endocytic marker can be seen traveling with the processive formin For2A when For2A is polymerizing an actin filament. Quantification of the activity of For2A along the length of tip growing cells reveals that For2A preferentially generates actin filaments towards the tip of the cell. This provides an actin array that is predominantly tip-oriented and could serve as a scaffold for myosins to transport cargo along towards the cell tip.
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