University of Minnesota
School of Physics & Astronomy

Biophysics Seminar

Wednesday, October 21st 2015
10:10 am:
Biophysics Seminar in 120 PAN
Speaker: Naomi Courtemanche, University of Minnesota, College of Biological Sciences
Subject: Mechanistic studies of formin-mediated actin assembly

Actin dynamics drive many cellular processes, including motility, cytokinesis and endocytosis. A host of actin-binding proteins controls actin filament nucleation, elongation, capping, branching and bundling. Members of the formin family of proteins nucleate new filaments and remain processively attached to actin filament barbed ends while promoting their elongation. Because we lack many of the details required to understand how formins function in cells, it is not known how they promote healthy cellular proliferation. I will present the results of three studies aimed at elucidating the mechanism of actin polymerization mediated by the S. cerevisiae formin Bni1p. First, I will describe experiments I performed using total internal reflection fluorescence microscopy to address the role of sequence organization in Formin Homology (FH) 1 domain-mediated transfer of profilin-actin to a formin-bound filament end. Second, I will describe a technique I developed called “actin curtains”, which I used to study the effect of linear force on formin-mediated polymerization. I will show that small linear forces dramatically slow formin-mediated polymerization in the absence of profilin, suggesting that force shifts the conformational equilibrium of the end of a formin-bound filament, but that profilin-actin associated with FH1 domains reverses this effect. Third, I will describe the effects of point-substitutions in the FH2 domain on the activity of Bni1p, which suggest that nucleation and elongation are separable functions of formins that involve different interactions with actin. These studies provide insight into the physical properties of formins, which will be useful in guiding future studies aimed at elucidating how sequence variations confer unique biological functions to formin isoforms expressed in the same cell.

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