1. Department of Systems Biology, Harvard Medical School, 200 Longwood Avenue, Boston, Massachusetts 02115, USA

Correspondence to: Alina M. Vrabioiu¹ Correspondence and requests for materials should be addressed to A.M.V. (Email: alina_vrabioiu@student.hms.harvard.edu).

Abstract

Septins are polymerizing GTPases¹ that function in cortical organization and cell division^{2, 3, 4}. In Saccharomyces cerevisiae they localize at the isthmus between the mother and the daughter cells, where they undergo a transition from a non-dynamic hourglass-shaped assembly⁵ to two separate rings, at the onset of cytokinesis^{6, 7}. Septins form filaments as pure protein⁸ and in vivo⁹, but the filament organization within the hourglass and ring structures is controversial^{9, 10}. Here, we use polarized fluorescence microscopy¹¹ of orientationally constrained green fluorescent protein to determine septin filament organization and dynamics in living yeast. We found that the hourglass is made of filaments aligned along the yeast bud neck. During the transition from hourglass to rings the filaments rotate through 90° in the membrane plane and become circumferential. These data resolve a long-standing controversy in the field and provide strong evidence that septins have a mechanical function in cell division.

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