Abstract

Cell division in many eukaryotes is driven by a ring containing actin and myosin. While much is known about the main proteins involved, the precise arrangement of actin filaments within the contractile machinery, and how force is transmitted to the membrane remains unclear. Here we use cryosectioning and cryo-focused ion beam milling to gain access to cryo-preserved actomyosin rings in Schizosaccharomyces pombe for direct three-dimensional imaging by electron cryotomography. Our results show that straight, overlapping actin filaments, running nearly parallel to each other and to the membrane, form a loose bundle of approximately 150 nm in diameter that \"saddles\" the inward-bending membrane at the leading edge of the division septum. The filaments do not make direct contact with the membrane. Our analysis of the actin filaments reveals the variability in filament number, nearest-neighbor distances between filaments within the bundle, their distance from the membrane and angular distribution with respect to the membrane.\n\nSignificance StatementMost eukaryotic cells divide using a contractile actomyosin ring, but its structure is unknown. Here we use new specimen preparation methods and electron cryotomography to image constricting rings directly in 3D, in a near-native state in the model organism Schizosaccharomyces pombe. Our images reveal the arrangement of individual actin filaments within the contracting actomyosin ring.