Tetrahymena fimbrin localized in the division furrow bundles actin filaments in a calcium-independent manner

In cytokinesis, the contractile ring constricts the cleavage furrow. However, the formation and properties of the contractile ring are poorly understood. Fimbrin has two actin-binding domains and two EF-hand Ca2+-binding motifs. Ca2+ binding to the EF-hand motifs inhibits actin-binding activity. In Tetrahymena, fimbrin is localized in the cleavage furrow during cytokinesis. In a previous study, Tetrahymena fimbrin was purified with an F-actin affinity column. However, the purified Tetrahymena fimbrin was broken in to a 60 kDa fragment of a 70 kDa full length fimbrin. In this study, we investigated the properties of recombinant Tetrahymena fimbrin. In an F-actin cosedimentation assay, Tetrahymena fimbrin bound to F-actin and bundled it in a Ca2+-independent manner, with a K-d of 0.3 muM and a stoichiometry at saturation of 1: 1.4 (Tetrahymena fimbrin: actin). In the presence of 1 molecule of Tetrahymena fimbrin to 7 molecules of actin, F-actin was bundled. Immunofluorecence microscopy showed that a dotted line of Tetrahymena fimbrin along the cleavage furrow formed a ring structure. The properties and localization of Tetrahymena fimbrin suggest that it bundles actin filaments in the cleavage furrow and plays an important role in contractile ring formation during cytokinesis.