The nuclear F-actin interactome of Xenopus oocytes

Xenopus oocyte nuclei are giant organelles whose mechanical integrity is provided by an extensive intranuclear F-actin network. Organization and maintenance of cytoplasmic equivalents involve large sets of accessory proteins. We here report on the development of a novel phalloidin-based affinity matrix that we used to identify the nuclear F-actin interactome of Xenopus oocytes. This approach revealed a compilation of proteins that strikingly differed from the respective cytoplasmic interactome. We designate previously known F-actin binders, chromatin remodelers and DNA binding proteins in a novel role as nuclear F-actin interactors. A major constituent of the nuclear F-actin network turned out to be a fully unexpected component, namely a novel kinesin that bundles F-actin filaments. This protein possesses features that allow contact to two different cytoskeletal systems. Indeed, following germinal vesicle break down, Kin3I contacts a structure called the transient microtubule array, which serves as precursor of the meiotic spindle. At the end of the first asymmetric cell division of the oocyte, we observed a striking localization of Kin3I to the resulting polar body, indicating that Kin3I might be involved in its extrusion.