Visualization of a highly organized intranuclear network of filaments in living mammalian cells

For 30 years, the mammalian cell nucleus has been hypothesized to contain a filamentous framework, the nuclear matrix or karyoskeleton, which regulates nuclear structure and function. However, such an organized network of filaments has never been observed in living cells. Here we show that human Cdc14B phosphatase in living cells tightly associates with long filaments that begin at the nucleolar periphery and extend to the nuclear envelope, frequently making close connections with nuclear pore complexes. We demonstrate that Cdc14B contains a bipartite signal that directs it to the intranuclear filaments, and we also detect a small amount of Cdc14B on interphase and mitotic centrosomes. Furthermore, we show that Cdc14B is critical for the maintenance of proper nuclear structure together with polo-like kinase Plk1. This work provides the first direct evidence for the existence of an intranuclear filamentous framework in living mammalian cells and implicates Cdc14B in the control of mammalian nuclear architecture.