The exchange of macromolecules between the nucleus and the cytoplasm is critical to establish and maintain order in eukaryotic cells, and constitutes an important step in the regulation of gene expression. Through a combination of genetic, biochemical and biophysical approaches in Saccharomyces cerevisiae and in metazoan cells, our laboratory seeks to characterize and analyze the molecular machinery that is responsible for the transport of macromolecules into and out of the nucleus.
Interestingly, components of the nuclear transport machinery function not only to transport cargo during interphase but also play essential roles in cell division. In particular, the small GTPase Ran has emerged as a key regulator, signaling multiple chromatin-mediated events during mitosis. We would like to understand the function of Ran in mitosis as well as identify and characterize factors that are regulated by Ran during cell division.
Nuclear transport. Compartmentalization in eukaryotes causes spatial separation of cellular processes such as DNA transcription and mRNA translation. This leads to the bi-directional exchange of a large number of macromolecules between the nuclear and cytoplasmic compartments and allows for additional regulation of eukaryotic gene expression (Weis, 2003). The nuclear pore complex (NPC), a gigantic, multi-protein structure, mediates nucleocytoplasmic transport. Our laboratory is working to understand the molecular principles governing transport events across the NPC. Using various approaches, we aim to dissect the roles of NPC components (Weirich et al., 2004; Zeitler and Weis, 2004) and soluble transport factors (Nachury and Weis, 1999; Stade et al., 1997), with the ultimate goal of understanding how protein and RNA complexes are directionally transported across the nuclear envelope.
Mitotic role of the RanGTPase. In collaboration with the laboratory of Rebecca Heald, we study regulation of cell division by the small GTPase Ran. Ran is a highly conserved and abundant protein, which, like other members of the family of small GTPases, continually cycles between GTP- and GDP-bound forms. Using a FRET-based RanGTP biosensor, we have shown that RanGTP is enriched around chromatin (Kalab et al., 2002) where it locally liberates cargoes from transport receptors, promoting spindle assembly during mitosis (Nachury et al., 2001). We have identified mitotic cargoes that are regulated by the RanGTP pathway, leading to the unexpected discovery that RNA is required for spindle organization and function (Blower et al., 2005). Currently, we are dissecting the novel functions of RNA in spindle assembly and continuing to characterize effectors of the Ran pathway during mitosis.