Nuclear pore complexes are central gatekeepers of the cell nucleus, and facilitate selective transport of macromolecules between the cytoplasm and nucleus. As such, they regulate crucial cellular processes such as the cell cycle, RNA metabolism, and gene expression. We apply x-ray crystallography to visualize the protein components of this large macromolecular machinery at atomic resolution. Our interdisciplinary approach integrates methods of chemistry, physics, medicine and biology and will give insights into cancer formation, cell division and differentiation of certain progenitor cells. Mutations of the protein components of the nuclear pore and nuclear export defects lead to cancer, especially several types of leukemia. Recently, based on structure guided drug design, selective inhibitors of nuclear export were developed, which have proven to be an effective treatment for several types of leukemia as well as other cancers (http://karyopharm.com/, http://karyopharm.com/sinetm-technology/overview/). Results from our research will help devise therapies for cancer and developmental diseases.
Transport of the nucleus. The cell nucleus is transported and positioned in a cell cycle specific manner, a process that is important for brain development and cell cycle control. Human disease mutations of proteins engaged in the transport of the nucleus cause cancer and severe developmental defects of the brain and spinal muscle, including microcephaly and spinal muscular atrophy, the most common genetic cause of death in infants. Thus, regulatory mechanisms for these transport events are promising targets to help devise therapies for these devastating neuromuscular and brain development diseases. Despite the importance of nuclear positioning for brain development, it remains largely elusive how the timely transport of the nucleus is initiated and orchestrated. Furthermore, the responsible motor protein complex is cytoplasmic dynein, which orchestrates a vast number of cellular transport events, including RNA/protein complexes, chromosomes, vesicles, mitochondria and organelles, but general principles how the correct cargo is selected at the correct time, have not been established. We plan to establish how the nucleus is recognized as cargo for transport in G2 phase, which is important for cell cycle control and brain development. Our approach combines x-ray crystallography, biophysical methods and cell biological studies. Recently, our lab has established a molecular mechanism how the G2 phase specific kinase cyclin-dependent kinase 1 regulates a pathway for nuclear positioning. This pathway is essential for the differentiation of brain progenitor cells.
Nuclear transport. Nuclear pore complexes consist of 30 proteins, the nups, and facilitate the selective exchange of macromolecules between the nucleus and the cytosol. Their transport channel is arguably the largest and most complex transport conduit in the eukaryotic kingdom, and it is likely composed of the three channel nups. A central question of nuclear transport is: how can the huge protein scaffold of the nuclear pore complex adjust the diameter of its transport channel from 10 to 50 nm to accommodate cargoes of different sizes, including ribosomal subunits and viruses? To address this question, we have determined the protein structures of portions of the channel nups. Based on these structures, we have proposed a 'ring cycle hypothesis' for dilating and constricting the transport channel of the nuclear pore complex from 10-50 nm (see figure above). This mechanism can help us understand how large cargo and viruses, such as HIV, cross the nuclear pore complex, and can inform therapies that block viral entry to the cell nucleus.
An animation of the Ring cycle is available for viewing at this link.
Our research is funded by National Institute of Health, National Institute of General Medical Sciences (NIH NIGMS) grant 1R15GM128119-01.