Nup358 recruits two opposing motors: dynein and kinesin-1 for transport of the nucleus
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 regulatory mechanisms for two pathways that facilitate nuclear positioning. Both pathways are essential for the differentiation of brain progenitor cells to neurons and other cell types.
Activation mechanism of the dynein adapter Bicaudal D2
A coiled-coil registry shift may activate the dynein adapter Bicaudal D2 for dynein recruitment: Dynein adaptors such as Bicaudal D2 (BicD2) recognize cargoes and link them to dynein. In the absence of cargo, BicD2 is autoinhibited and cannot recruit dynein. Our research has established mechanistic insights into activation. Based on our X-ray structures, M.D. simulations and other data, we propose that binding of cargo induces a coiled-coil registry shift in BicD2, i.e. a vertical displacement of the two helices against each other by one helical turn, which activates BicD2 for dynein recruitment. Activation of dynein adapters such as BicD2 is a key regulatory step for transport, as adapters are required to activate dynein for processive transport. Dynein facilitates a vast number of cellular transport events that are critical for chromosome segregation, signal transmission at synapses, as well as essential for brain and muscle development.
Ring cycle for dilation and constriction of the nuclear pore
Nucleo-cytoplasmic 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.