How is a symmetrical egg turned into an embryo with a head and a tail? How are neuronal connections strengthened or weakened in response to experience? Why don’t incompatible chemical reactions that are necessary for life interfere with each other? The solution to these seemingly unconnected questions lies in the ability of cells to target proteins and nucleic acids to specific subcellular domains. My laboratory is focused on identifying and characterizing novel mechanisms for generating subcellular organization and studying how those mechanisms are used in development, metabolism, and the nervous system. As part of this effort, we use a multidisciplinary approach that applies techniques from biochemistry, genetics, microscopy, and systems biology.
In order to identify novel types of intracellular organization, we are conducting a visual screen of the yeast GFP strain collection to identify proteins that assemble into previously unidentified intracellular structures. A pilot version of this screen has identified 38 novel intracellular structures the majority of which are comprised of proteins involved in metabolic pathways – suggesting that self-assembly is a major mechanism for regulating metabolic pathways in vivo.
Consistent with this, one of the proteins we identified, CTP synthase, forms a novel intracellular filament that is conserved from bacteria to mammals. We have also shown that its ability to self-assemble is connected to the regulation of CTP synthase activity. We propose that this form of enzyme regulation functions analogously to the way in which microtubules and actin operate – the core of the polymer is locked into a particular conformation by end limited disassembly. Since we have identified a number of enzymes that exhibit polymerization in vivo, it is likely that this mode of regulation is not unique to CTP synthase.
Interestingly, we have also found that CTP synthase filaments form in axons, but not in dendrites arguing that CTP synthase filaments may also have a cytoskeletal role in neurons apart from their role in regulating enzyme activity. We are currently characterizing CTP synthase filaments in neurons.
One mechanism for localizing cytoplasmic proteins is to transport the mRNA encoding the protein to the desired location, so that the synthesis of the protein is spatially restricted to particular cytoplasmic regions. Establishment of these domains is further enhanced by a translational control mechanism that ensures that only the properly targeted messages are translated. This sorting mechanism has been implicated in processes as diverse as stem cell differentiation, regulating synaptic strength in neurons and embryonic pattern – underscoring the importance of understanding mRNA localization for medicine, neuroscience, and developmental biology. While the importance of this method of sorting cytoplasmic proteins is clear, it is unclear how localized messages are targeted to different domains or how many cellular or developmental processes utilize mRNA localization. We have used a biochemical approach to purify mRNA transport/translational control complexes from Drosophila melanogaster. By exploiting the powerful genetic tools available in Drosophila, we have begun dissecting how these transport complexes are assembled and disassembled in order to establish the primary body axes of the Drosophila embryo.
Jim Wilhelm received his Ph.D. in Cell Biology from University of California, San Francisco. He then took a Staff Associate (independent postdoctoral position) in the Department of Embryology, Carnegie Institution of Washington where he received funding from the Life Sciences Research Foundation as a Howard Hughes Medical Institute Fellow. He is also a Sloan Research Fellow in Neuroscience, an Ellison Medical Foundation New Scholar in Aging, and March of Dimes Basil O'Connor Scholar.