We want to understand how complex macromolecular assemblies within cells are built, maintained, and remodeled at the right place and time. For most such assemblies, form is clearly critical for cellular function; consider, as examples, the barrel-like stacks of oligomeric rings found in nuclear pores and proteasomes. Numerous diseases, among them neurodegenerative disorders and systemic amyloidoses, arise from inappropriate folding of, or interactions between, protein subunits. Exactly what regulates higher-order assembly in normal and disease states is largely unknown, reflecting a gap between what traditional biochemical and cell biological approaches can tell us. We use new tools and approaches that fill this gap and enable the identification of molecular mechanisms by which functional higher-order structures are assembled from protein subunits and maintained or modified through cell division and differentiation.
Members of the septin family of proteins are found in nearly every eukaryote, typically as hetero-oligomeric complexes, the proper organization of which is required for their function. Although the mechanistic details of their roles remain largely unknown, septins participate in a variety of cellular processes, generally involving plasma membrane remodeling. Mutation or misregulation that upsets the stoichiometry of septin hetero-oligomers is a common feature of septin-associated human diseases, which include cancer, male infertility, and hereditary neuropathies. In budding yeast, where septins were first identified, hetero-octamers – whose subunit composition is strikingly similar to those within human septin complexes – polymerize into filaments arrayed at sites of cell division and morphogenesis. Septin-containing cellular structures in dividing and differentiating cells are dynamic, undergoing abrupt changes in organization in a temporally- and spatially-regulated manner. It is not known how assemblies with the proper arrangement of septin subunits are built in the cell, or how they are reorganized during cycles of proliferation and development. The budding yeast septins represent an elegant and powerful system with which to identify cellular mechanisms regulating the organization of these multi-subunit macromolecular assemblies.
We are currently focused on several projects:
1) In collaboration with the lab of Prof. Eva Nogales at the Univ. of California, Berkeley, elucidate at nanometer resolution the organization of septin subunits within higher-order assemblies inSaccharomyces cerevisiae.
2) Determine how the early events of folding, nucleotide binding/hydrolysis, and oligomerization contribute to higher-order septin assembly.
3) Identify the mechanisms by which pre-existing septin assemblies are remodeled during developmental transitions (e.g., sporulation).
4) Search for examples of "structural inheritance", in which the proper conformation or organization of a newly-made protein-based assembly depends upon the presence of pre-existing copies of the same structure.