Co-workers:

We study the function and regulation of the eukaryotic microtubule cytoskeleton, using the fission yeast Schizosaccharomyces pombe as a model system.

Microtubule nucleation in eukaryotic cells depends on the gamma-tubulin complex, a large multi-protein complex that is enriched at microtubule organizing centers such as the centrosome. In multicellular organisms, differentiated cells often have multiple, diverse microtubule organizing centers, but the mechanisms that target the gamma- tubulin complex to specific subcellular sites are largely unknown. We discovered two fission yeast proteins, Mto1 and Mto2, that form a complex (Mto1/2 complex) that targets the gamma-tubulin complex to different sites at different times during the cell cycle. Mutations in the human homolog of Mto1 lead to the brain disease microcephaly. In current work we are studying the molecular architecture of the Mto1/2 complex and how it is regulated by post-translational modifications through the cell cycle. We are also investigating how Mto1/2 interacts with the gamma-tubulin complex at the molecular level and how Mto1/2 is targeted to distinct subcellular sites during the cell cycle. To do this we use a combination of classical and molecular genetics, microscopy, and biochemistry/proteomics. In other work we are investigating our recent observation that the Mto1/2 complex may induce conformational changes in the gamma-tubulin complex that convert it from an inactive complex into an active microtubule nucleator.

One of the important roles of microtubules in fission yeast is to aid in the positioning of cell-polarity factors. Several proteins, including the polarity-regulator Tea1, associate with the growing ends of microtubules until the microtubules reach the cell tips, whereupon these proteins are deposited in the cell cortex. We discovered a membrane protein, Mod5, which interacts with Tea1, and we identified a positive-feedback loop in which Mod5 and Tea1 regulated each other’s localization at cell tips. Using a combination of genetics, FRAP microscopy, and mathematical modelling we have recently obtained evidence for a novel mechanism in which Mod5 plays a catalytic role in the anchoring of cortical Tea1. Further proteomic analysis suggests that Tea1 turnover at cell tips may be regulated by phosphorylation.

As technology is often a limiting factor in scientific progress, we also actively develop new methods and tools to help achieve our research goals in genetics, microscopy and proteomics.

Selected publications:

Snaith, H.A., Samejima, I., and Sawin, K.E. (2005). Multistep and multimode cortical anchoring of tea1p at cell tips in fission yeast. EMBO J 24, 3690-3699.

Anders, A., Lourenco, P.C., and Sawin, K.E. (2006). Non-core components of the fission yeast gamma-tubulin complex. Mol Biol Cell 17, 5075-5093.

Anders, A., Watt, S., Bahler, J., and Sawin, K.E. (2008). Improved tools for efficient mapping of fission yeast genes: identification of microtubule nucleation modifier mod22-1 as an allele of chromatin- remodelling factor gene swr1. Yeast 25, 913-925.

Samejima, I., Miller, V.J., Groocock, L.M., and Sawin, K.E. (2008). Two distinct regions of Mto1 are required for normal microtubule nucleation and efficient association with the gamma-tubulin complex in vivo. J Cell Sci 121, 3971-3980.

Microtubule nucleation and cytoskeletal organization