Motor proteins exert force on microtubules to position the nucleus, spindle, and other organelles in eukaryotic cells. The behavior of individual motor proteins has been studied extensively in vitro. In vivo, however, a large number of motors act together. A central question is how a multitude of motors organize their behavior to produce large-scale movements in the cell. To answer this question, we studied nuclear oscillations in fission yeast, which are driven by dynein motors pulling on microtubules. These oscillations facilitate chromosome pairing, which is a phenomenon conserved across species. We have shown that dynein motors dynamically redistribute from one part of the cell to the other, generating asymmetric patterns of motors and, consequently, of forces required for movement. By combining quantitative live cell imaging and laser ablation with a theoretical model, we demonstrated that the motor redistribution is driven by mechanical cues. Our work emphasizes that spatio-temporal pattern formation can occur due to a mechanism that differs from conventional molecular signaling, as well as from self-organization based on a combination of biochemical reactions and diffusion.

To exert forces, motor proteins bind with one end to cytoskeletal filaments, such as microtubules and actin, and with the other end to the cell cortex, a vesicle, or another motor. A general question is how motors target sites where they can exert force. We set up the experiments to observe the movement of single dyneins in vivo. We are able to follow single dyneins on the microtubule and in the cytoplasm. Unexpectedly, we are also able to directly visualize binding of dynein from the cytoplasm to the microtubule and to quantify this process. We are now using this straightforward approach based on direct observation of single motors to identify key steps of the redistribution of dynein and to measure the kinetics of this reaction-diffusion process. Finally, we will study how dynein undergoes activation and starts to walk along the microtubule, to perform its function of moving the nucleus.

For a mother cell to divide its genetic material equally between the two daughter cells, the chromosomes have to attach to microtubules, which will pull them apart. The linkers between chromosomes and microtubules are kinetochores, protein complexes on the chromosome. The key question is how microtubules find kinetochores. In fission yeast, kinetochore capture by microtubules can be observed when kinetochores are lost in the nucleoplasm, which can be induced by spindle disassembly during metaphase. To our surprise, we observed that the microtubule that captures a kinetochore does not grow directly towards the kinetochore. Instead, it starts to grow in an arbitrary direction and pivots around the spindle pole while growing, eventually reaching the kinetochore. By introducing a theoretical model, we are now investigating this novel mechanism where microtubules explore space by pivoting, as they search for intracellular targets.

The way that damaged proteins are segregated at cell division determines the survival and ageing of cells. We investigate the dynamics and segregation of Hsp104-associated aggregates of damaged proteins in fission yeast. We developed a mathematical model for aggregation based on the experimentally observed aggregate nucleation, diffusion, and fusion of aggregates with one another in the cytoplasm. At cell division, aggregates are segregated symmetrically to the two daughter cells under favorable growth conditions, whereas they are segregated asymmetrically under stress conditions, generating damage-free cells. We are currently exploring the mechanism underlying the transition from symmetric to asymmetric segregation, as well as the idea of this transition as a survival strategy.