Brunner Group

Figure 1: Interphase microtubule bundles in fission yeast cells.

 

Figure 2: Microtubule bundles in the epidermal cells during dorsal closure in Drosophila melanogaster.

Previous and current research 

To create a defined morphology, cells need to polarise and correctly orient their polarity axis. Both processes require a defined intracellular order based on the specific sub-cellular arrangement of actin and microtubule filaments. Our investigations focus on the contribution of microtubules, whose organisation varies tremendously between different cell types and also in individual cells during different developmental or cell cycle stages. Little is known about how this variability is achieved and how cells switch from one organisational state to another. We address these questions in two model organisms, the unicellular fission yeast Schizosaccharomyces pombe and the fruit fly Drosophila melanogaster.

In the cylindrical fission yeast cells we describe the cell autonomous machinery that organises and maintains a defined interphase microtubule distribution. In these cells, approximately 30 anti-parallel microtubules form 3-6 bundles that are arranged parallel to the long cell axis. Microtubule minus-ends overlap in the cell centre. From there the plus-ends grow to the cell poles where they switch to shrinkage, an event termed catastrophe (figure 1). This localised catastrophe is fundamental to proper cell morphology. Our findings suggest a model where conserved proteins at the growing microtubule plus-ends (+TIPs) mediate cell-pole targeting in two steps. First, the yeast EB1 homolog Mal3p promotes growth until cortical regions are encountered. In central regions of the cell cortex, the CLIP-170 homolog Tip1p then prevents premature catastrophes by suppressing Mal3p removal, which keeps microtubules growing below the cortex until the cell poles are reached.

A central question concerning +TIP function is how these proteins can accumulate at growing microtubule plus-ends. We discovered that Tip1p is transported there by the Tea2p motor protein. Mal3p in contrast, seems to 'treadmill', preferentially binding to plus-ends followed by rapid unbinding. In an attempt to further describe Mal3p plus-end binding, we discovered that Mal3p also binds and stabilises the microtubule lattice seam, explaining its weak localisation all along the microtubules. This provides a new twist to the model of how microtubule dynamics are controlled and shows that microtubules have two different surfaces for molecular interactions.

In fruit flies we explore to what extent the basic machinery found in fission yeast is used to maintain microtubule organisation in a multi-cellular organism and how non-autonomous cells achieve and coordinate changes in microtubule distribution. We have shown how microtubules become reorganised during embryonic dorsal closure (DC), a wound healing-related process. Thereby, antiparallel microtubules transiently form bundles in the epidermal cells that move dorsally to close a cavity (figure 2). Surprisingly, these microtubules are essential exclusively for the final step, the fusion of the epithelium.

Future projects and goals

In fission yeast we now focus on two topics. First, we want to understand how catastrophes are induced at cell poles and second we want to identify the critical molecules/processes for switching between the seven different microtubule arrangements found in S. pombe cells.

In flies, we are trying to identify the signals triggering microtubule reorganisation during DC and we want to uncover the molecular mechanisms driving the process.