Multicellular development of fission yeast

We use a combination of functional genomics and advanced microscopy to dissect the process of switiching to filamentous gowth in S. pombe. Using a collection of mutants in which each non-essential gene in the genome has been systematically deleted, we have identified many of the genes required for the process, and found that it an be proken down into three stages: adhesion, invasion and filamentous growth. Our current work concentrates on two of these stages.

The signal for invasion

We found that two factors contribute to trigering the switch to filamentous growth. First, a combination of low nitrogen byt high carbon levels promotes the process, via the intracellular signalling molecule cyclic AMP. Secondly, growth of a surface colony to hgih density is essential to trigger invasion; the continued presence of these cells is also essential for invasive grwoth to continue, even when the invading cells are some way from the surface. We are trying to identify the signal that send this information. Blocking this process with antagonists might be a new apporach to the development of antifungal agents.

Growth of S. pombe in filaments

As well as functional genomics, we have developed two apporaches which allow us to observe the proces of filamentous growth in real time. First, special culture dishes incorporating a microscope coverslip allow us to grow the filaments and carry out high-resolution mircoscopy simultaneously. Secondly, we identified an isolate of S. pombe which invades much more efficiently than the standard laboratory strain. 

The molecular basis of single-celled growth in S. pombe has been studied intensively. Both the microtubules and the actin cytoskeleton play critical roles. Immeidately after cell division, growth takes place only at one end - the 'old' end. At a fixed point in the cycle, growthbegins at the 'new' end; this is known as 'new-end take-off' or NETO. We found that NETO does take place in filaments. Growth continues at one end only, and hence cells become highly elongated. At the non-growing end, cell separation is not triggered, hence multicellualr structures and branching result.

Many of the molecules which orchestrate the pattern of growth have been identified; often these proteins localise to particular parts of the cell, such as the ends, at particular stages of the cycle. Using genes 'tagged' with Green Fluorescent Protein, we have compared the location of these proteins in filaments compared to single cells. Some show dramatic changes, beginning to indicate how the grwoth pattern of a eukaryotic cell can be reorganised at the molecular level.