Regulation of the Actin Cytoskeleton and Polarized Growth in Schizosaccharomyces pombe.

The fission yeast, Schizosaccharomyces pombe grows only in length in a highly polarized manner in a process tightly regulated by both the microtubule and actin cytoskeletons, which play key roles in the establishment of cell polarity and growth of the cell respectively. Previous studies have shown that within the microtubule cytoskeleton a large group of proteins accumulate at the cell poles, where they are believed to form a “polarisome” which coordinates the cell’s polarity. Whilst the main functions and key interactions between these proteins are known, many of the finer details remain unclear. The actin cytoskeleton is a highly dynamic network of different actin structures, with multiple changes occurring that are regulated by actin binding proteins (ABPs). One of the most important actin binding proteins is tropomyosin, which is responsible for stabilising interphase actin cables and the cytokinetic actomyosin ring (CAR). Within fission yeast tropomyosin is present in both acetylated and unacetylated forms which each localise to distinct actin structures.

Interphase actin cables and the CAR are nucleated by two different formins; For3 and Cdc12 respectively. Within this study it has been shown that it is these formins that determine which tropomyosin isoform associates with specific actin structures. Exchanging the localisation of these formins results in a corresponding exchange of tropomyosin present on actin filaments. This exchange also has a subsequent effect on actin dynamics and the interaction of other ABPs. In addition, when investigating the regulation of cell polarity, it was shown that this is a very complex process. Proteins from both the actin and microtubule cytoskeletons playing key roles in the transport, recruitment, tethering and turnover of each other as shown by deletions of one polarity protein often having a significant effect on the localisation and abundance of others.