Novel Modulation Of The S.Cerevsiae Actin-Cyctokeleton By Heterologous Expression Of The Short N.Crassa Tropomyosin

Tropomyosins are actin-regulatory proteins found in eukaryotes from yeast to man. They are best understood in their role in the regulation of muscle contraction in higher eukaryotes. However, only a small subset of the 20 or more isoforms of tropomyosin found in man are involved in muscle regulation. Their extensive diversity indicates their importance in regulating the actin cytoskeleton, a role in which their function is only beginning to be understood. Saccharomyces cerrevisiae provides a simple model system for trying to understand the functioning of tropomyosins in regulating the actin cytoskeleton. It has two tropomyosins, the larger 5 actin spanning TPM1 and smaller 4 actin spanning TPM2. Knockout of the major TPM1 isoform produces defects in cell division and hence cell size, whilst knockout of the minor TPM2 isoform has no morphological effect. A double knockout is lethal. Previous studies of heterologous expression of other tropomyosins in a ΔTPM1 background have all produced unremarkable ‘rescue’ of the knockout effects. We have previously reported that Neurospora crassa also has two tropomyosins, but in this case the larger is a 4 actin spanning and the shorter an unusual 3 actin spanning tropomyosin. Heterologous expression of this short N. crassa tropomyosin in ΔTPM1 cells does not produce rescue. Instead we observe a range of morphologicial effects indicating a significant modification of the functioning of the actin cyctoskeleton, with effects on both cell division and size / shape. This is the first time a clear effect has been observed by the expression of a heterologous tropomyosin in S. cerevisiae and indicates the short N. crassa tropomyosin may have unusual actin-regulatory properties as seen previously for an similar-sized artificial construct based on the yeast TPM1.