Win, Thein Zaw; (2001) Characterisation of two Class V myosins in the fission yeast Schizosaccharomyces pombe. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The fission yeast Schizosaccharomyces pombe is a cylindrically-shaped cell which grows by tip elongation and divides by medial fission. The actin cytoskeleton is intimately associated with both of these processes. The actin-activated motor protein, myosin, is involved in cellular processes that require the function of the actin cytoskeleton. S. pombe has five myosin genes spread amongst three different myosin classes. This thesis describes the identification and characterisation of two of these genes, myo51+ and myo52+, both of which belong to the class V group of the myosin superfamily. Gene inactivation studies reveal that myo51Δ cells display a phenotype that is indistinguishable from wild type whereas myo52Δ cells are temperature-sensitive with a rounded and pear-shaped morphology. The myo51Δ myo52Δ double mutant has the same characteristics as the myo52Δ single mutant. Overexpression of myo51+ produce elongated cells with multiple nuclei whereas overexpression of myo52+ produce branched cells with thickened septa. Preliminary localisation of Myo51 shows that the protein is localised to one of the cell poles. Detailed localisation studies of Myo52 shows that the protein is localised to the actin-rich poles during tip growth and is present at the equator in cells undergoing cytokinesis. The localisation of Myo52 is dependent on an intact actin cytoskeleton. Further analysis of myo52Δ shows that the null mutant is resistant to cell wall digestion by Zymolyase. Immunoblot analysis of the α-glucan synthase, Mok1, shows that the protein is upregulated in myo52Δ cells. Localisation studies reveal that Mok1 colocalises with Myo52 at the cell poles during tip growth and to a certain extent at the equator during cytokinesis. Mok1 is delocalised in myo52Δ cells. These findings suggest that the function of Myo52 is to localise factors involved in growth, such as Mok1, to defined sites in the cell by virtue of its interaction with the actin cytoskeleton.

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