AN ACTIN POINT-MUTATION NEIGHBORING THE HYDROPHOBIC PLUG CAUSES DEFECTS IN THE MAINTENANCE OF CELL POLARITY AND SEPTUM ORGANIZATION IN THE FISSION YEAST SCHIZOSACCHAROMYCES-POMBE

The fission yeast cps8 mutation gives rise to abnormally enlarged and dispolarized cells, each of which contains several nuclei with aberran t multisepta. Molecular cloning and sequence analysis of the cps8 gene indicated that it encodes an actin with an amino acid substitution of aspartic acid for glycine at residue 273 in the hydrophobic loop that is located between actin subdomains 3 and 4. Fluorescence microscopy using phalloidin and anti-actin antibody revealed changes in the F-act in structure and distribution in the mutant cells. These results indic ate that the hydrophobic loop plays an essential role for creating nor mal F-actin structure, only by which cell polarity and the late mitoti c events can be maintained properly.