THE ABERRANT POSITIONING OF NUCLEI AND THE MICROTUBULAR CYTOSKELETON IN SACCHAROMYCES-CEREVISIAE DUE TO IMPROPER ACTIN FUNCTION

An excentric position of the nuclei, random orientation of mitoses, an d multinuclear budding cells were identified in part of a population o f temperature-sensitive (ts) Saccharomyces cerevisiae actin mutants at the permissive temperature of 23 degrees C by fluorescence and electr on microscopy. The phenotype resembled that of mutants in beta-tubulin , dynein, JNM1, NUM1, ACT3, ACT5, myosins, profilin, tropomyosin 1, SL A2 and other genes. The question was addressed whether the cause was ( i) defects in cell polarity in some ts actin mutants, manifested by la ck of asymmetry of actin cortical patches, or (ii) lack of cytoplasmic or astral microtubules. The results indicated that in the cells with the nuclear defects, actin cortical patches showed the normal asymmetr ic distribution typical of undisturbed polarity. Cytoplasmic, astral a nd spindle microtubules were also preserved. The principal difference found between the wild-type and actin mutant cells was in actin cables , which in the actin mutants were developed insufficiently. It is sugg ested that actin cables serve as a 'suspensory apparatus' and/or 'intr acellular corridor', predetermining:the location of the nucleus in the central position in interphase; the axis of nuclear movement to the b ud neck before mitosis; the direction of the elongating nucleus during mitosis; and the motion of each nucleus from an excentric to a centra l position during cytokinesis, in cooperation with the above-mentioned and other gene products.