A mechanism for nuclear positioning in fission yeast based on microtubule pushing

The correct positioning of the nucleus is often important in defining the s patial organization of the cell, for example, in determining the cell divis ion plane. In interphase Schizosaccharomyces pombe cells, the nucleus is po sitioned in the middle of the cylindrical cell in an active microtubule (MT )-dependent process. Here, we used green fluorescent protein markers to exa mine the dynamics of MTs, spindle pole body, and the nuclear envelope in li ving cells. We find that interphase MTs are organized in three to four anti parallel MT bundles arranged along the long axis of the cell, with MT plus ends facing both the cell tips and minus ends near the middle of the cell. The MT bundles are organized from medial MT-organizing centers that may fun ction as nuclear attachment sites. When MTs grow to the cell tips, they exe rt transient forces produced by plus end MT polymerization that push the nu cleus. After an average of 1.5 min of growth at the cell tip, MT plus ends exhibit catastrophe and shrink back to the nuclear region before growing ba ck to the cell tip. Computer modeling suggests that a balance of these push ing MT forces can provide a mechanism to position the nucleus at the middle of the cell.