A Mechanism for Nuclear Positioning in Fission Yeast Based on Microtubule Pushing

TLDR

In fission yeast, the nucleus is actively positioned at the cell center during interphase through a microtubule‑dependent process that defines the division plane. Using GFP markers and computer modeling, the authors show that growing microtubules exert transient pushing forces at cell tips, and the balance of these forces positions the nucleus centrally. They observed that interphase microtubules form 3–4 antiparallel bundles with plus ends at the tips and minus ends near the middle, originating from medial organizing centers, and that plus‑end catastrophes after ~1.5 min of tip growth cause the bundles to retract toward the nucleus before regrowing.

Abstract

The correct positioning of the nucleus is often important in defining the spatial organization of the cell, for example, in determining the cell division plane. In interphase Schizosaccharomyces pombe cells, the nucleus is positioned in the middle of the cylindrical cell in an active microtubule (MT)-dependent process. Here, we used green fluorescent protein markers to examine the dynamics of MTs, spindle pole body, and the nuclear envelope in living cells. We find that interphase MTs are organized in three to four antiparallel 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 function as nuclear attachment sites. When MTs grow to the cell tips, they exert transient forces produced by plus end MT polymerization that push the nucleus. 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 back to the cell tip. Computer modeling suggests that a balance of these pushing MT forces can provide a mechanism to position the nucleus at the middle of the cell.