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

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.

Mid1/anillin and the spatial regulation of cytokinesis in fission yeast

“…Extension of the microtubule plus ends contributes to the centering of the nucleus by repulsive pushing force exerted on the cortex at the cell tips [Hagan and Hyams, 1988;Drummond and Cross, 2000;Tran et al, 2001]. Moreover, delivery to the cell tips of polarity factors, the Tea proteins, by microtubule plus ends contributes to polarity establishment and growth zone determination [see Martin, 2009; Huisman and Brunner, 2011 for reviews].…”

“…Since the nucleus is kept in the cell middle by the opposing pushing forces exerted by microtubule plus ends on the cell tips [Tran et al, 2001], the division site is therefore placed in the cell middle. In fact, this hypothesis was first formulated from the analysis of microtubule defective mutants displaying delocalized nuclei and septa [Toda et al, 1983;Radcliffe et al, 1998].…”

“…In particular, the actin cytoskeleton, assembled in cables and patches in the growing cell tips during interphase, reorganizes during metaphase to promote the assembly of the contractile ring in the cell middle [Marks et al, 1986]. On the other hand, during interphase, nuclear envelope-associated microtubule organizing centers assemble four or five dynamic antiparallel microtubule bundles with plus ends facing the cell tips.Extension of the microtubule plus ends contributes to the centering of the nucleus by repulsive pushing force exerted on the cortex at the cell tips [Hagan and Hyams, 1988;Drummond and Cross, 2000;Tran et al, 2001]. Moreover, delivery to the cell tips of polarity factors, the Tea proteins, by microtubule plus ends contributes to polarity establishment and growth zone determination [see Martin, 2009; Huisman and Brunner, 2011 for reviews].…”

Collision induced spatial organization of microtubules

“…During the growing state, microtubules interact with each other as well as with different cellular structures. The mechanical force opposing the growth of the plus-end may induce catastrophes [12,13]. Experiments involving growing microtubules and different immobile obstacles and barriers have shown that the opposing force reduces the growth velocity [14,15].…”

Coordinating cell polarity with cell division in space and time

“…In G2/M, phosphorylation of nuclear Mid1 by the mitotic kinase Pololike kinase (Plk1) triggers Mid1 nuclear export and its ability to recruit contractile ring components to the middle of the cell, temporally coordinating mitosis with cytokinesis [19]. Therefore, microtubule-dependent positioning of the nucleus [20] and inhibitory signals at the cell tips dictate the position of the division plane (reviewed in [21]). …”