Cytoplasmic microtubule organization in fission yeast

Abstract

During the cell cycle of the fission yeast Schizosaccharomyces pombe, striking changes in the organization of the cytoplasmic microtubule cytoskeleton take place. These may serve as a model for understanding the different modes of microtubule organization that are often characteristic of differentiated higher eukaryotic cells. In the last few years, considerable progress has been made in our understanding of the organization and behaviour of fission yeast cytoplasmic microtubules, not only in the identification of the genes and proteins involved but also in the physiological analysis of function using fluorescently-tagged proteins in vivo. In this review we discuss the state of our knowledge in three areas: microtubule nucleation, regulation of microtubule dynamics and the organization and polarity of microtubule bundles. Advances in these areas provide a solid framework for a more detailed understanding of cytoplasmic microtubule organization.

Figure 1. Microtubule organization in the fission yeast cell cycle.

A highly schematic illustration of microtubule (MT) distribution (green) in relation to microtubule organizing centers (MTOCs; red) and the nuclear envelope (blue). During interphase (A), MTOCs may be associated with the nuclear envelope or with existing MTs and may occasionally also be found free in the cytoplasm. MT minus ends (“-“) are generally found towards the cell center and MT plus ends (“+”) towards cell tips. During mitosis (B), intranuclear MTs form the mitotic spindle and astral MTs are nucleated from the SPBs. At the close of mitosis, during cell division (C), the equatoral MTOCs forms at the division site, to nucleate post-anaphase array MTs.

Figure 2. Model of interphase microtubule bundling in fission yeast.

A representation of how microtubules (MTs), motors, and microtubule-associated proteins (MAPs) may be necessary and sufficient to bundle and slide two MTs together into an antiparallel MT array. The model includes the following steps: 1) A cytoplasmic γ-tubulin complex (γ-TuC) satellite is recruited to the lattice of a preexisting mother MT. The mechanism of recruitment is not known. 2) The γ-TuC satellite nucleates a new daughter MT. The mechanism for recruitment-dependent nucleation is not known. 3) Ase1p bundles and stabilizes the antiparallel arrangement of daughter-mother MTs. Ase1p binding would be dynamic, so the bundling and stabilizing activities would still allow for MT sliding. 4) Klp2p is recruited to the growing plus end tip of the daughter MT, where it “pulls” the daughter MT toward the minus end of the mother MT. Pulling effectively slides the daughter and mother MT relative to each other. Sliding is attenuated as daughter MT continues to grow and new ase1p is recruited to the growing overlap region between daughter-mother MTs. The mechanism of klp2p attachment to the MT plus end tip is not known. 5) When klp2p reaches the end of the mother MT, no further sliding occurs, and the length of the daughter-mother overlap region is defined. 6) The daughter MT may then continue to grow beyond the mother’s minus-end, establishing an antiparallel and symmetric MT bundle with minus-ends bundled together at the middle, and plus-ends extending toward the cell tips (see Fig. 1).