A journey into space

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Abstract
Microtubules are important components for establishing positional information, and are required to establish a cellular axis and to position the nucleus. Several proteins have been identified that are required to establish an axis. In the absence of these proteins, fission yeast cells no longer grow in a straight line. These proteins include Tea1, Tea2, Tip1, Mal3, γ-tubulin, Alp4 and Alp6, all of which affect microtubule dynamics, and several proteins required for microtubule biogenesis. Many of the proteins identified in fission yeast are also conserved in other organisms. The CLIP170 protein family regulates microtubule dynamics and might be involved in a microtubule guidance mechanism, both in fission yeast and other eukaryotes. Three models have been proposed that might guide microtubules along an axis and identify the ends of the cell. One of these models depends on the presence of historical markers; one depends more on the self organization of microtubules, together with marker proteins that can act as catastrophe factors; and the third depends on the ability of microtubules to withstand pressure. These models are not mutually exclusive. The nucleus might be positioned at the cell centre by a balance of pushing forces generated by interphase microtubules. A second mechanism that positions the daughter nuclei at the cell centre after mitosis might depend on a pulling mechanism so that the nucleus is pulled to the cell centre by the post-anaphase array of microtubules. Microtubules and motor proteins are also required to position the nucleus in other organisms. Similar microtubular behaviour might be the basis of mechanisms to define the long axis of the cell and to position the nucleus at the cell centre.