Morphologically distinct microtubule ends in the mitotic centrosome of Caenorhabditis elegans

Abstract

During mitosis, the connections of microtubules (MTs) to centrosomes and kinetochores are dynamic. From in vitro studies, it is known that the dynamic behavior of MTs is related to the structure of their ends, but we know little about the structure of MT ends in spindles. Here, we use high-voltage electron tomography to study the centrosome- and kinetochore-associated ends of spindle MTs in embryonic cells of the nematode, Caenorhabditis elegans. Centrosome-associated MT ends are either closed or open. Closed MT ends are more numerous and are uniformly distributed around the centrosome, but open ends are found preferentially on kinetochore-attached MTs. These results have structural implications for models of MT interactions with centrosomes.

Quantitative analysis of changes in spatial distribution and plus-end geometry of microtubules involved in plant-cell cytokinesis

“…This experimental approach led to the identification of three different types of microtubule plus-end morphologies: (1) extended ends, which are typical of growing microtubules; (2) ram's-horn-like ends, which are characteristic of disassembling microtubules; and (3) blunt ends, which reflect a metastable state. More recently, a fourth microtubule plus-end morphology, flared, has been identified in electrontomographic studies of the mitotic kinetochores of Caenorhabditis elegans (O'Toole et al, 2003). To date, no systematic studies of microtubule plus-end geometries have been reported in plants.…”

Electron tomography reveals a flared morphology on growing microtubule ends

“…We next determined MT polarity by identifying capped MT ends, which represent minus ends that are protected by the -TURC (O'Toole et al, 1999;O'Toole et al, 2003). The other end of these MTs are almost certainly growing plus ends, given the MBC wash-out.…”

“…However, sheet-like MT plus ends are rare in yeasts and human cells. Here, a majority of MT ends are flared, meaning that most or all of their tubulin protofilaments splay out from the MT axis (O'Toole et al, 1999;O'Toole et al, 2003;McIntosh et al, 2008). In an effort to understand the diversity seen in these reports, we have studied growing MT ends in Schizosaccharomyces pombe after release from a MT-depolymerising drug, using electron tomography to answer the question how MTs grow inside a cell.…”

“…They undergo slow growth or rapid shortening, a trait termed dynamic instability (Mitchison and Kirschner, 1984). This behaviour is mostly confined to the MT plus end in vivo, because minus ends are commonly stabilised by the -tubulin ring complex (-TURC) (Moritz et al, 1995;O'Toole et al, 1999;O'Toole et al, 2003). The -TURC nucleates MTs and forms a cap on their minus end (Keating and Borisy, 2000;Moritz et al, 2000;Wiese and Zheng, 2000).…”

The perpetual movements of anaphase

“…Moreover, it has been demonstrated that, like depolymerizing microtubules in vitro, kinetochore microtubule plus-ends in yeast, worms, Drosophila and vertebrate somatic cells have bent protofilaments [142][143][144][145][146]. Definitive work by Grishchuk and McIntosh showed that deletion of all the three minus-end directed motors found in the fission yeast genome did not abolish the poleward movement of chromosomes during anaphase [94].…”