Localized and Ran-dependent accumulation of tubulin during semi-open mitosis in the Caenorhabditis elegans embryo 線虫胚における有糸分裂期のRan依存的なチューブリンの局所的集積
Localized and Ran-dependent accumulation of tubulin during semi-open mitosis in the Caenorhabditis elegans embryo
Mitosis is categorized as open or closed depending on whether loss of nuclear compartmentalization occurs or not. Some organisms adopt intermediate forms of mitosis, which are important to understand the commonality and diversity of the process. Caenorhabditis elegans undergoes semi-open mitosis. In this process, the permeability of the nuclear envelope increases during the prometaphase, but the membrane structure remains and encloses the mitotic spindle. The role of the remaining permeabilized nuclear envelope (in this thesis, I refer to the envelope as “spindle envelope”) is still unclear. Here, I report that unpolymerized tubulin subunits accumulate inside the spindle envelope in the C. elegans embryo. The accumulation coincided with increased nuclear envelope permeability, suggesting that the rise in permeability triggers the accumulation. A small GTPase protein, Ran, which plays a key role in providing spatial cues during open and closed mitosis, is required for tubulin accumulation. The boundaries of the region of accumulation coincided with the spindle envelope even when the shape of the envelope was deformed. Fluorescence recovery after photobleaching (FRAP) analysis revealed the enrichment of the immobile fraction in the nascent spindle region in a Ran-dependent manner. I propose stable binding of tubulin to an immobile structure as a mechanism of tubulin accumulation inside the spindle envelope. The accumulation was also observed for molecules other than tubulin, suggesting that this novel mechanism contributes to alter the molecular composition around the chromosome during semi-open mitosis in the C. elegans embryo.