In most animals, female meiotic spindles are assembled in the absence of centrosomes. How microtubules (MTs) are organized into acentrosomal meiotic spindles is poorly understood. In Caenorhabditis elegans, assembly of female meiotic spindles requires MEI-1 and MEI-2, constituting the microtubule-severing AAA-ATPase Katanin. However, the role of MEI-2 is not known and whether MT-severing is required for meiotic spindle assembly is unclear. Here we show that the essential role of MEI-2 is to confer MT-binding to the Katanin, which in turn stimulates the ATPase activity of MEI-1 leading to MT-severing. To test directly the contribution of MT-severing to meiotic spindle assembly, we engineered Katanin variants that retained MT-binding and MT-bundling activities but that were inactive for MT-severing. In vivo analysis of these variants showed disorganized microtubules that lacked focused spindle poles reminiscent of the Katanin loss of function phenotype, demonstrating that the MT-severing activity is essential for meiotic spindle assembly in C. elegans. Overall our results reveal the essential role of MEI-2 and provide the first direct evidence supporting an essential role of MT-severing in meiotic spindle assembly in C. elegans.
- Received June 8, 2016.
- Accepted August 12, 2016.