Naegleria’s mitotic spindles are built from unique tubulins and highlight core spindle features

• Published in: Current biology 2022-03, Vol.32 (6), p.1247-1261.e6
• Main Authors: Velle, Katrina B., Kennard, Andrew S., Trupinić, Monika, Ivec, Arian, Swafford, Andrew J.M., Nolton, Emily, Rice, Luke M., Tolić, Iva M., Fritz-Laylin, Lillian K., Wadsworth, Patricia
• Format: Article
• Language: English
• Subjects: cytoskeleton; Eukaryota; evolutionary cell biology; microtubules; Microtubules - chemistry; Microtubules - genetics; Microtubules - physiology; Mitosis; Naegleria; Naegleria - cytology; Naegleria - genetics; protist; spindle; Spindle Apparatus - chemistry; Spindle Apparatus - genetics; tubulin; Tubulin - genetics
• ISSN: 0960-9822; 1879-0445
• DOI: 10.1016/j.cub.2022.01.034

Naegleria gruberi is a unicellular eukaryote whose evolutionary distance from animals and fungi has made it useful for developing hypotheses about the last common eukaryotic ancestor. Naegleria amoebae lack a cytoplasmic microtubule cytoskeleton and assemble microtubules only during mitosis and thus represent a unique system for studying the evolution and functional specificity of mitotic tubulins and the spindles they assemble. Previous studies show that Naegleria amoebae express a divergent α-tubulin during mitosis, and we now show that Naegleria amoebae express a second mitotic α- and two mitotic β-tubulins. The mitotic tubulins are evolutionarily divergent relative to typical α- and β-tubulins and contain residues that suggest distinct microtubule properties. These distinct residues are conserved in mitotic tubulin homologs of the “brain-eating amoeba” Naegleria fowleri, making them potential drug targets. Using quantitative light microscopy, we find that Naegleria’s mitotic spindle is a distinctive barrel-like structure built from a ring of microtubule bundles. Similar to those of other species, Naegleria’s spindle is twisted, and its length increases during mitosis, suggesting that these aspects of mitosis are ancestral features. Because bundle numbers change during metaphase, we hypothesize that the initial bundles represent kinetochore fibers and secondary bundles function as bridging fibers. [Display omitted] •Naegleria expresses evolutionarily divergent α- and β-tubulins only during mitosis•Mitotic tubulins differ at key lateral and longitudinal microtubule interfaces•The mitotic spindle is a ring of regularly spaced microtubule bundles that twists•The length and number of microtubule bundles increase as mitosis proceeds Naegleria amoebae are profoundly different from other eukaryotes as they lack interphase microtubules. During cell division, Velle et al. show that Naegleria express α- and β-tubulins that are highly divergent at key structural positions. These tubulins form spindles with an unusual architecture: a ring of twisted microtubule bundles.