Alp7-Mtol and Alpl4 synergize to promote interphase microtubule regrowth from the nuclear envelope

Microtubules grow not only from the centrosome but also from various noncentrosomal microtubule-organizing centers(MTOCs),including the nuclear envelope(NE)and pre-existing microtubules.The evolutionarily conserved proteins Mtol/CDK5RAP2 and Alpl4/TOG/XMAP215 have been shown to be involved in promoting microtubule nucleation.However,it has remained elusive as to how the microtubule nucleation promoting factors are specified to various noncentrosomal MTOCs,particularly the NE,and how these proteins coordinate to organize microtubule assembly.Here,we demonstrate that in the fission yeast Schizosaccharomyces pombe,efficient interphase microtubule growth from the NE requires Alp7/TACC,Alpl4/TOG/XMAP215,and Mtol/CDK5RAP2.The absence of Alp7,A lp l4 t or Mtol compromises microtubule regrowth on the NE in cells undergoing microtubule repolymerization.We further demonstrate that Alp7 and Mtol interdependently localize to the NE in cells without microtubules and that A lp l4 localizes to the NE in an Alp7 and Mtol-dependent manner.Tethering Mtol to the NE in cells lacking Alp7 partially restores microtubule number and the efficiency of microtubule generation from the NE.Hence,our study delineates that Alp7,A lpl4,and Mtol work in concert to regulate interphase microtubule regrowth on the NE.

How do Plants Organize Microtubules Without a Centrosome?

A microtubule nucleates from a γ-tubuUn complex, which consists of γ-tubulin, proteins from the SPC971SPC98 family, and the WD40 motif protein GCP-WD. We analyzed the phylogenetic relationships of the genes encoding these proteins and found that the components of this complex are widely conserved among land plants and other eukaryotes. By contrast, the interphase and mitotic arrays of microtubules in land plants differ from those in other eukaryotes. In the interphase cortical array, the majority of microtubules nucleate on existing microtubules in the absence of conspicuous microtubule organizing centers （MTOCs）, such as a centrosome. During mitosis, the spindle also forms in the absence of conspicuous MTOCs. Both poles of the spindle are broad, and branched structures of microtubules called microtubule converging centers form at the poles. In this review, we hypothesize that the microtubule converging centers form via microtubule-dependent microtubule nucleation, as in the case of the interphase arrays. The evolutionary insights arising from the molecular basis of the diversity in microtubule organization are discussed.