A Splice Variant of Centrosomin Converts Mitochondria to Microtubule-Organizing Centers

Abstract

Summary Non-centrosomal Microtubule Organizing Centers (MTOCs) direct microtubule (MT) organization to exert diverse cell-type specific functions. In Drosophila spermatids, the giant mitochondria provide structural platforms for MT reorganization to support elongation of the extremely long sperm. However, the molecular basis for this mitochondrial MTOC and other non-centrosomal MTOCs have not been discerned. Here we report that Drosophila centrosomin (cnn) expresses two major protein variants: the centrosomal form (CnnC) and a non-centrosomal form in testes (CnnT). CnnC is established as essential for functional centrosomes, the major MTOCs in animal cells. We show that CnnT is expressed exclusively in testes by alternative splicing, and localizes to giant mitochondria in spermatids. In cell culture, CnnT targets to the mitochondrial surface, recruits the MT nucleator γ-TuRC, and is sufficient to convert mitochondria to MTOCs independent of core pericentriolar proteins that regulate MT assembly at centrosomes. We mapped two separate domains in CnnT. One that is necessary and sufficient to target it to mitochondria, and another that is necessary and sufficient to recruit γ-TuRCs and nucleate MTs. In elongating spermatids, CnnT forms speckles on the giant mitochondria that are required to recruit γ-TuRCs to organize MTs and support spermiogenesis. This molecular characterization of the mitochondrial MTOC defines a minimal molecular requirement for MTOC generation, and implicates the potent role of Cnn (or its related) proteins in the direct regulation of MT assembly and organization of non-centrosomal MTOCs.

Endosomal Wnt signaling proteins control microtubule nucleation in dendrites

“…Because cnn can activate microtubule nucleation [68,69], we hypothesized that ectopic γTub and cnn on mitochondria might convert mitochondria into nucleation sites. We A blue arrow is used to show the endosome from which an EB1 comet will initiate in subsequent frames, indicated with white arrows to track movement.…”

Microtubule minus-end regulation at a glance

“…Furthermore, in Drosophila oocytes γ-tubulin, Mud (NuMA homolog) and Asp participate in organizing the perinuclear MTOC (Januschke et al, 2006;Tissot et al, 2017). Other membrane organelles can also contribute to MTOC formation; for example, mitochondrial derivatives in Drosophila spermatids perform an MTOC function that is dependent on γ-TuRC and on a testis-specific non-centrosomal isoform of Cnn (CnnT; Chen et al, 2017). The presence of multicomponent complexes that form through multivalent interactions and exhibit properties of non-membrane-bound protein condensates or phase-separated, liquid droplets might be a general physicochemical feature of MTOCs, including spindle poles in animals (Borgal and Wakefield, 2018) or mitotic MTOCs, such as the polar caps and polar organizers in plants (Yi and Goshima, 2018).…”

Microtubules originate asymmetrically at the somatic Golgi and are guided via Kinesin2 to maintain polarity within neurons

“…S3A). Promoter one drives the best-studied isoform (that we term Cnn-P1) that localises to centrosomes during mitosis [63][64][65][66][67] ; promoter two drives isoforms (Cnn-P2) that are yet to be characterised; and promoter three drives isoforms that are expressed specifically within testes 29 and so have not been considered in this study. Immunostaining with previously-characterised antibodies against Cnn 68 revealed only very weak, if any, signal within the soma of the sensory neurons and few, if any, obvious puncta within their dendrites, although the presumptive basal bodies of the external sensory neurons displayed a strong Cnn signal (data not shown).…”