PgmNr D1009: A splice variant of Centrosomin converts mitochondria to MTOCs to facilitate sperm tail elongation in Drosophila.

Authors:
T. L. Megraw; J. V. Chen; L. R. Kao


Institutes
Florida State University, Tallahassee, FL.


Keyword: cytoskeleton

Abstract:

In Drosophila spermatids, the giant mitochondrion (nebenkern) provides a structural platform for microtubule (MT) organization to support sperm tail elongation. centrosomin (cnn) expresses several variants that fall into two major forms: a ubiquitously expressed centrosomal form (CnnC) and a non-centrosomal form in testes (CnnT).  CnnC is essential for functional centrosomes, the major microtubule-organizing centers (MTOCs) in animal cells. We found that CnnT is expressed exclusively in testes and localizes to nebenkerns in spermatids. When expressed ectopically in flies or in mammalian cell culture, CnnT localizes to mitochondria in those cells too. We show that CnnT targets to the mitochondrial surface and recruits the γ-TuRC MT-nucleating complex to assemble MTs on mitochondria, converting mitochondria to MTOCs. We have mapped two separate domains on CnnT that are necessary and sufficient to 1) target it to mitochondria, and 2) to recruit the γ-TuRC and nucleate MTs, respectively. Disrupting the conserved CM1 domain in CnnT (which is shared with CnnC) abolishes the MT-nucleating function but does not block γ-TuRC recruitment, indicating that CM1 is essential for the activation of bound γ-TuRCs independent from their recruitment to MTOCs. In vivo, CnnT forms speckles on the surface of the nebenkern in spermatids, where it is required to recruit γ-tubulin. cnnT mutant males have significantly smaller seminal vesicles, shorter mature sperm tails, and reduced fertility. CnnT therefore assembles unique non-centrosomal MTOCs on nebenkerns to facilitate the morphogenesis of the extremely long sperm that are found in Drosophila.



Flybase Genetic Index:
1. FlyBase gene symbol: cnn