Non-centrosomal microtubule-organizing centres (ncMTOCs) have a variety of roles presumed to serve the diverse functions of the range of cell types in which they are found. ncMTOCs are diverse in their composition, subcellular localization, and function. Here we report a perinuclear MTOC in Drosophila fat body cells that is anchored by Msp300/Nesprin at the cytoplasmic surface of the nucleus. Msp300 recruits the MT minus-end protein Patronin/CAMSAP, which functions redundantly with Ninein to further recruit the MT polymerase Msps/XMAP215 to assemble non-centrosomal MTs and does so independently of the widespread MT nucleation factor γ-tubulin. Functionally, the fat body ncMTOC and the radial MT arrays it organizes is essential for nuclear positioning and for secretion of basement membrane components via retrograde dynein-dependent endosomal trafficking that restricts plasma membrane growth. Together, this study identifies a perinuclear ncMTOC with unique architecture and MT regulation properties that serves vital functions.

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.

The centrosome is the best-understood microtubule-organizing center (MTOC) and is essential in particular cell types and at specific stages during Drosophila development. The centrosome is not required zygotically for mitosis or to achieve full animal development. Nevertheless, centrosomes are essential maternally during cleavage cycles in the early embryo, for male meiotic divisions, for efficient division of epithelial cells in the imaginal wing disc, and for cilium/flagellum assembly in sensory neurons and spermatozoa. Importantly, asymmetric and polarized division of stem cells is regulated by centrosomes and by the asymmetric regulation of their microtubule (MT) assembly activity. More recently, the components and functions of a variety of non-centrosomal microtubule-organizing centers (ncMTOCs) have begun to be elucidated. Throughout Drosophila development, a wide variety of unique ncMTOCs form in epithelial and non-epithelial cell types at an assortment of subcellular locations. Some of these cell types also utilize the centrosomal MTOC, while others rely exclusively on ncMTOCs. The impressive variety of ncMTOCs being discovered provides novel insight into the diverse functions of MTOCs in cells and tissues. This review highlights our current knowledge of the composition, assembly, and functional roles of centrosomal and non-centrosomal MTOCs in Drosophila.

The centrosome is a non‐membrane‐bound organelle present in most animal cells and it functions as the major microtubule‐organising centre (MTOC). Recent findings have revealed the detailed molecular and structural features of the centrosome, and architectural and functional changes at the centrosome during the cell cycle. The centriole, the organisational heart of the centrosome, duplicates once each cell cycle and depends on a hierarchy of regulatory and assembly factors for its biogenesis. The centrosome plays important roles in dividing and nondividing cells. This importance is reflected in the appearance of several human developmental disorders when genes encoding centrosomal proteins are mutated. The centriole is essential for the formation of cilia, the cell's ‘antennae’ that receive and transmit signals and sensory inputs critical for animal development and physiology. Impairment of cilium structure or function leads to a spectrum of diseases called ciliopathies. Key Concepts The centrosome typically contains a centriole pair, the mother and its daughter, and the pericentriolar material (PCM). Centrioles have a ninefold radial symmetry and are required for organising a functional centrosome. The PCM is a large multi‐protein complex that regulates microtubule (MT) assembly at centrosomes in dividing and nondividing cells. Centrioles duplicate only once each cell cycle, and the protein kinase PLK4 is a key early regulator of centriole duplication and biogenesis. The cartwheel at the proximal end of the centriole is assembled early in centriole biogenesis, and its ninefold symmetry is determined by the intrinsic properties of the key structural protein Sas‐6. The centrosome is involved in the asymmetric division of stem cells. In nondividing cells, the basal body, a modified mother centriole, serves as a platform for organising the MT‐based axoneme, forming a primary or motile cilium. Most vertebrate cells contain a primary cilium that is critical for signalling pathways including Hedgehog (Hh) signalling and left–right asymmetry designation during development. Centrosomal and ciliary dysfunctions have been linked to two types of diseases: microcephaly/primordial dwarfisms, and ciliopathies, respectively.

Zika virus (ZIKV) became a global health concern in 2016 due to its links to congenital microcephaly and other birth defects. Flaviviruses, including ZIKV, reorganize the endoplasmic reticulum (ER) to form a viroplasm, a compartment where virus particles are assembled. Microtubules (MTs) and microtubule-organizing centers (MTOCs) coordinate structural and trafficking functions in the cell, and MTs also support replication of flaviviruses. Here we investigated the roles of MTs and the cell’s MTOCs on ZIKV viroplasm organization and virus production. We show that a toroidal-shaped viroplasm forms upon ZIKV infection, and MTs are organized at the viroplasm core and surrounding the viroplasm. We show that MTs are necessary for viroplasm organization and impact infectious virus production. In addition, the centrosome and the Golgi MTOC are closely associated with the viroplasm, and the centrosome coordinates the organization of the ZIKV viroplasm toroidal structure. Surprisingly, viroplasm formation and virus production are not significantly impaired when infected cells have no centrosomes and impaired Golgi MTOC, and we show that MTs are anchored to the viroplasm surface in these cells. We propose that the viroplasm is a site of MT organization, and the MTs organized at the viroplasm are sufficient for efficient virus production.