BackgroundAstral microtubules emanating from the mitotic centrosomes play pivotal roles in defining cell division axis and tissue morphogenesis. Previous studies have demonstrated that human transforming acidic coiled-coil 3 (TACC3), the most conserved TACC family protein, regulates formation of astral microtubules at centrosomes in vertebrate cells by affecting γ-tubulin ring complex (γ-TuRC) assembly. However, the molecular mechanisms underlying such function were not completely understood.ResultsHere, we show that Aurora A site-specific phosphorylation in TACC3 regulates formation of astral microtubules by stabilizing γ-TuRC assembly in human cells. Mutation of the most conserved Aurora A targeting site, Ser 558 to alanine (S558A) in TACC3 results in robust loss of astral microtubules and disrupts localization of the γ-tubulin ring complex (γ-TuRC) proteins at the spindle poles. Under similar condition, phospho-mimicking S558D mutation retains astral microtubules and the γ-TuRC proteins in a manner similar to control cells expressed with wild type TACC3. Time-lapse imaging reveals that S558A mutation leads to defects in positioning of the spindle-poles and thereby causes delay in metaphase to anaphase transition. Biochemical results determine that the Ser 558- phosphorylated TACC3 interacts with the γ-TuRC proteins and further, S558A mutation impairs the interaction. We further reveal that the mutation affects the assembly of γ-TuRC from the small complex components.ConclusionsThe results demonstrate that TACC3 phosphorylation stabilizes γ- tubulin ring complex assembly and thereby regulates formation of centrosomal asters. They also implicate a potential role of TACC3 phosphorylation in the functional integrity of centrosomes/spindle poles.

Centrioles are essential components of centrosome, the main microtubule-organizing center of animal cells required for robust spindle bipolarity [1,2]. They are duplicated once during the cell cycle [3], and the duplication involves assembly of a cartwheel on the pre-existing centriole followed by assembly of triplet microtubules around the cartwheel [4,5]. Although the molecular details of cartwheel formation are understood [6][7][8][9][10][11][12][13], the mechanisms initiating the formation of centriolar microtubules are not known. Here, we show that the central component of cartwheel, HsSAS-6 plays a crucial role in the formation of centriolar microtubules by interacting with the microtubule nucleation machinery, g-tubulin ring complex (g-TuRC) in human cells. The globular N terminus and the central coiled-coil domain of SAS-6 are required for formation of the cartwheel [7,14], whereas the function of its C-terminal outer cartwheel region in centriole duplication remains unclear. We find that deletion of HsSAS-6 C terminus disrupts microtubule formation in daughter centriole, and as a result, cells fail to form the new centriole. Consequently, this results in mitotic cells having only two centrioles localized at a single site. Detailed molecular analyses showed that HsSAS-6 interacts with the g-TuRC proteins and associates with the g-TuRC at the centrosome, and furthermore, the C terminus is essential for this association. High-resolution microscopy revealed localization of the g-TuRC protein, g-tubulin as multiple lobes surrounding the HsSAS-6-containing central hub in the centriole. Together, the results indicate that HsSAS-6 regulates centriolar microtubule assembly by anchoring g-TuRCs to the pro-centriole at the onset of daughter centriole formation.

Centrosome plays essential roles in maintaining genetic stability, ciliogenesis and cell polarization. The core of centrosome is made of two centrioles that duplicate precisely once during every cell cycle to generate two centrosomes that are required for bipolar spindle assembly and chromosome segregation. Abundance of centriole proteins at optimal levels and their recruitment to the centrosome are tightly regulated in time and space in order to restrict aberrant duplication of centrioles, a phenomenon that is observed in many cancers.Recent advances have conclusively evidenced that dedicated ubiquitin ligase-dependent protein degradation machineries are involved in governing centriole duplication. These studies revealed intricate mechanistic insights into how the ubiquitin ligases target different centriole proteins. In certain cases, a specific ubiquitin ligase targets a number of substrate proteins that co-regulate centriole assembly, prompting the possibility that substrate targeting occurs during formation of the sub-centriolar structures. There are also instances where a specific centriole duplication protein is targeted by several ubiquitin ligases at different stages of the cell cycle, suggesting synchronized actions. Recent evidence also indicated a direct association of E3 ubiquitin ligase with the centrioles, supporting the notion that substrate-targeting occurs in the organelle itself. In this review, we highlight these advances by underlining the mechanisms how different ubiquitin ligase machineries control centriole duplication and discuss our views on their coordination. This article is protected by copyright. All rights reserved tumorigenesis and also certain developmental diseases such as, microcephaly [1][2][3][4][5]. Supernumerary centrioles/centrosomes are hallmarks of many cancer cells and promote cancer transformation [6]. Structurefunction aberration of centrosomes/centrioles is also known as the causal factor of various ciliary dysfunctions (ciliopathies), and male infertility. Centriole, in a specially matured form, the basal body, serves as the platform for cilia and flagella assembly [7]. During the G1 to S phase entry of the cell cycle, each mother centriole of centrosome initiates assembly of a daughter centriole at its proximal end, which further elongates to a size precisely up to that of the mother centriole during the S till G2 phase. After the new centrioles attain the optimal size and maturation, the older (mother) centrioles are disengaged from each other and thereby, the duplicated centrosomes each consisting the pair of the old and the new centriole are produced [8]. Centriole duplication in somatic cells is extremely accurate such that only one daughter centriole is formed from the mother and the mother duplicates strictly once during the cell cycle. The mechanisms how the daughter centriole is generated from the pre-existing mother centriole and how cells restrict centriole reduplication or over-duplication by limiting formation of a single daughter centriole per mother ce...