Proper organization of microtubule arrays is essential for intracellular trafficking and cell motility. It is generally assumed that most if not all microtubules in vertebrate somatic cells are formed by the centrosome. Here we demonstrate that a large number of microtubules in untreated human cells originate from the Golgi apparatus in a centrosome-independent manner. Both centrosomal and Golgi-emanating microtubules need gamma-tubulin for nucleation. Additionally, formation of microtubules at the Golgi requires CLASPs, microtubule-binding proteins that selectively coat noncentrosomal microtubule seeds. We show that CLASPs are recruited to the trans-Golgi network (TGN) at the Golgi periphery by the TGN protein GCC185. In sharp contrast to radial centrosomal arrays, microtubules nucleated at the peripheral Golgi compartment are preferentially oriented toward the leading edge in motile cells. We propose that Golgi-emanating microtubules contribute to the asymmetric microtubule networks in polarized cells and support diverse processes including post-Golgi transport to the cell front.

Error-free chromosome segregation requires stable attachment of sister kinetochores to the opposite spindle poles (amphitelic attachment). Exactly how amphitelic attachments are achieved during spindle assembly remains elusive. We employed photoactivatable GFP and high-resolution live-cell confocal microscopy to visualize for the first time complete 3-D movements of individual kinetochores throughout mitosis in non-transformed human cells. Combined with electron microscopy, molecular perturbations, and immunofluorescence analyses, this approach reveals unexpected new details of chromosome behavior. Our data demonstrate that unstable lateral interactions between kinetochores and microtubules dominate during early prometaphase. These transient interactions lead to the reproducible arrangement of chromosomes in an equatorial ring on the surface of the nascent spindle. A computational model predicts that this toroidal distribution of chromosomes exposes kinetochores to a high-density of microtubules which facilitates subsequent formation of amphitelic attachments. Thus, spindle formation involves a previously overlooked stage of chromosome prepositioning which promotes formation of amphitelic attachments.

Summary The centrosome is the principal microtubule organizing center (MTOC) of animal cells [1]. Accurate centrosome duplication is fundamental for genome integrity and entails the formation of one procentriole next to each existing centriole, once per cell cycle. The procentriole then elongates to eventually reach the same size as that of the centriole. The mechanisms that govern elongation of the centriolar cylinder and their potential relevance for cell division are not known. Here, we show that the SAS-4-related protein CPAP [2] is required for centrosome duplication in cycling human cells. Furthermore, we demonstrate that CPAP overexpression results in the formation of abnormal long centrioles. This also promotes formation of more than one procentriole in the vicinity of such overly long centrioles, eventually resulting in the presence of supernumerary MTOCs. This in turn leads to multipolar spindle assembly and cytokinesis defects. Overall, our findings suggest that centriole length must be carefully regulated to restrict procentriole number and thus ensure accurate cell division.

Significance Found in most eukaryotic cells, a centriole is a cylindrically shaped subcellular structure that plays an important role in various cellular processes, including mitotic spindle formation and chromosome segregation. Centriole duplication occurs only once per cell cycle, thus ensuring accurate control of centriole numbers to maintain genomic integrity. Although a growing body of evidence suggests that a Ser/Thr protein kinase, polo-like kinase 4 (Plk4), is a key regulator of centriole duplication, how Plk4 is recruited to centrosomes remains largely unknown. Here we showed that Plk4 dynamically localizes to distinct subcentrosomal regions by interacting with two hierarchically regulated scaffolds, Cep192 and Cep152. Highlighting the importance of these interactions, mutational disruption of either one of these interactions was sufficient to cripple Plk4-dependent centriole biogenesis.

Controlling the number of its centrioles is vital for the cell as supernumerary centrioles result in multipolar mitosis and genomic instability 1,2 . Normally, just one daughter centriole forms on each mature (mother) centriole 3,4 ; however, a mother centriole can produce multiple daughters within a single cell cycle 5,6 . The mechanisms that prevent centriole 'overduplication' are poorly understood. Here we use laser microsurgery to test the hypothesis that attachment of the daughter centriole to the wall of the mother inhibits formation of additional daughters 7,8 . We show that physical removal of the daughter induces reduplication of the mother in S-arrested cells. Under conditions when multiple daughters simultaneously form on a single mother, all of these daughters must be removed to induce reduplication. Intriguingly, the number of daughter centrioles that form during reduplication does not always match the number of ablated daughter centrioles. We also find that exaggeration of the pericentriolar material (PCM) via overexpression of the PCM protein pericentrin 9 in S-arrested CHO cells induces formation of numerous daughter centrioles. We propose that that the size of the PCM cloud associated with the mother centriole restricts the number of daughters that can form simultaneously.A typical centrosome in an animal cell consists of two microtubule-based cylindrical structures, termed the centrioles, surrounded by a cloud of pericentriolar material (PCM). Most of the components responsible for the major centrosomal functions, for example, the Υ-tubulin ring complex, reside in the PCM. However, in the absence of centrioles, the PCM cloud becomes structurally unstable, and eventually disperses 10 . Thus, the number of centrioles ultimately defines the number of centrosomes in the cell.Normally, in somatic cells new ('daughter') centrioles form in association with mature ('mother') centrioles. This process, known as "centriole duplication", is initiated when cells enter S phase, and the daughter centriole remains associated with its mother, in a strictly orthogonal configuration (i.e., 'diplosome'), until the second half of the ensuing mitosis 11-13 . Recent work suggested that as long as the daughter remains attached to the mother formation of additional daughters is not possible 7,8 . *Correspondence should be addressed to AK (Wadsworth Center, PO Box 509, Albany, NY 12201−0509, phone: (518)486−5339; fax: (518)486−4901; E-mail: khodj@wadsworth.org). Author contributions Experiments were conducted by J.L.; P.H. was responsible for EM preparation and data collection; V.M. designed, assembled, and maintained the laser microsurgery workstation; A.K. directed the work. Experiments were planned by J.L. and A.K. Competing financial interestsThe authors declare that they have no competing financial interests. Fig. S3). NIH Public AccessUpon ablation of the daughter centriole within a diplosome, the remaining mother consistently (20 of 20 experiments) developed a new daughter (Figs. 1A, S4). The interval be...

Summary Supernumerary centrioles lead to abnormal mitosis [1,2] which in turn promotes tumorigenesis [3,4]. Thus, centriole duplication must be coordinated with the cell cycle to ensure that the number of centrioles in the cell doubles precisely during each cell cycle [5]. However, in some transformed cells centrioles undergo multiple rounds of duplication (reduplication) during prolonged interphase [6-8]. Mechanisms responsible for centriole reduplication are poorly understood. Here, we report that centrioles reduplicate consistently in cancerous and non-transformed human cells during G2 arrests and this reduplication requires the activity of Polo-like kinase 1 (Plk1). We also find that cell’s ability to reduplicate centrioles during S-arrests depends on the presence of activated (T210-phosphorylated) Plk1 at the centrosome. In the absence of activated Plk1, nascent procentrioles remain associated with mother centrioles, which prevent centriole reduplication. In contrast, if Plk1(pT210) appears at the centrosome, procentrioles mature, disengage from mother centrioles, and ultimately duplicate. Plk1 activity is not required for the assembly of procentrioles, however. Thus, the role of Plk1 is to coordinate centriole duplication cycle with the cell cycle. Activation of Plk1 during late-S-G2 induces procentriole maturation and after this point the centriole cycle can be completed autonomously, even in the absence of cell cycle progression.

The inheritance of the centrosome during human fertilization remains mysterious. Here we show that the sperm centrosome contains, in addition to the known typical barrel-shaped centriole (the proximal centriole, PC), a surrounding matrix (pericentriolar material, PCM), and an atypical centriole (distal centriole, DC) composed of splayed microtubules surrounding previously undescribed rods of centriole luminal proteins. The sperm centrosome is remodeled by both reduction and enrichment of specific proteins and the formation of these rods during spermatogenesis. In vivo and in vitro investigations show that the flagellum-attached, atypical DC is capable of recruiting PCM, forming a daughter centriole, and localizing to the spindle pole during mitosis. Altogether, we show that the DC is compositionally and structurally remodeled into an atypical centriole, which functions as the zygote’s second centriole. These findings now provide novel avenues for diagnostics and therapeutic strategies for male infertility, and insights into early embryo developmental defects.

Centrioles are vital cellular structures that form centrosomes and cilia. The formation and function of cilia depends on a set of centriole’s distal appendages. In this study, we use correlative super resolution and electron microscopy to precisely determine where distal appendage proteins localize in relation to the centriole microtubules and appendage electron densities. Here we characterize a novel distal appendage protein ANKRD26 and detail, in high resolution, the initial steps of distal appendage assembly. We further show that distal appendages undergo a dramatic ultra-structural reorganization before mitosis, during which they temporarily lose outer components, while inner components maintain a nine-fold organization. Finally, using electron tomography we reveal that mammalian distal appendages associate with two centriole microtubule triplets via an elaborate filamentous base and that they appear as almost radial finger-like protrusions. Our findings challenge the traditional portrayal of mammalian distal appendage as a pinwheel-like structure that is maintained throughout mitosis.