Chromosome Segregation Machinery Research Articles

Metabolic contribution to age-related chromosome missegregation in mammalian oocytes.

Advanced maternal age is a major cause of infertility, miscarriage, and congenital abnormalities. This is principally caused by a decrease in oocyte quality and developmental competence with age. Oocyte ageing is characterised by an increase in chromosome missegregation and aneuploidy. However, the underlying mechanisms of age-related aneuploidy have not been fully elucidated and are still under active investigation. In addition to chromosome missegregation, oocyte ageing is also accompanied by metabolic dysfunction. In this review, we integrate old and new perspectives on oocyte ageing, chromosome segregation and metabolism in mammalian oocytes and make direct links between these processes. We consider age-related alterations to chromosome segregation machinery, including the loss of cohesion, microtubule stability and the integrity of the spindle assembly checkpoint. We focus on how metabolic dysfunction in the ageing oocyte disrupts chromosome segregation machinery to contribute to and exacerbate age-related aneuploidy. More specifically, we discuss how mitochondrial function, ATP production and the generation of free radicals are altered during ageing. We also explore recent developments in oocyte metabolic ageing, including altered redox reactions (NAD+ metabolism) and the interactions between oocytes and their somatic nurse cells. Throughout the review we integrate the mechanisms by which changes in oocyte metabolism influence age-related chromosome missegregation.

Expression of the checkpoint kinase BUB1 is a predictor of response to cancer therapies

The identification of clinically-relevant biomarkers is of upmost importance for the management of cancer, from diagnosis to treatment choices. We performed a pan-cancer analysis of the mitotic checkpoint budding uninhibited by benzimidazole 1 gene BUB1, in the attempt to ascertain its diagnostic and prognostic values, specifically in the context of drug response. BUB1 was found to be overexpressed in the majority of cancers, and particularly elevated in clinically aggressive molecular subtypes. Its expression was correlated with clinico-phenotypic features, notably tumour staging, size, invasion, hypoxia, and stemness. In terms of prognostic value, the expression of BUB1 bore differential clinical outcomes depending on the treatment administered in TCGA cancer cohorts, suggesting sensitivity or resistance, depending on the expression levels. We also integrated in vitro drug sensitivity data from public projects based on correlation between drug efficacy and BUB1 expression to produce a list of candidate compounds with differential responses according to BUB1 levels. Gene Ontology enrichment analyses revealed that BUB1 overexpression in cancer is associated with biological processes related to mitosis and chromosome segregation machinery, reflecting the mechanisms of action of drugs with a differential effect based on BUB1 expression.

Defining a core configuration for human centromeres during mitosis

The centromere components cohesin, CENP-A, and centromeric DNA are essential for biorientation of sister chromatids on the mitotic spindle and accurate sister chromatid segregation. Insight into the 3D organization of centromere components would help resolve how centromeres function on the mitotic spindle. We use ChIP-seq and super-resolution microscopy with single particle averaging to examine the geometry of essential centromeric components on human chromosomes. Both modalities suggest cohesin is enriched at pericentromeric DNA. CENP-A localizes to a subset of the α-satellite DNA, with clusters separated by ~562 nm and a perpendicular intervening ~190 nM wide axis of cohesin in metaphase chromosomes. Differently sized α-satellite arrays achieve a similar core structure. Here we present a working model for a common core configuration of essential centromeric components that includes CENP-A nucleosomes, α-satellite DNA and pericentromeric cohesion. This configuration helps reconcile how centromeres function and serves as a foundation to add components of the chromosome segregation machinery.

Robust ParB Binding to Half-parS Sites in Pseudomonas aeruginosa-A Mechanism for Retaining ParB on the Nucleoid?

Chromosome segregation in Pseudomonas aeruginosa is assisted by the tripartite ParAB-parS system, composed of an ATPase (ParA), a DNA-binding protein (ParB) and its target parS sequence(s). ParB forms a nucleoprotein complex around four parSs (parS1-parS4) that overlaps oriC and facilitates relocation of newly synthesized ori domains inside the cells by ParA. Remarkably, ParB of P. aeruginosa also binds to numerous heptanucleotides (half-parSs) scattered in the genome. Here, using chromatin immunoprecipitation-sequencing (ChIP-seq), we analyzed patterns of ParB genome occupancy in cells growing under conditions of coupling or uncoupling between replication and cell division processes. Interestingly, a dissipation of ParB-parS complexes and a shift of ParB to half-parSs were observed during the transition from the exponential to stationary phase of growth on rich medium, suggesting the role of half-parSs in retaining ParB on the nucleoid within non-dividing P. aeruginosa cells. The ChIP-seq analysis of strains expressing ParB variants unable to dislocate from parSs showed that the ParB spreading ability is not required for ParB binding to half-parSs. Finally, a P. aeruginosa strain with mutated 25 half-parSs of the highest affinity towards ParB was constructed and analyzed. It showed altered ParB coverage of the oriC region and moderate changes in gene expression. Overall, this study characterizes a novel aspect of conserved bacterial chromosome segregation machinery.

Kinetochore deterioration concommitant with centromere weakening during aging in mouse oocyte meiosis-I.

Age-related oocyte aneuploidy occurs as a result of chromosome segregation errors in female meiosis-I and meiosis-II, and is caused by a progressive age-related deterioration of the chromosome segregation machinery. Here, we assess the impact of age upon the kinetochore, the multi-protein structure that forms the link between the chromosome and spindle microtubules. We find that in meiosis-I the outer kinetochore assembles at germinal vesicle breakdown, but that a substantially smaller outer kinetochore is assembled in oocytes from aged mice. We show this correlates with a weaker centromere in aged oocytes and, using nuclear transfer approaches to generate young-aged hybrid oocytes, we show that outer kinetochore assembly always mirrors the status of the centromere, regardless of cytoplasmic age. Finally, we show that weaker kinetochores in aged oocytes are associated with thinner microtubule bundles, that are more likely to be mis-attached. We conclude that progressive loss of the centromere with advancing maternal age underpins a loss of the outer kinetochore in meiosis-I, which likely contributes to chromosome segregation fallibility in oocytes from older females.

Abstract 1421: Targeting ASPM suppresses the tumorigenicity of human hepatocellular carcinoma cells via ASPM-TPX2 axis disruption and result in increased chromosome segregation error

Abstract Aneuploidy and chromosomal instability (CIN) are common abnormalities in human cancer, might arise from a lesion in the chromosome segregation machinery. CIN has a tumour suppressive role because of excessive genomic damage and subsequent apoptosis or other forms of cell death. Therefore, induced massive chromosome instability and led to cellular death in cancer cells can be one of the most successful strategies for cancer therapy. ASPM is a critical regulator of cell-cycle progression, DNA damage and genome stability. In our previous work, we showed that ASPM was often overexpressed in human HCC and associated with tumor malignance progression and poor prognosis. However, the molecular roles of ASPM in the liver cancer cells still unclear. In this study, our results indicated that targeting ASPM caused a rising impaired chromosomal instability resulting in multinuclearity, mitosis progression prolong, increased multicentrosomes, chromosome segregation error and led to chromosome numbers variation in liver cancer cells. Finally, ASPM inhibition led to cell cycle progression arrest and apoptosis in liver cancer cells. Moreover, in vitro tumor spheroid assay and in vivo xenografts mouse model showed a therapeutic opportunity. Our results indicated the effect of ASPM silencing was led to chromosome segregation error in liver cancer cell. In mechanism analysis, the TPX2 was similar binary expression with ASPM in HCC and as downstream target. In our previous work, targeting TPX2 suppresses the tumorigenesis of hepatocellular carcinoma cells resulting in arrested mitotic phase progression and increased genomic instability. However, silenced ASPM or TPX2 both led to deregulated cyclin B2 protein expression. These results indicated ASPM-TPX2-Cyclin B2 axis was co-expression and TPX2-Cyclin B2 should be the potential downstream targets of ASPM. Moreover, in this study, ASPM was interacted with TPX2 and co-localized at mitotic spindle pore. Overexpression of EGFP-TPX2 was semi-restored the cell fitness caused by ASPM inhibition in liver cancer cells. Taken together, our results indicated that ASPM play important roles in cell proliferation, tumorigenesis and cell cycle progression in HCC cells. These results suggested a new insight into the mechanism of ASPM depletion through deregulated TPX2 and result in Cyclin B2 protein downregulation and contributed to cancer cell growth arrest, chromosome instability and cell death in hepatocellular carcinoma. Citation Format: Hung-Wei Pan, Zhong-Zhe Lin, Guan-Jin Huang. Targeting ASPM suppresses the tumorigenicity of human hepatocellular carcinoma cells via ASPM-TPX2 axis disruption and result in increased chromosome segregation error [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1421.

The adaptive strategies of cells in the face of CIN.

An increased frequency of chromosome segregation errors, known as chromosomal instability (CIN), leads to accumulation of aneuploid cells with abnormal chromosomal numbers, which impairs viability through negative effects on survival and proliferation under most conditions. Two recent papers find by independent approaches that the key to surviving high levels of CIN is reducing the instability itself, showcasing the remarkable adaptability of the chromosome segregation machinery, in particular the microtubule-kinetochore interface, and highlighting the crucial role that maintaining chromosomal stability plays in cell proliferation.

P326 DNA replication initiator proteins stabilize the kinetochore in the human fungal pathogen Cryptococcus neoformans

Poster session 3, September 23, 2022, 12:30 PM - 1:30 PMObjectivesDNA replication licensing ensures maintenance of the ploidy state by limiting DNA replication once per cell cycle. The mini-chromosome maintenance (MCM) complex is an evolutionarily conserved DNA helicase in eukaryotes that aids this process. Upon activation in the S phase by several protein kinases, the MCM complex unwinds the double-stranded DNA and moves away from DNA replication origins preventing re-initiation of replication in the remainder of the cell cycle. Deregulation of MCM function can lead to malignant transformation of proliferating cells as indicated by their upregulated expression in human cancer and pre-cancerous cells. MCMs, therefore, serve as important biomarkers and potential targets for anti-cancer drugs. This study aims to decipher the effect of deregulated expression of the MCM complex on the cell cycle events of the human fungal pathogen, Cryptococcus neoformans, which is more similar to metazoans than other budding yeast. Aneuploidy-mediated drug resistance is a common mechanism in many major human fungal pathogens including Cryptococcus and Candida.MethodsWe generated conditional mutants of individual subunits of the MCM complex in C. neoformans and assayed the impact of their altered expression on cell cycle events. We also tested the alternative role of MCM subunits in the assembly of the kinetochore, a vital component of the chromosome segregation machinery.ResultsOur screen with MCM conditional mutants identified two in vivo functional subcomplexes that comprise the MCM complex in C. neoformans. Although upregulated expression of either Mcm2 or Mcm3 does not affect cell cycle progression, overexpression of either Mcm6 or Mcm7 led to the accumulation of cells in the large bud stage with nuclear segregation defects. This work provides evidence for the first time for a mitotic role of pre-replication complex proteins, MCM 2-7 complex in Cryptococcus. Depletion of Mcm2 led to arrest of cells in the S and G2/M stages of the cell cycle with defects in nuclear segregation. Localization and expression of several kinetochore proteins upon depletion of Mcm2 established that Mcm2 is vital for kinetochore assembly/integrity. Although the centromeric histone H3 variant, CENP-A, remains largely clustered, the outer kinetochore did not mature indicating that Mcm2 alone or as part of the MCM complex plays a role in kinetochore assembly/integrity and thereby in chromosome segregation.ConclusionA conserved eukaryotic DNA helicase, MCM 2-7 complex, has an unexplored non-canonical role in kinetochore assembly/integrity and chromosome segregation.

The cell cycle, cancer development and therapy.

The process of cell division plays a vital role in cancer progression. Cell proliferation and error-free chromosomes segregation during mitosis are central events in life cycle. Mistakes during cell division generate changes in chromosome content and alter the balances of chromosomes number. Any defects in expression of TIF1 family proteins, SAC proteins network, mitotic checkpoint proteins involved in chromosome mis-segregation and cancer development. Here we discuss the function of organelles deal with the chromosome segregation machinery, proteins and correction mechanisms involved in the accurate chromosome segregation during mitosis.

Genus-Specific Interactions of Bacterial Chromosome Segregation Machinery Are Critical for Their Function.

Most bacteria use the ParABS system to segregate their newly replicated chromosomes. The two protein components of this system from various bacterial species share their biochemical properties: ParB is a CTPase that binds specific centromere-like parS sequences to assemble a nucleoprotein complex, while the ParA ATPase forms a dimer that binds DNA non-specifically and interacts with ParB complexes. The ParA-ParB interaction incites the movement of ParB complexes toward the opposite cell poles. However, apart from their function in chromosome segregation, both ParAB may engage in genus-specific interactions with other protein partners. One such example is the polar-growth controlling protein DivIVA in Actinomycetota, which binds ParA in Mycobacteria while interacts with ParB in Corynebacteria. Here, we used heterologous hosts to investigate whether the interactions between DivIVA and ParA or ParB are maintained across phylogenic classes. Specifically, we examined interactions of proteins from four bacterial species, two belonging to the Gram positive Actinomycetota phylum and two belonging to the Gram-negative Pseudomonadota. We show that while the interactions between ParA and ParB are preserved for closely related orthologs, the interactions with polarly localised protein partners are not conferred by orthologous ParABs. Moreover, we demonstrate that heterologous ParA cannot substitute for endogenous ParA, despite their high sequence similarity. Therefore, we conclude that ParA orthologs are fine-tuned to interact with their partners, especially their interactions with polarly localised proteins are adjusted to particular bacterial species demands.

Building the microtubule cytoskeleton via phase transitions

How does a cell construct its microtubule cytoskeleton? According to Feynman’s principle “what I cannot create, I do not understand”, my lab pursues this question by building the chromosome segregation machinery from scratch. I will first tell you how the microtubule framework is generated in a cell. Upon deciphering the function of the most important microtubule accessory proteins, I will present how we use those building blocks to reconstitute a spindle substructure in vitro and determine its building plan. Surprisingly, the spindle substructure relies on a protein condensate, which forms on the surface of a microtubule and arranges into a droplet pattern like a fluid.By studying how the MT cytoskeleton is built, I hope to help explain how hundreds of proteins can self‐assemble on the nm scale into a complex molecular machine 1000‐fold larger than its constituents, a challenge for the biochemistry of the 21st century.

Defining the consequences of endogenous genetic variation within a novel family of Schizosaccharomyces pombe heterochromatin nucleating sequences.

Centromeres are essential for genetic inheritance—they prevent aneuploidy by providing a physical link between DNA and chromosome segregation machinery. In many organisms, centromeres form at sites of repetitive DNAs that help establish the chromatin architecture required for centromere function. These repeats are often rapidly evolving and subject to homogenization, which causes the expansion of novel repeats and sequence turnover. Thus, centromere sequence varies between individuals and across species. This variation can affect centromere function. We utilized Schizosaccharomyces pombe to assess the relationship between centromere sequence and chromatin structure and determine how sensitive this relationship is to genetic variation. In S. pombe, nucleating sequences within centromere repeats recruit heterochromatin via multiple mechanisms, which include RNA-interference (RNAi) . Heterochromatin, in turn, contributes to centromere function through its participation in three essential processes; establishment of a kinetochore, cohesion of sister chromatids, and suppression of recombination. Here, we show that a centromere element containing RevCen, a target of the RNAi pathway, establishes heterochromatin and gene silencing when relocated to a chromosome arm. Within this RevCen-containing element (RCE), a highly conserved domain is necessary for full heterochromatin nucleation but cannot establish heterochromatin independently. We characterize the 10 unique RCEs in the S. pombe centromere assembly, which range from 60% to 99.6% identical, and show that all are sufficient to establish heterochromatin. These data affirm the importance of centromere repeats in establishing heterochromatin and suggest there is flexibility within the sequences that mediate this process. Such flexibility may preserve centromere function despite the rapid evolution of centromere repeats.

Random Chromosome Partitioning in the Polyploid Bacterium Thermus thermophilus HB27.

Little is known about chromosome segregation in polyploid prokaryotes. In this study, whether stringent or variable chromosome segregation occurs in polyploid thermophilic bacterium Thermus thermophilus was analyzed. A stable heterozygous strain (HL01) containing two antibiotic resistance markers at one gene locus was generated. The inheritance of the two alleles in the progeny of the heterozygous strain was then followed. During incubation without selection pressure, the fraction of heterozygous cells decreased and that of homozygous cells increased, while the relative abundance of each allele in the whole population remained constant, suggesting chromosome segregation had experienced random event. Consistently, in comparison with Bacillus subtilis in which the sister chromosomes were segregated equally, the ratios of DNA content in two daughter cells of T. thermophilus had a broader distribution and a larger standard deviation, indicating that the DNA content in the two daughter cells was not always identical. Further, the protein homologs (i.e., ParA and MreB) which have been suggested to be involved in bacterial chromosome partitioning did not actively participate in the chromosome segregation in T. thermophilus. Therefore, it seems that protein-based chromosome segregation machineries are less critical for the polyploid T. thermophilus, and chromosome segregation in this bacterium are not stringently controlled but tend to be variable, and random segregation can occur.

DNA Segregation in Natural and Synthetic Minimal Systems.

Faithful segregation of replicated genomes to dividing daughter cells is a major hallmark of cellular life and needs to be part of the future design of the robustly proliferating minimal cell. So far, the complexity of eukaryotic chromosome segregation machineries has limited their applicability to synthetic systems. Prokaryotic plasmid segregation machineries offer promising alternative tools for bottom-up synthetic biology, with the first three-component DNA segregation system being reconstituted a decade ago. In this review, the mechanisms underlying DNA segregation in prokaryotes, with a particular focus on segregation of plasmids and chromosomal replication origins are reviewed, along with a brief discussion of archaeal and eukaryotic systems. In addition, this review shows how in vitro reconstitution has allowed deeper insights into these processes and discusses possible applications of these machineries for a minimal synthetic segrosome as well as the challenge of its coupling to a minimal replisome.

Electron cryotomography analysis of Dam1C/DASH at the kinetochore-spindle interface in situ.

In dividing cells, depolymerizing spindle microtubules move chromosomes by pulling at their kinetochores. While kinetochore subcomplexes have been studied extensively in vitro, little is known about their in vivo structure and interactions with microtubules or their response to spindle damage. Here we combine electron cryotomography of serial cryosections with genetic and pharmacological perturbation to study the yeast chromosome segregation machinery in vivo. Each kinetochore microtubule has one (rarely, two) Dam1C/DASH outer kinetochore assemblies. Dam1C/DASH contacts the microtubule walls and does so with its flexible "bridges"; there are no contacts with the protofilaments' curved tips. In metaphase, ∼40% of the Dam1C/DASH assemblies are complete rings; the rest are partial rings. Ring completeness and binding position along the microtubule are sensitive to kinetochore attachment and tension, respectively. Our study and those of others support a model in which each kinetochore must undergo cycles of conformational change to couple microtubule depolymerization to chromosome movement.

Centromere and its associated proteins-what we know about them in Plasmodium falciparum.

The complex life cycle of intracellular parasitic protozoans entails multiple rounds of DNA replication and mitosis followed by cytokinesis to release daughter parasites. To gain insights into mitotic events it is imperative to identify the biomarkers that constitute the chromosome segregation machinery in the parasite. Chromosomal loci called centromeres and their associated proteins play an essential role in accurate chromosome segregation. Although new information on the centromere-kinetochore proteins has been added to the existing pool of knowledge, a paucity of biomarkers for nuclear division prevents a global view of chromosome segregation mechanism in the malaria parasite. In Plasmodium falciparum, except CENH3 and CENP-C homologues, other centromere associated proteins responsible for centromere functions and kinetochore assembly are not known. The focus of this review is to summarize the current understanding on the centromere organization and its associated proteins in eukaryotes with the emerging information in P. falciparum. © 2018 IUBMB Life, 70(8):732-742, 2018.

Bactofilin-mediated organization of the ParABS chromosome segregation system in Myxococcus xanthus

In bacteria, homologs of actin, tubulin, and intermediate filament proteins often act in concert with bacteria-specific scaffolding proteins to ensure the proper arrangement of cellular components. Among the bacteria-specific factors are the bactofilins, a widespread family of polymer-forming proteins whose biology is poorly investigated. Here, we study the three bactofilins BacNOP in the rod-shaped bacterium Myxococcus xanthus. We show that BacNOP co-assemble into elongated scaffolds that restrain the ParABS chromosome segregation machinery to the subpolar regions of the cell. The centromere (parS)-binding protein ParB associates with the pole-distal ends of these structures, whereas the DNA partitioning ATPase ParA binds along their entire length, using the newly identified protein PadC (MXAN_4634) as an adapter. The integrity of these complexes is critical for proper nucleoid morphology and chromosome segregation. BacNOP thus mediate a previously unknown mechanism of subcellular organization that recruits proteins to defined sites within the cytoplasm, far off the cell poles.

Targeted TPX2 increases chromosome missegregation and suppresses tumor cell growth in human prostate cancer.

Prostate cancer is a complex disease that can be relatively harmless or extremely aggressive. Although androgen-deprivation therapy is a commonly used treatment for men with prostate cancer, the adverse effects can be detrimental to patient health and quality of life. Therefore, identifying new target genes for tumor growth will enable the development of novel therapeutic intervention. TPX2 plays a critical role in chromosome segregation machinery during mitosis. Low rates of chromosome missegregation can promote tumor development, whereas higher levels might promote cell death and suppress tumorigenesis. Hence, the strategy of promoting cell death by inducing massive chromosome missegregation has been a therapeutic application for selectively eliminating highly proliferating tumor cells. RNAi was used for TPX2 protein expression knockdown, and a clonogenic assay, immunostaining, double thymidine block, image-cytometry analysis, and tumor spheroid assay were used to analyze the role of TPX2 in tumor cell growth, cell cycle progression, multinuclearity, ploidy, and tumorigenicity, respectively; finally, Western blotting was used to analyze anticancer mechanisms in TPX2 targeting. We demonstrated that targeting TPX2 reduced cell cycle regulators and chromosome segregation genes, resulting in increased cell micronucleation. Moreover, TPX2 depletion led to prostate cancer cell growth inhibition, increased apoptosis, and reduced tumorigenesis. These results confirmed the therapeutic potential of targeting TPX2 in prostate cancer treatment. Moreover, we found that TPX2 silencing led to deregulation of CDK1, cyclin B, securin, separase, and aurora A proteins; by contrast, p21 mRNA was upregulated. We also determined the molecular mechanisms for TPX2 targeting in prostate cancer cells. In conclusion, our study illustrates the power of TPX2 as a potential novel target gene for prostate cancer treatment.

Trypanosome outer kinetochore proteins suggest conservation of chromosome segregation machinery across eukaryotes.

Kinetochores are multiprotein complexes that couple eukaryotic chromosomes to the mitotic spindle to ensure proper segregation. The model for kinetochore assembly is conserved between humans and yeast, and homologues of several components are widely distributed in eukaryotes, but key components are absent in some lineages. The recent discovery in a lineage of protozoa called kinetoplastids of unconventional kinetochores with no apparent homology to model organisms suggests that more than one system for eukaryotic chromosome segregation may exist. In this study, we report a new family of proteins distantly related to outer kinetochore proteins Ndc80 and Nuf2. The family member in kinetoplastids, KKT-interacting protein 1 (KKIP1), associates with the kinetochore, and its depletion causes severe defects in karyokinesis, loss of individual chromosomes, and gross defects in spindle assembly or stability. Immunopurification of KKIP1 from stabilized kinetochores identifies six further components, which form part of a trypanosome outer kinetochore complex. These findings suggest that kinetochores in organisms such as kinetoplastids are built from a divergent, but not ancestrally distinct, set of components and that Ndc80/Nuf2-like proteins are universal in eukaryotic division.

Unique Function of the Bacterial Chromosome Segregation Machinery in Apically Growing Streptomyces - Targeting the Chromosome to New Hyphal Tubes and its Anchorage at the Tips.

The coordination of chromosome segregation with cell growth is fundamental to the proliferation of any organism. In most unicellular bacteria, chromosome segregation is strictly coordinated with cell division and involves ParA that moves the ParB nucleoprotein complexes bi- or unidirectionally toward the cell pole(s). However, the chromosome organization in multiploid, apically extending and branching Streptomyces hyphae challenges the known mechanisms of bacterial chromosome segregation. The complex Streptomyces life cycle involves two stages: vegetative growth and sporulation. In the latter stage, multiple cell divisions accompanied by chromosome compaction and ParAB assisted segregation turn multigenomic hyphal cell into a chain of unigenomic spores. However, the requirement for active chromosome segregation is unclear in the absence of canonical cell division during vegetative growth except in the process of branch formation. The mechanism by which chromosomes are targeted to new hyphae in streptomycete vegetative growth has remained unknown until now. Here, we address the question of whether active chromosome segregation occurs at this stage. Applied for the first time in Streptomyces, labelling of the chromosomal replication initiation region (oriC) and time-lapse microscopy, revealed that in vegetative hyphae every copy of the chromosome is complexed with ParB, whereas ParA, through interaction with the apical protein complex (polarisome), tightly anchors only one chromosome at the hyphal tip. The anchor is maintained during replication, when ParA captures one of the daughter oriCs. During spore germination and branching, ParA targets one of the multiple chromosomal copies to the new hyphal tip, enabling efficient elongation of hyphal tube. Thus, our studies reveal a novel role for ParAB proteins during hyphal tip establishment and extension.