## 4 Discussion  
Centrosomes generally serve as MTOCs in animal cells. MTOC functions are mostly localized in concentric layers of PCM recruited around centrioles. Stable centrosome cohesion in interphase holds two disengaged MTOC-competent centrosomes together as a single entity before centriole duplication. In late G\({}_{2}\)-phase, centrosome cohesion is relieved, and centrosomes separate to define the two poles (MTOCs) of the mitotic spindle. Balanced production of dynamic MTs from the two MTOCs in M-phase is essential for proper formation and function of the spindle. A set of a few conserved proteins turned out to be consistently essential in most metazoan species for centriole duplication: CEP192/SPD2, PLK4/ZYG1, SAS6, STIL/SAS5, and CPAP/SAS4 in humans. However, additional factors aid centriole stability, centrosome cohesion and splitting, the establishment of PCM around newly formed centrioles (centriole to centrosome conversion, CCC) starting in late mitosis, and the further increase (centrosome maturation) in PCM at mitotic onset. Previously, chicken DT40 cells were used to analyze essential functions of Cep135, a direct interaction partner of Sas-6, in a vertebrate model cell line [24]. Homozygous loss of _cep135_ resulted in atypical centriolar structure without no obvious defects in centrosome composition and spindle formation. In contrast, knockdown of human CEP135 not only caused centriole duplication defects but strongly compromised spindle assembly and chromosome segregation, resulting in frequent mitotic failure [25]. More recently, high (\"super\")-resolution light microscopy has greatly facilitated the analysis of individual centrosonal protein functions in human cells. Using 3D-SIM, as well as STED microscopy, [PERSON] and colleagues could demonstrate that Bld10, the ortholog of human CEP135 in flies, precedes recruitment of Anal/Cep295 and Asterless/Cep152 to establish centriole-to  
Figure 8: The two poles of metaphase spindles are balanced in _cep135_ KO cells. (**A**) Representative images showing balanced spindle poles in metaphase. Scale bar: 10 \(\upmu\)m. (**B–D**) Signal intensity of metaphase \(\gamma\)-tubulin (**B**), CDKSRAP2 (**C**), and \(\alpha\)-tubulin (**D**) signals. A total of 150 cells were measured in total for each cell line. The graph shows mean values \(\pm\) SD from three independent experiments. The \(p\)-value was calculated with a two-tailed Student’s \(t\)-test. * indicates \(p<0.05\). (**E–G**) The distinct \(\gamma\)-tubulin, CDKSRAP2, and \(\alpha\)-tubulin signals of mitotic centrosomes/spindle poles were quantified, and their ratios (higher/lower value) were determined. The plot shows the distribution of ratios ranging from 1 (equal signals) to 0 (extreme asymmetry between the two signals). A total of 150 cells were measured in total for each experimental condition. centrosome conversion (CCC) [26]. Acting early in the assembly of a new (daughter) centriole, Cep135/Bld10 reaches out from the carstwheel structure beyond centriolar MT, enabling further protein interactions with Ana1 and Asl to refine daughter centrioles for the motherhood competent in the next cell cycle [27]. Analogously, CEP135, CEP295, and CEP152 have been shown to establish CCC in human cells [28]. This sets the ground for centriole duplication in the following S-phase. A picture emerges in which CEP135 is spotted as the communication point (\"pinpoint\" [29]) of the central carstwheel structure and the centriolar MT, stabilizes the centriole after carstwheel removal, fosters CCC, and improves competence for further duplication. In biochemical experiments, the WD40 repeat-containing protein WDR8 is the most consistently found interaction partner of CEP135 ([[https://thebiogrid.org/115018/summary/](https://thebiogrid.org/115018/summary/)]([https://thebiogrid.org/115018/summary/](https://thebiogrid.org/115018/summary/)) homo-sapiens/cep135.html, accessed on 8 March 2022). WDR8, in turn, communicates with the centriolar satellite protein SSX2 IP.