## 1 Introduction  
Microtubules (MTs) play crucial roles in in all eukaryotic cells throughout the cell cycle. In interphase, MTs organize the cytoplasm, helping to shape cell morphology and serving as transport routes for intracellular cargoes. In mitosis and meiosis (M-phase), however, tubulin dimers assemble into the highly dynamic bipolar spindle that drives chromosome segregation. Microtubule reorganization upon M-phase entry is facilitated by global alterations in the molecular composition of centrosomes, the major microtubule organizing centers (MTOCs) in animal cells [1]. Centrosomes generally comprise a pair of centrioles surrounded by layers of pericentriolar material (PCM) [2, 3, 4]. The PCM serves as a platform for proteins and protein complexes that govern MT nucleation and organization such as the \(\gamma\)-tubulin ring complex (\(\gamma\)-TuRC) [5]. The amount of \(\gamma\)-TuRC and other MT organizers in the PCM strongly increases at M-phase onset and enables overall increased MT assembly required for proper spindle formation. Recently, we exploited _Xenopus_ oocytes entering maturation (meiosis) to screen for novel factors required for MT functions in M-phase [6]. While vertebrate oocytes eliminate centrosomes during their development, the zygote forms a pair of new (zygotic) centrosomes from the paternal centriole (sperm basal body) and from a pool of materially expressed centrosomal proteins that are already present in mature oocytes (eggs) immediately prior to fertilization. _Xenopus_ egg lysates are, therefore, ideally suited for the identification of building blocks of centrioles and PCM. This fostered the identification of a previously uncharacterized centrosonal protein complex whose central moiety consists of two conserved proteins, Ssx2 ip (synovial sarcoma X-breakpoint 2-interacting protein) and Wrap73/Wdr8 (WD repeat-containing, antisense to p73/WD repeat-containing protein 8; here referred to as WDR8/Wdr8) [6; 7; 8; 9; 10; 11; 12]. Ssx2 ip, as a ubiquitously expressed 70 kDa \(\alpha\)-helix-rich protein, localizes to centriolar satellites and maintains the nucleation capacity of centrosomes during embryonic cleavage divisions [6]. Substantial cell division defects are readily observed after downregulation of Ssx2 ip in medaka embryos while defects are only partially reflected in knockdown of human somatic cells which display mild chromosome segregation errors [6]. In vertebrate somatic cells and embryonic cell extracts, Ssx2 ip consistently interacts with a WD40 repeat protein, Wdr8. Orthologs of Wdr8 are highly conserved from yeast to man; frog and human sequences display 76% identity at the amino-acid sequence level [7; 9; 10; 11]. Recent data show that the knockdown of WDR8 in human cells destabilizes SsX2 IP and results in the dispersal of other centriolar satellite proteins including the pericentriolar material protein-1 (PCM-1). WDR8 knockdown yields measurable but non-engraving mitotic defects including compromised spindle positioning and spindle architecture [9; 12]. In contrast, medaka knockout (ko) fish display strong chromosome segregation errors, abnormal timing of spindle formation, defects in its architecture, and impaired centrosome assembly, finally leading to early embryonic death after the loss of maternal Wdr8 [13]. In human somatic cells, WDR8 has been shown to interact with the centriolar protein CEP135 [10; 11], while Cep135 expression and interaction with Wdr8 in embryonic systems have not been established. Here, we show that Wdr8 interacts with newly expressed Cep135 in mature _Xenopus_ oocytes before zygotic centrosome reformation. We further address essential functions of CEP135, WDR8, and SsXZIP in human somatic cells and generate homozygous KOs of all genes in human RPE-1 cells. Unlike early embryos, somatic cells tolerate individual gene losses although _cep135_ KO cells show premature centrosome splitting and reduced PCM recruitment even in early interphase.