It takes three to tango at the centriole

Proper centriole assembly and recruitment of the surrounding pericentriolar material (PCM) are crucial for the generation of centrosomes, but the first stages of centriolar assembly are not well understood, given that the lack of any of the core components prevents centriole formation. Jordan Raff and colleagues previously reported that overexpression of the centriole proteins Spindle assembly abnormal protein 6 (Sas-6) and Anastral spindle 2 (Ana2, STIL in vertebrates) in Drosophila embryos and eggs leads to the formation of so-called Sas-6/Ana2 particles (SAPs). Now, they use this system (Gartenmann et al., 2020) to dissect the initial stages of centriole assembly and show here that SAPs resemble the structure and microtubule-organising capability of centrosomes and can aggregate other known centrosome proteins, such as Spindle defective 2 (Spd-2, Cep192 in vertebrates) and Asterless (Asl, Cep152 in vertebrates). Sas-4 is essential to promote the Sas-6–Ana2 interaction, and these three proteins are able to self-organise. Surprisingly, this assembly does not depend on Polo-like kinase 4, a kinase that is normally linked to centriole assembly. Finally, the authors show that Asl aids SAP assembly but is not required for its maintenance. Interestingly, Asl is required for recruitment of PCM to SAPs but, in its absence, this function can be covered by Spd-2. This study thus demonstrates the use of a simple system such as SAPs to decipher centriole and centrosome assembly.