The centriculum, a membrane reticulum that surrounds Caenorhabditis elegans centrosomes, might serve as a microtubule filter.

Centrosomes are cytoplasmic microtubule-nucleating structures. They are considered membraneless organelles, but, in several cell types, they are surrounded by endoplasmic reticulum-derived membrane. In Caenorhabditis elegans early embryos, this membrane forms a dense membrane reticulum, named the centriculum, that affects centrosome structure and microtubule-nucleating capacity. The centriculum is adjacent to the pericentriolar material (PCM) of the centrosome and to abundant, short, peri-centrosomal microtubules. Here, we show that when microtubule abundance is reduced, centriculum and PCM size decrease, while the density of the PCM protein SPD-5 increases, suggesting that the PCM can be compacted. We further show that centriculum size is determined by microtubules, and that the centriculum likely limits the length of peri-centrosomal microtubules. Finally, we find that the centriculum is more porous where spindle microtubules pass compared to where astral microtubules pass. These data are consistent with the centriculum serving as a microtubule filter, blocking the extension of most, but not all, centrosome-nucleated microtubules. Finally, if microtubule-centriculum collisions result in microtubule catastrophe, the filter function of the centriculum could also explain the high concentration of soluble tubulin at the C. elegans centrosome.