Microtubule minus-end anchorage at centrosomal and non-centrosomal sites: the role of ninein

The novel concept of a centrosomal anchoring complex, which is distinct fro m the gamma-tubulin nucleating complex, has previously been proposed follow ing studies on cochlear epithelial cells, In this investigation we present evidence from two different cell systems which suggests that the centrosoma l protein ninein is a strong candidate for the proposed anchoring complex, Ninein has recently been observed in cultured fibroblast cells to localise primarily to the post-mitotic mother centriole, which is the focus for a cl assic radial microtubule array, We show here by immunoelectron microscopica l analyses of centrosomes from mouse L929 cells that ninein concentrates at the appendages surrounding the mother centriole and at the microtubule min us-ends. We further show that localisation of ninein in the cochlear suppor ting epithelial cells, where the vast majority of the microtubule minus-end s are associated with apical non-centrosomal sites, suggests that it is not directly involved in microtubule nucleation. Ninein seems to play an impor tant role in the positioning and anchorage of the microtubule minus-ends in these epithelial cells. Evidence is presented which suggests that ninein i s released from the centrosome, translocated with the microtubules, and is responsible for the anchorage of microtubule minus-ends to the apical sites . We propose that ninein is a non-nucleating microtubule minus-end associat ed protein which may have a dual role as a minus-end capping and anchoring protein.