Distribution of the microtubule-related protein ninein in developing neurons.
Ninein associates with the centrosome in many cell types, where it recaptures minus-ends of microtubules after their release. In more complex and polarized cells, ninein has also been observed at noncentrosomal locales, where its function is not as well understood. We have found that cultured neurons contain both centrosomal and noncentrosomal ninein, and that the noncentrosomal ninein, typically observed as small particles, is both abundant and widespread. Noncentrosomal ninein is also dispersed throughout the cytoplasm of non-neuronal cells present within the cultures, but is particularly rich in the cytoplasm of neurons, where it may compete with centrosomal ninein to impede the recapture of microtubules by the centrosome after their release. Interestingly, noncentrosomal ninein is concentrated in regions of both neurons and non-neuronal cells undergoing retraction, such as in the trailing processes that retract during neuronal migration. These results suggest that noncentrosomal ninein may contribute to the configuration of the microtubule array underlying alterations in cellular morphology, and that such a contribution is likely to be particularly important for neuronal cells.