Ubiquitin is a component of the microtubule network.

Immunofluorescence microscopy was used to study the intracellular localization of ubiquitin. Baby hamster kidney cells (BHK cells) and several other cell lines were probed with a well characterized monoclonal antibody to ubiquitin. The antibody stained a complex cellular structure that we identified as the microtubule network. The anti-ubiquitin antibody bound to the microtubule network at all stages of the cell cycle, and we showed that the apparent association of ubiquitin with the microtubule network is not an artifact of crosslinking of free ubiquitin to the cell structure. Immunoblot procedures demonstrated that tubulin itself was not ubiquitinated. We propose that ubiquitin and/or ubiquitin-protein conjugates are associated with those networks as a new class of microtubule-associated protein. The targeting of ubiquitin to specific sites within the cell by its association with the microtubule network may regulate some of the functions of ubiquitin.