Lumenal components of cytoplasmic microtubules

The lumen of cytoplasmic microtubules is a poorly explored expanse of intracellular space. Although typically represented in textbooks as a hollow tube, studies over several decades have shown that the microtubule lumen is occupied by a range of morphologically diverse components. These are predominantly globular particles of varying sizes which appear to exist either in isolation, bind to the microtubule wall, or form discontinuous columns that extend through the lumenal space. Actin filaments with morphologies distinct from the canonical cytoplasmic forms have also now been found within the microtubule lumen. In this review, we examine the historic literature that observed these lumenal components in tissues from diverse species and integrate it with recent cryo-electron tomography studies that have begun to identify lumenal proteins. We consider their cell and tissue distribution, possible mechanisms of incorporation, and potential functions. It is likely that continuing work in this area will open a new frontier in cytoskeletal biology.