In situ cryo-electron tomography reveals filamentous actin within the microtubule lumen

Paul et al. use cryo-ET to show that microtubules can contain actin filaments in their lumen. Two types of F-actin structure are distinguished, Class I and Class II, with slightly different helical symmetries and outer microtubule diameters. They call these filaments microtubule lumenal actin (ML-actin).
Microtubules and filamentous (F-) actin engage in complex interactions to drive many cellular processes from subcellular organization to cell division and migration. This is thought to be largely controlled by proteins that interface between the two structurally distinct cytoskeletal components. Here, we use cryo-electron tomography to demonstrate that the microtubule lumen can be occupied by extended segments of F-actin in small molecule–induced, microtubule-based, cellular projections. We uncover an unexpected versatility in cytoskeletal form that may prompt a significant development of our current models of cellular architecture and offer a new experimental approach for the in situ study of microtubule structure and contents.