Microtubules (MTs) play a critical role in many cellular processes, most significantly in intracellular trafficking. It has been proposed that signals that regulate motor-driven intracellular transport are deposited onto MT tracks, either by post-translational modifications (PTMs) and isotypes of tubulin (tubulin-code) or MT-associated proteins (MAP-code). In vitro studies showed that MT motors are tightly regulated by MAPs but not by tubulin PTMs. Therefore, it remains unclear whether tubulin modifications directly regulate MAPs and motors on the MT. Using recombinant tubulin, we investigated the interplay between the tubulin- and MAP-codes and how they impact motor activity in vitro. We found that different tubulin modifications have a minor impact on kinesin motility when MTs are stabilized by taxol, because taxol expands the MT lattice and outweighs the effects of tubulin PTMs. However, these changes can significantly affect MT-MAP decoration, which in turn strongly affects motility. In dynamically growing MT experiments, we observed different tubulin modifications selectively tune the recruitment of MAPs and motors to the MT. We showed that tubulin PTMs affect the MT affinity of tau by altering its affinity to bind the MT in dynamic assays. Our results are consistent with a two-layered code, in which the tubulin code selectively recruits MAPs onto MTs, and the MAP code dictates which motors can transport cargo along those tracks.
Fernandes et al. (Sun,) studied this question.