Microtubule-associated proteins (MAPs) decorate the microtubule cytoskeleton, control its stability and architecture. MAPs exhibit distinct localization patterns across polarized cells, such as neurons, and regulate intracellular transport driven by motor proteins. However, several MAPs, such as MAP9, remain poorly characterized to date, despite having crucial roles during metazoan development and cytokinesis. Here, we identify MAP9 as a well-conserved, essential regulator of neuronal morphogenesis. Cryo-electron microscopy revealed that, unlike other MAPs that lie parallel to the microtubule protofilament, MAP9 can bind across up to five protofilaments through consecutive microtubule-binding repeats that engage identical sites on β-tubulin. This unique binding mode enables MAP9 to “staple” adjacent protofilaments together and suppresses the catastrophe of dynamic microtubules in vitro in a manner superior to other MAPs. MAP9 knockdown abolishes the outgrowth of neurites, a phenotype which is not observed through loss of other neuronal MAPs. We also found that MAP9 is positioned adjacent to loop-8 of the kinesin motor domain when bound to the same tubulin site. This positioning allows MAP9 to selectively permit kinesin-3 motility while hindering kinesin-1, due to differences in the chemical properties of their loop-8 sequences. Collectively, our findings establish MAP9 as a key regulator of neuronal growth through its distinct role in stabilizing microtubules and uncover the mechanisms by which it differentially regulates transport kinesins.
Cetin et al. (2026) studied this question.