UC Davis

Microtubules and actin are two essential cytoskeletal networks in neurons, providing structural support and serving as tracks for intracellular transport during neuronal morphogenesis and maintenance. The crosstalk between actin filaments and microtubules is important for various cellular processes such cell division, migration, vesicle and organelle transport, and axonal outgrowth. Microtubules provide platforms for many non-enzymatic microtubule-associated proteins (MAPs) and enzymatic motor proteins, which are responsible for intracellular transport. One non-enzymatic MAP, MAP1B, has been shown to have important functions in neural development due to its early expression in neurons. Previous studies have shown that knockdown of MAP1B results in defects in axon elongation and dendrite formation in neurons. Mutations in MAP1B are also associated with epilepsy. However, the molecular mechanism of how MAP1B regulates microtubule-based activities is still unclear. To understand the basic function of MAP1B, we investigated its role in regulating actin- and microtubule-based activities. Our data revealed that both the Heavy Chain (HC) and Light Chain (LC) of MAP1B strongly bind to microtubules, but LC displayed a more diffuse behavior due to interactions with C-terminal tubulin tails. Both HC and LC also shield microtubules from severing by Spastin, although HC's protection was relatively weaker, possibly because it remains statically bound to microtubules rather than competing with Spastin for C-terminal tail of tubulins. MAP1B was also found to be a general inhibitor of various motor proteins, including kinesin-3, kinesin-4, and cytoplasmic dynein, by preventing access by these motors to the microtubule lattice. The effect of MAP1B on actin was also explored. While both MAP1B LC and non-phosphorylated HC interact with actin filaments, HC's formation of an HC-LC complex hinders the binding of LC to actin filaments, thereby negatively regulating this interaction. Additionally, the phosphorylation of MAP1B HC 1-1500 by DYRK1A was shown to decrease its affinity for actin significantly, without a substantial impact on microtubule binding. In contrast, MAP1B LC's actin binding ability remained unaffected despite post-translational modifications. Furthermore, MAP1B LC was found to bind actin monomers and directly facilitate actin polymerization over microtubules, creating a link between the two cytoskeletons. This action also enabled unanchored polymerized actin filaments to attach to adjacent microtubules, thereby connecting the two cytoskeletal networks, even without LC being statically bound to actin filaments. In addition, we have identified DYRK1A as key kinase that negatively regulates MAP1B HC's affinity for actin filaments. In summary, this comprehensive study delves into the molecular mechanisms underlying MAP1B's involvement in mediating the interaction between actin and microtubules. It sheds light on the effects of both MAP1B HC and LC and their regulation by post-translational modifications, providing insights into its role during neuronal development and the pathology of associated neurological disorders.