Caltubin regulates microtubule stability via Ca2+-dependent mechanisms favouring neurite regrowth
Microtubule regulation is highly controlled in nerve regeneration. Caltubin, a novel Lymnaea stagnalis protein, contains putative EF-hand calcium-binding motifs and promotes neuronal outgrowth in Lymnaea and mouse. Here, we generated cell-permeable caltubin proteins to investigate mechanisms underlying this effect. We observed increased neurite extension and outgrowth following injury in caltubin-treated mouse neurons compared to vehicle controls. Purified caltubin bound -tubulin between its L391-V405 amino acids and promoted microtubule assembly. Caltubin competitively inhibited binding of tubulin tyrosine ligase, which catalyzes tubulin retyrosination, and increased the ratio of detyrosinated to tyrosinated tubulin. Our crystal structure analysis confirmed that caltubin has four Ca2+-binding EF-hand motifs, like calmodulin but has distinct peptide binding domains. Our work suggests a unique Ca2+-dependent regulatory mechanism of microtubule assembly by caltubin. This may represent an essential mechanism of axonal regulation, which may optimize its activity in response to various calcium states, both physiological and following injury.