Mal3 is a fission yeast homolog of EB1, a plus-end tracking protein (+. TIP). We have generated a mutation (89R) replacing glutamine with arginine in the calponin homology (CH) domain of Mal3. Analysis of the 89R mutant. in vitro has revealed that the mutation confers a higher affinity to microtubules and enhances the intrinsic activity to promote the microtubule-assembly. The mutant Mal3 is no longer a +. TIP, but binds strongly the microtubule lattice. Live cell imaging has revealed that while the wild type Mal3 proteins dissociate from the tip of the growing microtubules before the onset of shrinkage, the mutant Mal3 proteins persist on microtubules and reduces a rate of shrinkage after a longer pausing period. Consequently, the mutant Mal3 proteins cause abnormal elongation of microtubules composing the spindle and aster. Mal3 is phosphorylated at a cluster of serine/threonine residues in the linker connecting the CH and EB1-like C-terminal motif domains. The phosphorylation occurs in a microtubule-dependent manner and reduces the affinity of Mal3 to microtubules. We propose that because the 89R mutation is resistant to the effect of phosphorylation, it can associate persistently with microtubules and confers a stronger stability of microtubules likely by reinforcing the cylindrical structure.
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