The number of repeat sequences in microtubule-associated protein 4 affects the microtubule surface properties

The microtubule-binding domain of MAP4, a ubiquitous microtubule-associated protein, contains a Repeat region with tandemly organized repeat sequences. In this study, we focused on the variations of the Repeat region, and searched for MAP4 isoforms with diverse Repeat region organizations. We successfully isolated four types of MAP4 cDNAs, which differed from each other in both the number and the arrangement of the repeat sequences, from a single source (bovine adrenal gland). To examine the functional differences among the isoforms, we prepared the microtubule-binding domain polypeptides of three of the four isoforms, and examined their activities. The isoform fragments showed similar degrees of microtubule assembly promoting activity and microtubule binding affinity. This result suggested that the Repeat region variation is not important for the control of microtubule dynamics, which is believed to be the main function of MAPs. On the other hand, the microtubule bundle-forming activity differed among the isoform fragments. The bundle formation was augmented by increasing the number of repeat sequences in the fragments. Based on these results, we propose the hypothesis that the role of the MAP4 isoforms is to regulate the surface charge of microtubules.