The Number of Repeat Sequences in Microtubule-associated Protein 4 Affects the Microtubule Surface Properties

Abstract

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.

Microtubule-associated protein (MAP) 4 interacts with microtubules in an intrinsically disordered manner

“…However, more recent quantitative analysis revealed that one microtubule-binding domain binds to only one tubulin molecule, 10) and that the binding affinity is independent of the number of AP sequences. 5) Consequently, the current views on not only MAP4, 5,10) but also MAP2 30) and τ, 31) imply that multiple AP sequences do not function simultaneously. This apparent discrepancy can be resolved by considering the intrinsically disordered nature of MAP4.…”

“…[2][3][4][5][6] However, we also emphasized the enhancement of the binding affinity by the Pro-rich region.…”

“…1(A)), 5) each consisting of about 18 consecutive amino acid residues. The AP sequence is considered to be essential for microtubule-binding activity.…”

Protein kinase A phosphorylation of tau-serine 214 reorganizes microtubules and disrupts the endothelial cell barrier

“…Although tau is a relatively small protein, it shares a conserved 18-amino acid microtubule-binding repeat in common with larger microtubule-associated proteins (3,44). Tau isoforms vary in their number of repeat domains (20), with the longest tau40 isoform having 4 microtubule-binding repeats at residues 256 -273, 287-304, 318 -335, and 350 -367.…”

MAP4 (microtubule-associated protein 4)

“…Three to five repeats of the AP sequence have been described in MAP4 (Aizawa et al, 1990). In vitro experiments with bovine MAP4 have shown that the number of repeat sequences affects the microtubule surface properties (Tokuraku et al, 2003).…”