Molecular cloning of XTP, a tau-like microtubule-associated protein from Xenopus laevis tadpoles

The microtubules of the mammalian nervous system are stabilised by several microtubule-associated proteins (MAPs), including the tau and MAP-2 protein families. The most prominent feature of mammalian tau and MAP-2 proteins is a common and highly homologous microtubule-binding region consisting of three or four imperfect tandem repeats. In this paper we report the cloning and characterisation of a Xenopus laevis tau-like protein (XTP) from tadpole tails. This protein encompasses two isoforms of 673 or 644 amino acids with four tandem repeats that are highly homologous to mammalian tau repeats. Both isoforms share a common amino terminal half, whereas the carboxyl terminus downstream of the repeat region is unique for each isoform. Northern blot analysis revealed that both isoforms are preferentially expressed in the tail of X. laevis tadpoles, whereas a shorter version of XTP is expressed in the head. Recombinant proteins of both XTP isoforms were able to bind microtubules. The longest isoform, however, was more effective at promoting tubulin polymerisation, indicating that sequences downstream of the repeat region affect the microtubule assembling capacity. These results demonstrate that tau-like proteins are found in non-mammalian vertebrate species, where they may support the stability of microtubules.