PTL-1, A MICROTUBULE-ASSOCIATED PROTEIN WITH TAU-LIKE REPEATS FROM THE NEMATODE CAENORHABDITIS-ELEGANS

Tau, MAP2 and MAP4 are structural microtubule-associated proteins (MAP s) that promote the assembly and stability of microtubules. They share three or four imperfect tandem repeats of an amino acid motif, which is involved in the binding to microtubules. All sequences to date cont aining this motif are of mammalian origin. We report here the cloning and functional characterisation of a new member of this family of prot eins from the nematode Caenorhabditis elegans, This protein exists as two isoforms of 413 and 453 amino acids with four or five tandem repea ts that are 50% identical to the tau/MAP2/MAP4 repeats, Both isoforms bind to microtubules and promote microtubule assembly, with the five-r epeat isoform being more effective at promoting assembly than the four -repeat isoform, When expressed in COS cells, the five-repeat isoform co-localises with microtubules and induces the formation of microtubul e bundles, whereas its expression in Sf9 cells leads to the extension of long unipolar processes. In view of its length, amino acid sequence and functional characteristics, we have named this invertebrate struc tural MAP 'Protein with Tau-Like Repeats' (PTL-1), In C. elegans PTL-1 is expressed in two places known to require microtubule function, It is first seen in the embryonic epidermis, when circumferentially orien ted microtubules help to distribute forces generated during elongation . Later, it is found in mechanosensory neurons which contain unusual 1 5 protofilament microtubules required for the response to touch, These findings indicate that MAPs of the tau/MAP2/MAP4 family are found thr oughout much of the animal kingdom, where they may play a role in spec ialised processes requiring microtubules.