The major microtubule-associated protein (MAP) of sea urchins and several other echinoderms is a polypeptide of M(r) 77,000. The echinoderm MAP (EMAP) is abundant in embryonic and differentiated cells, as well as in mitotic and interphase microtubule arrays. To characterize the molecular structure and function of the EMAP, we isolated a full-length cDNA clone, which has one open reading frame that predicts a polypeptide of 686 amino acids with a calculated M(r) of 75,488. On the basis of charge distribution, EMAP can be divided into two distinctive domains: The NH2-terminal basic region (amino acids 1-137, pI = 10.0) and a slightly acidic, COOH-terminal region (amino acids 138-686, pI = 5.8). This charge distribution is typical of many microtubule-binding proteins, but no significant sequence homology has been detected with any known MAPs. The EMAP, however, does show significant sequence similarity with the beta-subunit of the heterotrimeric G-protein, transducin. The homology lies in a series of 10 imperfect, 43-amino acid repeats (WD-40 repeats) that have been found in many proteins of diverse functions, including beta-transducins, Drosophila Enhancer of split, the yeast STE4, CDC4, CDC20, PRP4, and Tup1 gene products, and the dTAFII80 subunit of Drosophila TFIID. The function of these repeats still remains unknown. It is possible that these repeats are involved in protein-protein interactions, perhaps with the tetratricopeptide repeat-containing protein family. Alternatively, the EMAP may be an important link between signal transduction events and a change in microtubule organization during the cell cycle.