Molecular cloning and domain structure of human myosin-VIIa, the gene product defective in Usher syndrome 1B.

Myosin-VIIa is an unconventional myosin with relatively restricted expression. Cloned first from an intestinal epithelium cell line, it occurs most notably in the testis, in the receptor cells of the inner ear, and in the pigment epithelium of the retina. Defects in myosin-VIIa cause the shaker-1 phenotype in mice and Usher syndrome 1B in human, which are characterized by deafness, lack of vestibular function, and (in human) progressive retinal degeneration. Because the described cDNAs encode less than half of the protein predicted from immunoblots, we have cloned cDNAs encoding the rest of human myosin-VIIa. Two transcripts were found, one encoding the predicted 250-kDa protein and another encoding a shorter form. Both transcripts were found in highest abundance in testis, although the shorter transcript was much less abundant. Both could be detected in lymphocytes by RT-PCR. The myosin tail encoded by the long transcript includes a long repeat of approximately 460 amino acids. Each repeat contains a novel "MyTH4" domain similar to domains in three other myosins, and a domain similar to the membrane-associated portion of talin and other members of the band-4.1 family.