Molecular Cloning and Domain Structure of Human Myosin-VIIa , the Gene Product Defective in Usher Syndrome 1 B

sin, VIIa and VIIb, were first identified in a human Myosin-VIIa is an unconventional myosin with relaintestinal epithelium cell line, on the basis of partial tively restricted expression. Cloned first from an intesamino acid sequence of the conserved head domain tinal epithelium cell line, it occurs most notably in the (Bement et al., 1994). A portion of the myosin-VIIa testis, in the receptor cells of the inner ear, and in the head domain was also cloned from the sensory epithepigment epithelium of the retina. Defects in myosinlium of the bullfrog inner ear (Solc et al., 1994). VIIa cause the shaker-1 phenotype in mice and Usher Larger portions of the mouse and human myosinsyndrome 1B in human, which are characterized by VIIa sequences were described when it was found deafness, lack of vestibular function, and (in human) that defects in myosin-VIIa cause the mouse shakerprogressive retinal degeneration. Because the de1 phenotype and the human Usher Syndrome 1B, scribed cDNAs encode less than half of the protein prewhich are characterized by deafness, lack of vestibudicted from immunoblots, we have cloned cDNAs enlar function, and (in human) progressive retinal decoding the rest of human myosin-VIIa. Two transcripts generation (Gibson et al., 1995; Weil et al., 1995). A were found, one encoding the predicted 250-kDa prostill larger portion of the human sequence was detein and another encoding a shorter form. Both transcribed by Hasson et al. (1995), who also used a spescripts were found in highest abundance in testis, alcific antibody to the tail domain to characterize the though the shorter transcript was much less abundant. protein. Immunoblots indicated that myosin-VIIa is Both could be detected in lymphocytes by RT-PCR. The myosin tail encoded by the long transcript ina protein of molecular weight 230–250 kDa which is cludes a long repeat of Ç460 amino acids. Each repeat expressed most highly in testis but also in the retina contains a novel ‘‘MyTH4’’ domain similar to domains and the cochlea. Immunocytochemical labeling in three other myosins, and a domain similar to the showed that myosin-VIIa is expressed in the receptor membrane-associated portion of talin and other memcells of the inner ear and the pigment epithelium bers of the band-4.1 family. q 1996 Academic Press, Inc. cells of the retina, consistent with the sensory deficits of Usher syndrome 1B (Hasson et al., 1995). The published sequence still accounts for less than INTRODUCTION half of the predicted coding sequence of myosin-VIIa. Whereas the ‘‘motor’’ head domain is like other memMyosin-VII is a member of the myosin superfamily, bers of the superfamily, containing the conserved actinwhich now includes 11 or more different branches and ATP-binding regions, the uncharacterized portion (Cheney and Mooseker, 1992; Hasson and Mooseker, includes most of the unique tail domain that gives the 1995). It is considered an ‘‘unconventional’’ myosin, molecule its specificity. To understand the interaction distinct from the conventional myosin-II family origiof myosin-VIIa with other proteins, specific probes nally characterized in muscle. Two forms of this myomust be generated from unique segments of the tail. In addition, gene defects in the head domain have been 1 To whom correspondence should be addressed at WEL414, Masfound in fewer than half of the families with Usher 1B. sachusetts General Hospital, Boston, MA 02114. Telephone: (617) 726-6147. Fax: (617) 726-5256. Additional defects in the tail may be expected in the