Novel Myosin VI Isoform is Abundantly Expressed in Retina

Several forms of sensory deficit have been associated with unconventional myosin defects in humans and other animals. Normal hearing in mammals has been shown to require functional myosin VI (Avraham et al., 1995) and myosin VIIA (Gibson et al., 1995; Liu et al., 1997), and the combined blindness and deafness of Usher syndrome type IB has been shown to be produced by specific defects in myosin VIIA (Weil et al., 1997). Here we report the cloning and characterization of two distinct myosin VI isoforms (FMVIA and FMVIB) initially identified in a degenerate PCR screen of retinal cDNA from the striped bass, Morone saxatilis. Open reading frames for FMVIA and FMVIB encode predicted proteins of 1304 and 1270 amino acids respectively, which are 83% identical at the amino acid level. Both fish isoforms are likewise approximately 83-86% identical to mammalian class VI myosins (Hasson and Mooseker, 1994). Northern blot analysis revealed that FMVIA mRNA is broadly expressed and most abundant in kidney, a pattern similar to that previously reported for mammalian myosin VI. FMVIB expression is dramatically more abundant in retina than in any other tissue examined. Antibodies directed against pig myosin VI (Hasson and Mooseker, 1994) detect a doublet at approximately 150 kDa in bass retina and RPE. Since both fish VIA and VIB isoforms share high sequence identity with pig myosin VI within the domain used for antibody production, it seems likely that this antibody crossreacts with both FMVIA and FMVIB. Immunocytochemistry with this same affinity-purified rabbit anti-myosin VI antibody shows that myosin VI isoforms are primarily localized in photoreceptors, horizontal cells and Müller cells in both fish and primate retinas. This report is the first demonstration that two myosin VI genes are expressed in the same organism and the same cell type (RPE). The relatively high abundance of FMVIB expression in retina suggests that it may play an important role in retinal motility events.