The molecular cloning and characterization of Drosophila melanogaster myosin-IA and myosin-IB

In this paper we describe the isolation and characterization of myosin-IA and myosin-IB, two distinct class I myosins from Drosophila melanogaster. A polymerase chain reaction based strategy using degenerate primers directed against two highly-conserved regions in the head domain of most myosins resulted in the isolation of these two novel myosins-I in addition to a number of previously identified myosins from three Drosophila cDNA libraries. A approximately 3.9 kilobase cDNA clone encoding the putative full-length myosin-IA gene product was isolated from an early embryonic library. Its deduced amino acid sequence predicts a protein of 1011 residues (117,094 Da) with a typical although highly basic myosin head, a neck composed of two IQ motifs, and a unique tail. A approximately 3.4 kilobase cDNA clone encoding the putative full-length myosin-IB gene product was isolated from an adult head library. Its deduced amino acid sequence predicts a protein of 1026 residues (117,741 Da) with a canonical head, three IQ motifs constituting the neck, and a distinct tail. Although both are myosins-I from fly, myosin-IA at cytological locus 31D-F and myosin-IB at cytological locus 61F appear to be more similar to their vertebrate homologs than they are to each other. Primary sequence analyses of both the head and tail domains of the known class I myosins illustrate a division of the metazoan myosin-I family into four distinct subclasses with myosin-IA and myosin-IB as members of two of these groups. Just as the sequence comparisons demonstrate a disparity between myosin-IA and myosin-IB, Northern blot analysis of these two unconventional myosins indicates distinct patterns of temporal expression.