An essential cell division gene of Drosophila , absent from Saccharomyces , encodes an unusual protein with tubulin-like and myosin-like peptide motifs

Null mutations at themisato locus ofDrosophila melanogaster are associated with irregular chromosomal segregation at cell division. The consequences for morphogenesis are that mutant larvae are almost devoid of imaginal disk tissue, have a reduction in brain size, and die before the late third-instar larval stage. To analyze these findings, we isolated cDNAs in and around themisato locus, mapped the breakpoints of chromosomal deficiencies, determined which transcript corresponded to themisato gene, rescued the cell division defects in transgenic organisms, and sequenced the genomic DNA. Database searches revealed thatmisato codes for a novel protein, the N-terminal half of which contains a mixture of peptide motifs found in α-, β-, and γ-tubulins, as well as a motif related to part of the myosin heavy chain proteins. The sequence characteristics ofmisato indicate either that it arose from an ancestral tubulin-like gene, different parts of which underwent convergent evolution to resemble motifs in the conventional tubulins, or that it arose by the capture of motifs from different tubulin genes. TheSaccharomyces cerevisiae genome lacks a true homolog of themisato gene, and this finding highlights the emerging problem of assigning functional attributes to orphan genes that occur only in some evolutionary lineages.