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 the misato locus of Drosophila melanogaster are asso ciated with irregular chromosomal segregation at cell division. The co nsequences for morphogenesis are that mutant larvae are almost devoid of imaginal disk tissue, have a reduction in brain size, and die befor e the late third-instar larval stage. To analyze these findings, we is olated cDNAs in and around the misato locus, mapped the breakpoints of chromosomal deficiencies, determined which transcript corresponded to the misato gene, rescued the cell division defects in transgenic orga nisms, and sequenced the genomic DNA, Database searches revealed that misato codes for a novel protein, the N-terminal half of which contain s a mixture of peptide motifs found in alpha-, beta-, and gamma-tubuli ns, as well as a motif related to part of the myosin heavy chain prote ins, The sequence characteristics of misato indicate either that it ar ose from an ancestral tubulin-like gene, different parts of which unde rwent convergent evolution to resemble motifs in the conventional tubu lins, or that it arose by the capture of motifs from different tubulin genes, The Saccharomyces cerevisiae genome lacks a true homolog of th e misato gene, and this finding highlights the emerging problem of ass igning functional attributes to orphan genes that occur only in some e volutionary lineages.