Drosophila D-Titin is required for myoblast fusion and skeletal muscle striation

An ethylmethane sulfonate (EMS) mutagenesis of Drosophila melanogaster aime d at discovering novel genes essential for neuromuscular development identi fied six embryonic lethal alleles of one genetic locus on the third chromos ome at 62C. Two additional lethal P element insertion lines, l(3)S02001 and l(3)j1D7, failed to complement each other and each of the six EMS alleles, Analysis of genomic sequence bracketing the two insertion sites predicted a protein of 16,215 amino acid residues, encoded by a 70 kb genomic region. This sequence includes the recently characterized kettin, and includes all known partial D-Titin sequences. We call the genetic locus, which encodes both D-Titin and kettin, D-Titin. D-Titin has 53 repeats of the immunoglobu lin C2 domain, 6 repeats of the fibronectin type III domain and two large P EVK domains. Kettin appears to be the NH2-terminal one third of D-Titin, pr esumably expressed via alternative splicing. Phenotype assays on the alleli c series of D-Titin mutants demonstrated that D-Titin plays an essential ro le in muscle development. First, D-Titin has an unsuspected function in myo blast fusion during myogenesis and, second, D-Titin later serves to organiz e myofilaments into the highly ordered arrays underlying skeletal muscle st riation. We propose that D-Titin is instrumental in the development of the two defining features of striated muscle: the formation of multi-nucleate s yncitia and the organization of actin-myosin filaments into striated arrays .