D-TITIN: a giant protein with dual roles in chromosomes and muscles

Previously, we reported that chromosomes contain a giant filamentous protei n, which we identified as titin, a component of muscle sarcomeres. Here, we report the sequence of the entire titin gene in Drosophila melanogaster, D -Titin, and show that it encodes a two-megadalton protein with significant colinear homology to the NH2-terminal half of vertebrate titin. Mutations i n D-Titin cause chromosome undercondensation, chromosome breakage, loss of diploidy, and premature sister chromatid separation. Additionally, D-Titin mutants have defects in myoblast fusion and muscle organization. The phenot ypes of the D-Titin mutants suggest parallel roles for titin in both muscle and chromosome structure and elasticity, and provide new insight into chro mosome structure.