The complete primary structure of human nebulin and its correlation to muscle structure

Nebulin is a giant filamentous protein specific for vertebrate skeletal muscles. The correlation of its size to thin filament lengths in vertebrates suggests that nebulin may function as a molecular ruler to determine thin filament length. We have isolated a full-length cDNA of 20.8 kb encoding human nebulin and determined its sequence. The cDNA's predicted peptide has a molecular weight of 773 kDa, and 97% of its mass consists of 185 copies of -35-residue module. Within the molecule, different sub-families of modules can be distinguished, and their arrangement is correlated to the structure of the thin filament. The central 154 copies are grouped into 22 seven-module super repeats corresponding to 38.5 nm thin filament repeats. In the thin filament ruler region, multiple isoforms are generated by alternative exon usage which is likely to explain the developmental and tissue-specific size variations of nebulins previously found in vertebrate skeletal muscles. We propose that different types of nebulin molecular rulers are expressed in the different types of skeletal muscles by differential splicing. Outside the super repeat region, the presence of distinct module arrangements implies functional diversity of the nebulin module family. A novel "simple repeat" family together with an SH3 domain at the C-terminus appear to anchor the nebulin filament system in the Z-disc. Nebulin's SH3 domain is highly related in sequence to the SH3 domains in yeast actin binding protein ABP-1 and to the src substrate p80/85 in chicken, both proteins which are involved in regulating actin assembly of the cytoskeleton in non-muscle cells. Study of nebulins terminal sequences is likely to reveal how integration of the nebulin filament into the sarcomere is regulated.