Identification and localisation of nebulin as a thin filament component ofinvertebrate chordate muscles

The giant actin-binding protein nebulin is regarded as a component of the t hin filaments in vertebrate skeletal muscles, whereas the existence of nebu lin in invertebrate muscles has not yet been demonstrated. Using the cross- reactivities of polyclonal antibodies raised against nebulin from muscles o f trout and lamprey. we were able to identify nebulin in the myofibrils of the cephalochordate Branchiostoma lanceolatum (lancelet) by immunoblot and immunofluorescence techniques. The similar to 720-kDa protein is localised in the I-bands of the sarcomere, where vertebrate nebulin has previously al so been shown to br localised. Since lancelets have a phylogenetically key position at the vertebrate/invertebrate boundary, the detection of a high-m olecular-weight nebulin indicates that nebulin-like proteins may be common to striated muscles in all chordates and increases the probability that non -chordate invertebrates also possess nebulin-related proteins.