Pappas et al. demonstrate that the giant disease-related protein nebulin stabilizes actin filaments to control their length in skeletal muscle.
Nebulin is big enough to stretch the entire length of muscle thin filaments, binding individual actin subunits via a series of repeated domains. Alternative splicing produces nebulin molecules in various sizes that match the filament lengths of different muscle tissues, suggesting that nebulin acts as a “molecular ruler,” setting filament length by binding a defined number of actin monomers. The protein's size has hampered efforts to test its function directly, so Pappas et al. synthesized a truncated “mini-nebulin” to replace the longer version.
Muscle cells expressing mini-nebulin contained filaments that were longer than mini-nebulin itself, indicating that the protein doesn't measure out filament length directly. Instead, nebulin regulates thin filament size by stabilizing them: both full-length and mini-nebulin protected filaments from the depolymerizing drug latrunculin A. Filaments never depolymerized to lengths shorter than mini-nebulin, suggesting that the protein binds the filaments to set their minimum size. But photobleaching experiments revealed that mini-nebulin also stabilized filament ends not directly bound by the protein, allowing the filaments to grow longer still.
Senior author Carol Gregorio now wants to investigate how mini-nebulin stabilizes parts of the actin filament it has no contact with. The shortened protein will also enable studies of nebulin mutations that cause nemaline myopathy, a human disease characterized by protein aggregates and muscle weakness, and sometimes short, thin filaments.