Department of Cell Biology and Anatomy, University of Arizona, 1501 N. Campbell Avenue, Tucson, AZ 85724, USA.
Nebulin is a giant, modular sarcomeric protein and although it was discovered over 2 decades ago, it remains one of the most nebulous components of striated muscle. Previously, several groups identified nebulin as the prime candidate molecule for functioning as a "ruler" to specify the precise lengths of the actin (thin) filaments in skeletal muscle, yet this proposal has never been proven. This article reviews the evidence implicating nebulin as a thin filament ruler, including the most recent studies highlighting its potentially extensive isoform diversity and exciting reports revealing its expression in cardiac tissue. Also examined are novel findings indicating that nebulin is actually a multifunctional filament system, perhaps playing roles in signal transduction, contractile regulation, and myofibril force generation; these ideas are especially intriguing given the growing number of mutations in this giant molecule that are associated with human myopathies.