A Calcium Signaling Cascade Essential for Myosin Thick Filament Assembly in Xenopus Myocytes

Spontaneous calcium release from intracellular stores occurs during myofibrillogenesis, the process of sarcomeric protein assembly in striated muscle. Preventing these Ca²⁺ transients disrupts sarcomere formation, but the signal transduction cascade has not been identified. Here we report that specific blockade of Ca²⁺ release from the ryanodine receptor (RyR) activated Ca²⁺ store blocks transients and disrupts myosin thick filament (A band) assembly. Inhibition of an embryonic Ca²⁺/calmodulin-dependent myosin light chain kinase (MLCK) by blocking the ATP-binding site, by allosteric phosphorylation, or by intracellular delivery of a pseudosubstrate peptide, also disrupts sarcomeric organization. The results indicate that both RyRs and MLCK, which have well-described calcium signaling roles in mature muscle contraction, have essential developmental roles during construction of the contractile apparatus.