Calmodulin-sensitive interaction of human nebulin fragments with actin and myosin

Nebulin is a giant protein ruler of the actin filament of skeletal muscle. This modular protein primarily consists of a repeating sequence (module) of 35 residues and a superrepeat of seven modules. These modules are thought to be actin binding domains along the length of nebulin. Cloned human nebulin fragments of 7-8 modules bind with high affinity to calmodulin, actin, myosin, and myosin head. The stoichiometry of high-affinity binding is approximately one actin monomer bound per nebulin fragment and one myosin bound per nebulin fragment, as determined by cosedimentation binding assays. These observations raise the intriguing possibility that nebulin might have regulatory functions on actomyosin interactions. Nebulin fragments from the N-terminal half situated in the actomyosin overlap region of the sarcomere inhibit actomyosin ATPase activity and the sliding velocity of actin over myosin in motility assays, while a nebulin fragment near the C-terminus, which is localized to the Z-line, does not prevent actin sliding. Calmodulin reverses the inhibition of ATPase and accelerates actin sliding in calcium-dependent manner. Calmodulin with calcium greatly reduces the binding of nebulin fragments to both actin and myosin. The data suggest that the nebulin can interact with both actin and myosin in a calcium/calmodulin-dependent manner and might have regulatory functions in skeletal muscle contraction in a fashion analogous to caldesmon in smooth muscle.