A 7.5-kb 3′-Terminal cDNA Sequence of Chicken Skeletal Muscle Nebulin Reveals Its Actin Binding Regions

Muscle Giants: Molecular Scaffolds in Sarcomerogenesis

“…Similarly, a super-repeat should extend ~38.5 nm, the length of a single tropomyosin molecule associated with seven actin monomers in the thin filament (182). Modeling studies as well as experiments with small fragments of nebulin support this idea by demonstrating the repetitive organization of nebulin repeats along the thin filaments, and the direct binding of short sets of repeats with F-actin (162, 163, 221, 301, 302, 350, 389). Modeling of the association of nebulin repeats with F-actin, ultrastructural studies, as well as consideration of the steric requirements placed on its association by the need for tropomyosin and the troponins to regulate actin-myosin binding in response to Ca 2+ , indicate that nebulin is oriented parallel to the long axis of the F-actin filament and the tropomyosin-troponin complex (221, 302, 350).…”

“…Nebulin modules are ~35 amino acids in length. The large majority are contained within the repeats numbered 9–162 and share the sequence SDXXYK, which promotes binding to actin (186, 192, 302, 350). Groups of 7 tandem modules within this region are further organized into 22 “super-repeats,” most of which share the sequence WLKGIGW (186, 192, 350).…”

“…The large majority are contained within the repeats numbered 9–162 and share the sequence SDXXYK, which promotes binding to actin (186, 192, 302, 350). Groups of 7 tandem modules within this region are further organized into 22 “super-repeats,” most of which share the sequence WLKGIGW (186, 192, 350). The order of the super-repeats reflects the pattern of duplication of exons within the NEB gene (see above), which is likely to have occurred by duplication of units of seven repeats, resulting in the preservation not only of the sequence homologies among the repeats and super-repeats but also their exon-intron boundaries (30).…”

“…The pattern of repeats and super-repeats suggested to early investigators that individual 35-amino acid modules can interact with actin and that a group of 7 repeats organized into a super-repeat interact with a stretch of F-actin containing 7 G-actin subunits bound to a tropomyosin-troponin complex (163, 186, 192, 193, 302, 350). Specifically, if nebulin repeats are α -helical, each repeat should extend ~5.5 nm, equivalent to the diameter of a G-actin monomer within the thin filament.…”

Structure of giant muscle proteins

“…Almost 20 years ago, CD and NMR studies showed that the entirety of the nebulin repeat assumes an α-helical conformation (Figure 5) (Pfuhl et al, 1994). Such a structure allows each nebulin repeat to extend ~5.5 nm, which is also the length of the actin monomer within the thin filament (Labeit and Kolmerer, 1995; Suzuki et al, 2000). There are two nebulin molecules associated with each actin polymer, one for each face of the filament.…”