title: The N-terminal end of nebulin interacts with tropomodulin at the pointed ends of the thin filaments.
abstract: Strict regulation of actin thin filament length is critical for the proper functioning of sarcomeres, the basic contractile units of myofibrils. It has been hypothesized that a molecular template works with actin filament capping proteins to regulate thin filament lengths. Nebulin is a giant protein ( approximately 800 kDa) in skeletal muscle that has been proposed to act as a molecular ruler to specify the thin filament lengths characteristic of different muscles. Tropomodulin (Tmod), a pointed end thin filament capping protein, has been shown to maintain the final length of the thin filaments. Immunofluorescence microscopy revealed that the N-terminal end of nebulin colocalizes with Tmod at the pointed ends of thin filaments. The three extreme N-terminal modules (M1-M2-M3) of nebulin bind specifically to Tmod as demonstrated by blot overlay, bead binding, and solid phase binding assays. These data demonstrate that the N terminus of the nebulin molecule extends to the extreme end of the thin filament and also establish a novel biochemical function for this end. Two Tmod isoforms, erythrocyte Tmod (E-Tmod), expressed in embryonic and slow skeletal muscle, and skeletal Tmod (Sk-Tmod), expressed late in fast skeletal muscle differentiation, bind on overlapping sites to recombinant N-terminal nebulin fragments. Sk-Tmod binds nebulin with higher affinity than E-Tmod does, suggesting that the Tmod/nebulin interaction exhibits isoform specificity. These data provide evidence that Tmod and nebulin may work together as a linked mechanism to control thin filament lengths in skeletal muscle.
authors.affiliation: Department of Cell Biology and Anatomy, University of Arizona, Tucson, Arizona 85724, USA.
authors.forename: A S
authors.initials: AS
authors.lastname: McElhinny
authors.forename: B
authors.initials: B
authors.lastname: Kolmerer
authors.forename: V M
authors.initials: VM
authors.lastname: Fowler
authors.forename: S
authors.initials: S
authors.lastname: Labeit
authors.forename: C C
authors.initials: CC
authors.lastname: Gregorio
mesh_terms: D008841:Actin Cytoskeleton / Q000737:chemistry* / Q000378:metabolism; D000199:Actins / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D019921:Biotinylation; D002352:Carrier Proteins / Q000378:metabolism*; D005455:Fluorescent Antibody Technique; D006801:Humans; D008840:Microfilament Proteins / Q000378:metabolism*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009124:Muscle Proteins / Q000737:chemistry* / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000378:metabolism; D010446:Peptide Fragments / Q000737:chemistry / Q000378:metabolism; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D051381:Rats; D011994:Recombinant Proteins / Q000378:metabolism; D012996:Solutions; D013379:Substrate Specificity; D051348:Tropomodulin; D014335:Tropomyosin / Q000378:metabolism
chemical_list: D000199:Actins; D002352:Carrier Proteins; D008840:Microfilament Proteins; D009124:Muscle Proteins; D010446:Peptide Fragments; D011994:Recombinant Proteins; D012996:Solutions; C495829:TMOD1 protein, human; C495832:Tmod1 protein, rat; D051348:Tropomodulin; D014335:Tropomyosin; C035620:nebulin