title: The contributions of filaments and cross-bridges to sarcomere compliance in skeletal muscle.
abstract: Force generation in the muscle sarcomere is driven by the head domain of the myosin molecule extending from the thick filament to form cross-bridges with the actin-containing thin filament. Following attachment, a structural working stroke in the head pulls the thin filament towards the centre of the sarcomere, producing, under unloaded conditions, a filament sliding of ∼ 11 nm. The mechanism of force generation by the myosin head depends on the relationship between cross-bridge force and movement, which is determined by compliances of the cross-bridge (C(cb)) and filaments. By measuring the force dependence of the spacing of the high-order myosin- and actin-based X-ray reflections from sartorius muscles of Rana esculenta we find a combined filament compliance (Cf) of 13.1 ± 1.2 nm MPa(-1), close to recent estimates from single fibre mechanics (12.8 ± 0.5 nm MPa(-1)). C(cb) calculated using these estimates is 0.37 ± 0.12 nm pN(-1), a value fully accounted for by the compliance of the myosin head domain, 0.38 ± 0.06 nm pN(-1), obtained from the intensity changes of the 14.5 nm myosin-based X-ray reflection in response to 3 kHz oscillations imposed on single muscle fibres in rigor. Thus, a significant contribution to C(cb) from the myosin tail that joins the head to the thick filament is excluded. The low C(cb) value indicates that the myosin head generates isometric force by a small sub-step of the 11 nm stroke that drives filament sliding at low load. The implications of these results for the mechanism of force generation by myosins have general relevance for cardiac and non-muscle myosins as well as for skeletal muscle.
authors.affiliation: Laboratorio di Fisiologia, Dipartimento di Biologia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy.
authors.forename: Elisabetta
authors.initials: E
authors.lastname: Brunello
authors.affiliation: Laboratorio di Fisiologia, Dipartimento di Biologia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy.
authors.forename: Marco
authors.initials: M
authors.lastname: Caremani
authors.affiliation: Laboratorio di Fisiologia, Dipartimento di Biologia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy.
authors.forename: Luca
authors.initials: L
authors.lastname: Melli
authors.affiliation: Laboratorio di Fisiologia, Dipartimento di Biologia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy.
authors.forename: Marco
authors.initials: M
authors.lastname: Linari
authors.affiliation: European Synchrotron Radiation Facility, BP220, F-38043, Grenoble, France.
authors.forename: Manuel
authors.initials: M
authors.lastname: Fernandez-Martinez
authors.affiliation: European Synchrotron Radiation Facility, BP220, F-38043, Grenoble, France.
authors.forename: Theyencheri
authors.initials: T
authors.lastname: Narayanan
authors.affiliation: Randall Division, King's College London, London, SE1 1UL, UK.
authors.forename: Malcolm
authors.initials: M
authors.lastname: Irving
authors.affiliation: Laboratorio di Fisiologia, Dipartimento di Biologia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy.
authors.forename: Gabriella
authors.initials: G
authors.lastname: Piazzesi
authors.affiliation: Laboratorio di Fisiologia, Dipartimento di Biologia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy vincenzo.lombardi@unifi.it.
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Lombardi
authors.affiliation: Laboratorio di Fisiologia, Dipartimento di Biologia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy Consorzio Nazionale Interuniversitario per le Scienze Fisiche della Materia, UdR Firenze, Italy.
authors.forename: Massimo
authors.initials: M
authors.lastname: Reconditi
mesh_terms: D000199:Actins / Q000737:chemistry / Q000378:metabolism*; D000595:Amino Acid Sequence; D000818:Animals; D008969:Molecular Sequence Data; D009038:Motion; D009119:Muscle Contraction; D009218:Myosins / Q000737:chemistry / Q000378:metabolism*; D011893:Rana esculenta; D012518:Sarcomeres / Q000378:metabolism*
chemical_list: D000199:Actins; D009218:Myosins