SKELETAL MyBP-C ISOFORMS TUNE THE MOLECULAR CONTRACTILITY OF DIVERGENT SKELETAL MUSCLE SYSTEMS

Abstract

Skeletal muscle myosin-binding protein C (MyBP-C) is a myosin thick filament-associated protein; localized through its C terminus to distinct regions (C-zones) of the sarcomere. MyBP-C modulates muscle contractility, presumably through its N terminus extending from the thick filament and interacting with either the myosin head region and/or the actin thin filament. Two isoforms of MyBP-C (fast- and slow-type) are expressed in a muscle-type specific manner. Are the expression, localization, and Ca2+-dependent modulatory capacities of these isoforms different in fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (SOL) muscles derived from Sprague-Dawley rats? By mass spectrometry, four MyBP-C isoforms (one fast-type MyBP-C and three N-terminally spliced slow-type MyBP-C) were expressed in EDL but only the three slow-type MyBP-C isoforms in SOL. Using EDL and SOL native thick filaments in which the MyBP-C stoichiometry and localization are preserved, native thin filament sliding over these thick filaments showed that only in the C-zone, MyBP-C Ca2+-sensitizes the thin filament and slows thin filament velocity. These modulatory properties depended on MyBP-Cs N-terminus, as N-terminal proteolysis attenuated MyBP-Cs functional capacities. To determine each MyBP-C isoforms contribution to thin filament Ca2+-sensitization and slowing in the C-zone, we used a combination of in vitro motility assays using expressed recombinant N-terminal fragments and in silico mechanistic modeling. Our results suggest that each skeletal MyBP-C isoforms N terminus is functionally distinct and has modulatory capacities that depend on the muscle-type in which they are expressed, providing the potential for molecular tuning of skeletal muscle performance through differential MyBP-C expression.\n\nSIGNIFICANCEMyosin-binding protein C (MyBP-C) is a critical component of the skeletal muscle sarcomere, muscles smallest contractile unit. MyBP-Cs importance is evident by genetic mutations leading to human myopathies, such as distal arthrogryposis (i.e. club foot). However, the molecular basis of MyBP-Cs functional impact on skeletal muscle contractility is far from certain. Complicating matters further is the expression of fast- and slow-type MyBP-C isoforms that depend on whether the muscle is fast- or slow-twitch. Using multi-scale proteomic, biophysical and mathematical modeling approaches, we define the expression, localization, and modulatory capacities of these distinct skeletal MyBP-C isoforms in rat skeletal muscles. Each MyBP-C isoform appears to modulate muscle contractility differentially; providing the capacity to fine-tune muscles mechanical performance as physiological demands arise.