Fig 1: Regulation of muscle contraction by MyBP-C isoforms was determined using recombinant proteins representing the N-terminal region of slow-skeletal, fast-skeletal, and cardiac isoforms of MyBP-C. (A) Schematic diagram of full-length slow-skeletal, fast-skeletal, and cardiac MyBP-C isoforms (ssMyBP-C, fsMyBP-C, and cMyBP-C, respectively). Domains are numbered C0 to C10 from the N-terminus, and proline-alanine region (PA) is common to all isoforms. Circles denote immunoglobulin-like domains, and pentagons represent fibronectin type 3 domains. Yellow lines identify known phosphorylation sites; red lines indicate cardiac-specific insert in C5 domain of cMyBP-C. (B) SDS-PAGE demonstrates the relative size and purity of each MyBP-C recombinant protein, encompassing the N-terminal region, up to and including the C2 domain. (C–F) Permeabilized ventricular rat papillary muscles were left untreated (control, white column) or were incubated with 10 µM ssC1C2 (red), fsC1C2 (blue), and C0C2 (black) and allowed to undergo muscle contraction analysis by the Force-ATPase assay. (C) Relative force-pCa curves and (D) quantification of pCa50 values from the relative force-pCa curves demonstrate significant increases of fsC1C2 and C0C2 in Ca2+-sensitivity of force development. (E) Rate of tension redevelopment (ktr) was determined using a rapid release and restretch maneuver at maximal Ca2+ levels (pCa 4.5) and submaximal Ca2+ levels (pCa 6). Top trace is of (F) at pCa 6, and ktr was significantly enhanced by fsC1C2 and C0C2. Graphs represented as mean ± SEM, *p < 0.05 vs. controls, **p < 0.01 vs. controls, #p < 0.05 vs. ssC1C2/fsC1C2, n = 7–9 animals.