The N-terminal region of skeletal myosin light chain-1 (MLC-1) binds to the C terminus of actin, yet the functional significance of this interaction is unclear. We studied a fragment (MLC-pep; residues 5-14) of the ventricular MLC-1. When added to rat cardiac myofibrils, 10 nM MLC-pep induced a supramaximal increase in the MgATPase activity at submaximal Ca2+ levels with no effect at low and maximal Ca2+ levels. A nonsense, scrambled sequence peptide had no effect at any pCa value. MLC-pep did not affect myosin KEDTA and CaATPase activities or actin-activated MgATPase activities in the absence or presence of tropomyosin. The MLC-pep did not alter the ability of troponin I to inhibit MgATPase activity. Moreover, when troponin I and troponin C were extracted from the myofibrils, the MLC-pep lost its ability to stimulate the ATPase rate. This effect was fully restored upon reconstitution of the extracted myofibrils with troponin I-troponin C complex. Thus, activation of MgATPase activity by the peptide required a full complement of thin filament regulatory proteins. Interestingly, the stimulatory effect occurred at a ratio of 4 peptides to 1 thin filament, suggesting that the peptide engages in a highly cooperative process that may involve activation of the entire thin filament.