Cardiac troponin I (cTnI) is an essential element in activation of myofilaments by Ca2+ binding to cardiac troponin C (cTnC). Yet, its role in transduction of the Ca2+ binding signal to cardiac troponin T (cTnT) and tropomyosin-actin remain poorly understood. We have recently discovered that regions of cTnI C-terminal to a previously defined inhibitory peptide are essential for full inhibitory activity and Ca(2+)-sensitivity of cardiac myofilaments (Rarick et al., 1997). However, apart from its role in structural binding to cTnC, there is little knowledge concerning the role of the N-terminus of cTnI in the activation and regulation of cardiac myofilaments. To address this question, we generated wild-type mouse cardiac TnI (WT-cTnI; 211 residues) and two N-terminal deletion mutants of mouse cTnI, cTnI54-211 (missing 53 residues), and cTnI80-211 (missing 79 residues). The cTnI54-211 mutant retained the ability to bind to cTnT, but lost the ability to bind to cTnC, whereas the cTnI80-211 mutant lost the ability to bind to cTnT, but bound weakly to cTnC. Both mutants bound to F-actin. In the absence of Ca2+, cTnI54-211 was able to inhibit the unregulated MgATPase activity of myofibrils lacking endogenous cTnI-cTnC to the same extent as WT-cTnI, whereas cTnI80-211 had some impairment of its inhibitory capability. Reconstitution with cTnI54-211/cTnC complex did not restore Ca(2+)-activation of myofibrillar MgATPase activity at all, however, the cTnI80-211/cTnC complex restored Ca(2+)-activation to nearly 50% of that obtained with WT-cTnI/cTnC. These data provide the first evidence of a significant function of a cTnT-binding domain on cTnI. They also indicate that the structural cTnC binding site on cTnI is required for Ca(2+)-dependent activation of cardiac myofilaments, and that cTnT binding to the N-terminus of cTnI is a negative regulator of activation.