Phosphatase inhibitor-1 (I-1) inhibits the catalytic subunit of protein phosphatase type 1 (PP1c) in its protein kinase A (PKA)-phosphorylated form (I-1(P)). It thereby amplifies PKA signaling, which, in the heart, mediates both beneficial (acute) and adverse (chronic) effects of catecholamines. Genetic deletion of I-1 was associated with protection against catecholamine toxicity, making the PP1c-I-1(P) complex a potential therapeutic target for chronic heart disease. Here, we sought to define targetable interaction sites of I-1 and PP1c, concentrating on the N-terminal domain of I-1 which includes the PP1c binding motif ((9)KIQF(12)) as well as a poly-Arg stretch. Substitution of (9)KIQ(11) residues for analogous amino acids, (9)RLN(11), resulted in doubling of the IC50 values, deletion of (9)KIQF(12) prevented I-1 PKA-dependent phosphorylation and thus activation. Mutation of the Arg residues preceding the PKA phosphorylation site (Thr35) to Ala (R/A(30-33)) abolished I-1 phosphorylation and its binding to and inhibition of PP1c. A series of synthetic peptides (4-11 residues) indicated that the KIQF motif as well as the surrounding anchoring residues was essential for interfering with the inhibitory effect of I-1(P) on PP1c, whereas the four Arg residues were not. Unexpectedly, the most effective nonapeptide (SPRKIQFTV) also antagonized the inhibitory effect of the non-conditional PP1 inhibitor-2 with similar affinity. Incubation of neonatal rat cardiac myocytes with a poly-Arg-modified SPRKIQFTV (10 μM) reduced catecholamine-induced phosphorylation of phospholamban, a well-known PKA downstream target sensitive to PP1c. Our data reiterate the importance of the KIQF motif and provide a tool for antagonizing I-1 inhibitory effects on PP1c, i.e., activating PP1 in vivo.