This process is tightly regulated by the rates of myosin light chain phosphorylation through the myosin light chain kinase and subsequent dephosphorylation via myosin phosphatase (MLCP) 2 (1-6). MLCP is a heterotrimer comprised of a 130-kDa myosin binding subunit (MBS), a 37-kDa protein phosphatase inhibitor catalytic subunit, and a 20-kDa subunit with no ascribed function (1-5). The importance of cGMP-dependent PKG-I␣ in vascular smooth muscle tone is supported by extensive studies demonstrating that PKG-I␣ regulates pathways of NO-dependent vascular relaxation that involve the MBS of MLCP (6). Protein-protein interactions between PKG-I␣ and MLCP are believed to be mediated by the N-terminal leucine zipper (LZ) domain of 59 residues in PKG-I␣ (PKG-I␣ 1-59 ) and C-terminal 180 residues of MBS (MBS CT180 ) in MLCP (1).In previous studies by Atkinson et al. (7), they described the dimer properties of PKG-I␣ (residues 1-39) and determined the structure of the PKG-I␣ monomer within this leucine-zipper dimer. We and others have shown that N-terminal PKG-I␣ is a homodimer in solution (7-9). Recently we have demonstrated that the LZ region of PKG-I␣, PKG-I␣ 1-59 , is indeed a coiled coil (CC) LZ dimer of ϳ38 residues commencing at residue Ala 9 and extending through to Gln 44 on the basis of our 15 N R 1 and R 2 relaxation data (10). Our NMR structure of this region of PKG-I␣ was determined using residual dipolar couplings as structural restraints, whereas the parallel orientation of this homodimer was determined using Prediction of Alignment from Structure (PALES), a method that evaluates the structural charge distribution of the monomer and dimer structures (10).The interaction between PKG-I␣ 1-59 and the CC and/or LZ domain of MBS CT180 has recently been documented by several workers (8,11,12). MBS CT180 is comprised of a predicted bind-

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