The mature human immunodeficiency virus type 1 protease rapidly folds into an enzymatically active stable dimer, exhibiting an intricate interplay between structure formation and dimerization. We now show by NMR and sedimentation equilibrium studies that a mutant protease containing the R87K substitution (PR(R87K)) within the highly conserved Gly(86)-Arg(87)-Asn(88) sequence forms a monomer with a fold similar to a single subunit of the dimer. However, binding of the inhibitor DMP323 to PR(R87K) produces a stable dimer complex. Based on the crystal structure and our NMR results, we postulate that loss of specific interactions involving the side chain of Arg(87) destabilizes PR(R87K) by perturbing the inner C-terminal beta-sheet (residues 96-99 from each monomer), a region that is sandwiched between the two beta-strands formed by the N-terminal residues (residues 1-4) in the mature protease. We systematically examined the folding, dimerization, and catalytic activities of mutant proteases comprising deletions of either one of the terminal regions (residues 1-4 or 96-99) or both. Although both N- and C-terminal beta-strands were found to contribute to dimer stability, our results indicate that the inner C-terminal strands are absolutely essential for dimer formation. Knowledge of the monomer fold and regions critical for dimerization may aid in the rational design of novel inhibitors of the protease to overcome the problem of drug resistance.