Previously, we have described a miniature protein-based approach to the design of molecules that bind DNA or protein surfaces with high affinity and specificity. In this approach, the small, wellfolded protein avian pancreatic polypeptide acts as a scaffold to present and stabilize an R-helical or PPII-helical recognition epitope. The first miniature protein designed in this way, a molecule called p007, presents the R-helical recognition epitope found on the bZIP protein GCN4 and binds DNA with nanomolar affinity and exceptional specificity. In this work we use alanine-scanning mutagenesis to explore the contributions of 29 p007 residues to DNA affinity, specificity, and secondary structure. Virtually every residue within the p007 R-helix, and most residues within the p007 PPII helix, contribute to both DNA affinity and specificity. These residues include those introduced to make specific and nonspecific DNA contacts, as well as those that complete the miniature protein core. Moreover, there exists a direct correlation between the affinity of a p007 variant for specific DNA and the ability of that variant to select for specific DNA over nonspecific DNA. Although we observe no correlation between R-helicity and affinity, we observe a limited correlation between R-helicity and sequence specificity that emphasizes the role of coupled binding/folding in the function of p007. Our results imply that formation of a highly evolved set of protein‚DNA contacts in the context of a well-packed hydrophobic core, and not the extent of intrinsic R-helical structure, is the primary determinant of p007 function.The development of general strategies for the selective recognition of macromolecular targets remains a major challenge for chemical biology and a fundamental postgenome goal. Molecules capable of tight and selective macromolecular recognition have utility in the validation of potential therapeutic targets and can, in certain cases, function as therapeutics in their own right (1-3). Our laboratory has developed a general approach toward the design of small, well-folded proteins that bind macromolecular targets with high affinity and selectivity (4-11). This approach is often referred to as protein grafting, and the molecules that result are called miniature proteins: miniature because they contain fewer than 40 amino acids and proteins because they often fold cooperatively. In a protein grafting experiment, those residues that comprise a natural R-helical or type II polyproline (PPII)-helical recognition epitope are introduced onto the solvent-exposed R-or PPII-helical face of the small yet stable protein avian pancreatic polypeptide (aPP) 1 (12, 13). Our laboratory has used this procedure, often in combination with directed evolution, to engineer miniature protein ligands with high affinity for a variety of targets including the duplex DNAs recognized by GCN4 and CREB (4, 5) and the Q50K engrailed homeodomain (7), the antiapoptotic proteins Bcl-X L (6) and Bcl-2 (10), the oncoprotein hDM2 (manuscript in prepara...