We recently described general principles for designing ideal protein structures stabilized by completely consistent local and nonlocal interactions. The principles relate secondary structure patterns to tertiary packing motifs and enable design of different protein topologies. To achieve fine control over protein shape and size within a particular topology, we have extended the design rules by systematically analyzing the codependencies between the lengths and packing geometry of successive secondary structure elements and the backbone torsion angles of the loop linking them. We demonstrate the control afforded by the resulting extended rule set by designing a series of proteins with the same fold but considerable variation in secondary structure length, loop geometry, β-strand registry, and overall shape. Solution NMR structures of four designed proteins for two different folds show that protein shape and size can be precisely controlled within a given protein fold. These extended design principles provide the foundation for custom design of protein structures performing desired functions.de novo design | protein design | ideal protein | control protein shape P rotein design holds promise for applications ranging from therapeutics to biomaterials, with recent progress in designing small molecule binding proteins (1, 2), inhibitors of protein-protein interactions (3, 4), and self-assembling nanomaterials (5-7). Most of these efforts have repurposed naturally occurring scaffolds, which are likely not optimal starting points for creating new functions because they generally contain sequence and structural idiosyncrasies that arose during evolutionary optimization for their natural functions (8). Robust design of new functional proteins would be considerably enabled by the capability of precisely designing from scratch arbitrary protein structures.We previously described general principles that allowed the de novo design of ideal protein structures with five different folds (9). In this paper, we focus on the "variations on a theme" problem of precisely controlling structural variation within the same fold. To achieve such control, we begin by characterizing the coupling between loop backbone geometry and the packing of the flanking secondary elements. We then use the resulting extended set of design principles to systematically vary structure for two different folds and describe the experimental characterization of five of these de novo designed proteins.
ResultsLocal Structure Building Blocks. The design rules described in our previous paper relate the packing orientation of ββ-, βα-, and αβ-units to the length of the loop connecting them (9). Here, we begin by extending these rules to the level of specific loop conformations to allow more detailed control over local geometry and overall protein topology.It is convenient to describe protein local geometry by using the ABEGO (10) alphabet illustrated in Fig. 1A. "A" indicates the alpha region of the Ramachandran plot (11); "B," the beta region; "G" and "E", the po...