Solution structure and dynamics of a de novo designed three-helix bundle protein

Although de novo protein design is an important endeavor with implications for understanding protein folding, until now, structures have been determin ed for only a few 25- to 30-residue designed miniproteins. Acre, the NMR so lution structure of a complex 73-residue three-helix bundle protein, alpha( 3)D, is reported. The structure of alpha(3)D was not based on any natural p rotein, and yet it shows thermodynamic and spectroscopic properties typical of native proteins. A variety of features contribute to its unique structu re, including electrostatics, the packing of a diverse set of hydrophobic s ide chains, and a loop that incorporates common capping motifs. Thus, it is now possible to design a complex protein with a well defined and predictab le three-dimensional structure.