CRYSTALLIZATION OF A DESIGNED PEPTIDE FROM A MOLTEN GLOBULE ENSEMBLE

Background: The design of amino acid sequences that adopt a desired th ree-dimensional fold has been of keen interest over the past decade, H owever, the design of proteins that adopt unique conformations is stil l a considerable problem, Until very recently, all of the designed pro teins that have been extensively characterized por;sess the hallmarks of the molten globular state. Molten globular intermediates have been observed in both equilibrium and kinetic protein folding/stability stu dies, and understanding the forces that determine compact non-native s tates is critical for a comprehensive understanding of proteins. This paper describes the solution and early solid state characterization of peptides that form molten globular ensembles, Results & Conclusions: Crystals diffracting to 3.5 Angstrom resolution have been grown of a 1 6-residue peptide (alpha(1),A) designed to form a tetramer of alpha-he lices. In addition, a closely related peptide, <alpa>(1), has previous ly been shown to yield crystals that diffract to 1.2 Angstrom resoluti on, The solution properties of these two peptides were examined to det ermine whether their well defined crystalline conformations were retai ned in solution. On the basis of an examination of their NMR spectra, sedimentation equilibria, thermal unfolding, and ANS binding, it is co ncluded that the peptides form alpha-helical aggregates with propertie s similar to those of the molten globule state. Thus, for these peptid es, the process of crystallization bears many similarities to models o f protein folding, Upon dissolution, the peptides rapidly assume compa ct molten globular states similar to the molten globule like intermedi ates that are formed at short times after refolding is initiated. Foll owing a rate-determining nucleation step, the peptides crystallize int o a single or a small number of conformations in a process that mimics the formation of native structure in proteins. (C) Current Biology Lt d