title: A designed well-folded monomeric four-helix bundle protein prepared by Fmoc solid-phase peptide synthesis and native chemical ligation.
abstract: The design and total chemical synthesis of a monomeric native-like four-helix bundle protein is presented. The designed protein, GTD-Lig, consists of 90 amino acids and is based on the dimeric structure of the de novo designed helix-loop-helix GTD-43. GTD-Lig was prepared by the native chemical ligation strategy and the fragments (45 residues long) were synthesized by applying standard fluorenylmethoxycarbonyl (Fmoc) chemistry. The required peptide-thioester fragment was prepared by anchoring the free gamma-carboxy group of Fmoc-Glu-allyl to the solid phase. After chain elongation the allyl moiety was orthogonally removed and the resulting carboxy group was functionalized with a glycine-thioester followed by standard trifluoroacetic acid (TFA) cleavage to produce the unprotected peptide-thioester. The structure of the synthetic protein was examined by far- and near-UV circular dichroism (CD), sedimentation equilibrium ultracentrifugation, and NMR and fluorescence spectroscopy. The spectroscopic methods show a highly helical and native-like monomeric protein consistent with the design. Heat-induced unfolding was studied by tryptophan absorbance and far-UV CD. The thermal unfolding of GTD-Lig occurs in two steps; a cooperative transition from the native state to an intermediate state and thereafter by noncooperative melting to the unfolded state. The intermediate exhibits the properties of a molten globule such as a retained native secondary structure and a compact hydrophobic core. The thermodynamics of GuHCl-induced unfolding were evaluated by far-UV CD monitoring and the unfolding exhibited a cooperative transition that is well-fitted by a two-state mechanism from the native to the unfolded state. GTD-Lig clearly shows the characteristics of a native protein with a well-defined structure and typical unfolding transitions. The design and synthesis presented herein is of general applicability for the construction of large monomeric proteins.
authors.affiliation: LEDSS 5, Université Joseph Fourier, BP 53, 38041 Grenoble Cedex 9, France. gunnar.dolphin@ujf-grenoble.fr
authors.forename: Gunnar T
authors.initials: GT
authors.lastname: Dolphin
mesh_terms: D019281:Dimerization; D019791:Guanidine / Q000737:chemistry; D018257:Helix-Loop-Helix Motifs; D006358:Hot Temperature; D010455:Peptides / Q000138:chemical synthesis*; D011489:Protein Denaturation; D017510:Protein Folding; D017433:Protein Structure, Secondary; D013816:Thermodynamics; D014461:Ultracentrifugation
chemical_list: D010455:Peptides; D019791:Guanidine