title: Rise of the Helix from a Collapsed Globule during the Folding of Monellin.
abstract: Early kinetic intermediates observed during the folding of many proteins are invariably compact and appear to possess some secondary structure. Consequently, it has been difficult to understand whether compaction drives secondary structure formation or secondary structure formation facilitates compaction during folding. In this study of the folding of single-chain monellin, it is shown that a kinetic molten globule (MG) is populated at 2 ms of folding. Far-UV circular dichroism (CD) measurements show that the kinetic MG is devoid of any helical structure even under the most stabilizing folding conditions. Multisite fluorescence resonance energy transfer (FRET) measurements show that the kinetic MG is compact with different segments having contracted to different extents. It is shown that the sequence segment that goes on to form the sole helix in the native protein is fully collapsed in the kinetic MG. This segment expands to accommodate the helix as the kinetic MG folds further to the native state, while other segments of the protein contract. Helix formation starting from the kinetic MG is shown to occur in multiple kinetic steps, whether measured by far-UV CD or by FRET.
authors.affiliation: National Centre for Biological Sciences, Tata Institute of Fundamental Research , Bengaluru 560065, India.
authors.forename: Rama Reddy
authors.initials: RR
authors.lastname: Goluguri
authors.affiliation: National Centre for Biological Sciences, Tata Institute of Fundamental Research , Bengaluru 560065, India.
authors.forename: Jayant B
authors.initials: JB
authors.lastname: Udgaonkar
mesh_terms: D019943:Amino Acid Substitution; D002942:Circular Dichroism; D003545:Cysteine / Q000737:chemistry; D031541:Fluorescence Resonance Energy Transfer; D057927:Hydrophobic and Hydrophilic Interactions; D007700:Kinetics; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D010940:Plant Proteins / Q000737:chemistry* / Q000235:genetics; D011487:Protein Conformation; D017510:Protein Folding; D055550:Protein Stability; D017433:Protein Structure, Secondary; D013050:Spectrometry, Fluorescence
chemical_list: D010940:Plant Proteins; C002541:monellin protein, Dioscoreophyllum cumminsii; D003545:Cysteine