Ligand-modulated Parallel Mechanical Unfolding Pathways of Maltose-binding Proteins

• Published in: The Journal of biological chemistry Vol. 286; no. 32; pp. 28056 - 28065
• Main Authors: Aggarwal, Vasudha, Kulothungan, S. Rajendra, Balamurali, M.M., Saranya, S.R., Varadarajan, Raghavan, Ainavarapu, Sri Rama Koti
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 12.08.2011; American Society for Biochemistry and Molecular Biology
• Subjects: Atomic Force Microscopy; Carrier Proteins - chemistry; Carrier Proteins - genetics; Carrier Proteins - metabolism; Escherichia coli - chemistry; Escherichia coli - genetics; Escherichia coli - metabolism; Escherichia coli Proteins - chemistry; Escherichia coli Proteins - genetics; Escherichia coli Proteins - metabolism; Force Spectroscopy; Kinetic Partitioning; Ligand-binding Protein; Ligands; Maltose - chemistry; Maltose - metabolism; Maltose-binding Protein; Parallel Pathways; Protein Conformation; Protein Folding; Protein Precursors - chemistry; Protein Precursors - genetics; Protein Precursors - metabolism; Protein Structure and Folding; Protein Structure, Tertiary; Protein Unfolding; Single Molecule Biophysics; Ligand-binding Protein; Parallel Pathways; Maltose-binding Protein; Protein Unfolding; Kinetic Partitioning; Single Molecule Biophysics; Protein Conformation; Force Spectroscopy; Atomic Force Microscopy; Protein Folding
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M111.249045

Protein folding and unfolding are complex phenomena, and it is accepted that multidomain proteins generally follow multiple pathways. Maltose-binding protein (MBP) is a large (a two-domain, 370-amino acid residue) bacterial periplasmic protein involved in maltose uptake. Despite the large size, it has been shown to exhibit an apparent two-state equilibrium unfolding in bulk experiments. Single-molecule studies can uncover rare events that are masked by averaging in bulk studies. Here, we use single-molecule force spectroscopy to study the mechanical unfolding pathways of MBP and its precursor protein (preMBP) in the presence and absence of ligands. Our results show that MBP exhibits kinetic partitioning on mechanical stretching and unfolds via two parallel pathways: one of them involves a mechanically stable intermediate (path I) whereas the other is devoid of it (path II). The apoMBP unfolds via path I in 62% of the mechanical unfolding events, and the remaining 38% follow path II. In the case of maltose-bound MBP, the protein unfolds via the intermediate in 79% of the cases, the remaining 21% via path II. Similarly, on binding to maltotriose, a ligand whose binding strength with the polyprotein is similar to that of maltose, the occurrence of the intermediate is comparable (82% via path I) with that of maltose. The precursor protein preMBP also shows a similar behavior upon mechanical unfolding. The percentages of molecules unfolding via path I are 53% in the apo form and 68% and 72% upon binding to maltose and maltotriose, respectively, for preMBP. These observations demonstrate that ligand binding can modulate the mechanical unfolding pathways of proteins by a kinetic partitioning mechanism. This could be a general mechanism in the unfolding of other large two-domain ligand-binding proteins of the bacterial periplasmic space.