Conformational Flexibility of a Human Immunoglobulin Light Chain Variable Domain by Relaxation Dispersion Nuclear Magnetic Resonance Spectroscopy: Implications for Protein Misfolding and Amyloid Assembly

The conformational flexibility of a human immunoglobulin κIV light-chain variable domain, LEN, which can undergo conversion to amyloid under destabilizing conditions, was investigated at physiological and acidic pH on a residue-specific basis by multidimensional solution-state nuclear magnetic resonance (NMR) methods. Measurements of backbone chemical shifts and amide ¹⁵N longitudinal and transverse spin relaxation rates and steady-state nuclear Overhauser enhancements indicate that, on the whole, LEN retains its native three-dimensional fold and dimeric state at pH 2 and that the protein backbone exhibits limited fast motions on the picosecond to nanosecond time scale. On the other hand, ¹⁵N Carr–Purcell–Meiboom–Gill (CPMG) relaxation dispersion NMR data show that LEN experiences considerable slower, millisecond time scale dynamics, confined primarily to three contiguous segments of about 5–20 residues and encompassing the N-terminal β-strand and complementarity determining loop regions 2 and 3 in the vicinity of the dimer interface. Quantitative analysis of the CPMG relaxation dispersion data reveals that at physiological pH these slow backbone motions are associated with relatively low excited-state protein conformer populations, in the ∼2–4% range. Upon acidification, the minor conformer populations increase significantly, to ∼10–15%, with most residues involved in stabilizing interactions across the dimer interface displaying increased flexibility. These findings provide molecular-level insights about partial protein unfolding at low pH and point to the LEN dimer dissociation, initiated by increased conformational flexibility in several well-defined regions, as being one of the important early events leading to amyloid assembly.