Urea denaturation studies were carried out as a function of transthyretin (TTR) concentration to quantify the thermodynamically linked quaternary and tertiary structural stability and to better understand the relationship between mutant folding energetics and amyloid disease phenotype. Urea denaturation of TTR involves at least two equilibria-dissociation of tetramers into folded monomers, and monomer unfolding. To deal with the thermodynamic linkage of these equilibria, we analyzed concentration-dependent denaturation data by global fitting to an equation that simultaneously accounts for the two-step denaturation process. Using this method, the quaternary and tertiary structural stabilities of well-behaved TTR sequences, wild type (WT) TTR and the disease-associated variant V122I, were scrutinized. The V122I variant is linked to late onset familial amyloid cardiomyopathy, the most common familial TTR amyloid disease. V122I TTR exhibits a destabilized quaternary structure and a stable tertiary structure relative to WT TTR. Three other variants of TTR were also examined, L55P, V30M, and A25T TTR. The L55P mutation is associated with the most aggressive familial TTR amyloid disease. L55P TTR has a complicated denaturation pathway that includes dimers and trimers, and so globally fitting its concentration-dependent urea denaturation data yielded error-laden estimates of stability parameters. Nevertheless, it is clear that L55P TTR is substantially less stable than WT TTR, primarily because its tertiary structure is unstable, although its quaternary structure is destabilized as well. V30M is the most common mutation associated with neuropathic forms of TTR amyloid disease. V30M TTR is certainly destabilized relative to WT TTR, but like L55P TTR it has a complex denaturation pathway that cannot be fit to the aforementioned two-step denaturation model. Literature data suggest that V30M † This research was supported by NIH Grant DK46335, the Skaggs Institute for Chemical Biology, and the Lita Annenberg Hazen Foundation. A.R.H.B. was supported by NIH Grants T32-AG00080 and F32-GM067348. *To whom correspondence should be addressed. Phone: (858) 784-9601; Fax: (858) 784-9610; E-mail: epowers@scripps.edu, jkelly@scripps.edu. 1 Abbreviations. TTR, transthyretin; WT, wild-type; M-TTR, a monomeric variant of transthyretin harboring F87M and L110M mutations; SSA, senile systemic amyloidosis; FAP, familial amyloid polyneuropathy; FAC, familial amyloid cardiomyopathy; Tris, Tris (hydroxymethyl)aminomethane; EDTA, ethylenediamine-N,N,N′,N′-tetraacetic acid; DTT, dithiothreitol; SDS-PAGE, sodium dodecyl sulfate-polyacrylamide gel electrophoresis; CSF, cerebrospinal fluid.
SUPPORTING INFORMATION AVAILABLEPlots showing the global fit of our three state model (native tetramer, folded monomer, unfolded monomer) to the concentration-dependent urea denaturation data for WT, V122I, and L55P TTR, and a plot showing the relatively poor global fit to a two state model (native tetramer, unfolded monomer) for WT TTR. This material is ...