ABSTRACT
Senile systemic amyloidosis (SSA) is caused by amyloid deposits of wild-type transthyretin in various organs. Amyloid deposits from SSA contain large amounts of the C-terminal fragments starting near amino acid residue 50 as well as full-length transthyretin. Although a number of previous studies suggest the importance of the C-terminal fragments in the pathogenesis of SSA, little is known about the structure and aggregation properties of the C-terminal fragments of transthyretin. To understand the role of C-terminal fragments in SSA, we examined the effects of the truncation of the N-terminal portions on the structure and aggregation properties of wild-type transthyretin. The deletion mutant lacking 50 N-terminal residues was largely unfolded in terms of secondary and tertiary structure, leading to self-assembly into spherical aggregations under nearly physiological conditions. By contrast, the deletion mutant lacking 37 N-terminal residues did not have a strong tendency to aggregate, although it also adopted a largely unfolded conformation. These results suggest that global unfolding of transthyretin by proteolysis near amino acid residue 50 is an important step of self-assembly into aggregations in SSA.
Subject(s)
Prealbumin/chemistry , Prealbumin/metabolism , Protein Folding , Sequence Deletion , Amino Acids/genetics , Benzothiazoles , Chromatography, Gel , Circular Dichroism , Fluorescence , Microscopy, Atomic Force , Mutant Proteins/chemistry , Prealbumin/genetics , Protein Structure, Quaternary , Protein Structure, Secondary , Sequence Analysis, Protein , Thiazoles/chemistry , Time FactorsABSTRACT
The secretion of transthyretin (TTR) variants contributes to the pathogenesis of amyloidosis because they form aggregates in the extracellular environment. However, the mechanism of how TTR variants pass the quality control system in the endoplasmic reticulum (ER) has not yet been elucidated. We investigated here the mechanism of how TTR passes ER monitoring. Monomeric mutation introduced in TTRs (M-TTRs) resulted in the ER retention of amyloidogenic M-TTRs but not non-amyloidogenic M-TTRs. Retention of amyloidogenic M-TTRs induced the unfolded protein response and upregulated the expression of ER chaperones BiP and glucose-regulated protein (GRP) 94. Additionally, we showed that the ER-retained amyloidogenic M-TTRs are subject to ER-associated degradation. On the other hand, the amyloidogenic TTR variants and non-amyloidogenic M-TTRs were secreted normally. These findings suggest that unlike for wild-type TTR, the ER quality control system may differentially regulate the fate of the TTR variants and their monomeric counterparts.
