Structural insights into a zinc-dependent pathway leading to Leu55Pro transthyretin amyloid fibrils

Human transthyretin (TTR) is a homotetrameric protein that is responsible for the formation of amyloid in patients with familiar amyloidotic polyneuropathy (FAP), familiar amyloid­otic cardiomyopathy (FAC) and senile systemic amyloidosis (SSA). Amyloid fibrils are characterized by a cross-β structure. However, details of how TTR monomers are organized to form such an assembly remain unknown. The effect of Zn²⁺ in increasing TTR L55P amyloidogenecity has been reported. Crystals of the TTR L55P–Zn²⁺ complex were grown under conditions similar to those leading to higher amyloidogenic potential of the variant protein and the three-dimensional structure of the complex was determined by X-­ray crystallo­graphy. Two different tetrahedral Zn²⁺-binding sites were identified: one cross-links two tetramers, while the other lies at the interface between two monomers in a dimer. The association of monomers involving the two Zn²⁺-binding sites leads to a bidimensional array with a cross-β structure. The formation of this structure and subsequent organization into amyloid fibrils was monitored by fluorescence spectroscopy and electron microscopy. The TTR L55P–Zn²⁺ structure offers the first molecular insights into the role of Zn²⁺ as a mediator of cross-β-type structure in TTR amyloidosis and the relevance of a Zn²⁺-dependent pathway leading to the production of early amyloidogenic intermediates is discussed.