# Probing Macromolecular Interactions by Loss of Solvent Content in Protein Crystals: Application to the Amyloid‑β Peptide Binding to Transthyretin

## TL;DR

This study uses protein crystal solvent content to probe weak interactions, revealing how amyloid-beta binds to transthyretin, relevant to Alzheimer's disease.

## Contribution

A novel method using crystal solvent content to study transient macromolecular interactions, specifically applied to amyloid-beta and transthyretin.

## Key findings

- The Aβ12–28 fragment interacts more strongly with TTR than N- and C-terminal fragments.
- Crystal solvent analysis indicates an equimolar interaction between TTR and Aβ12–28.
- Surface-exposed regions like the thyroxine binding pocket are likely involved in the interaction.

## Abstract

X-ray crystallography is commonly used to determine the
structure
of protein–protein complexes, revealing the atomic details
of the interactions between macromolecules in the crystal. However,
this technique has limited application in binary systems characterized
by transient or weak interactions. Here, we demonstrate that protein
crystals can still provide valuable information in such systems by
assessing crystal solvent content. We applied this method to investigate
the interaction between transthyretin (TTR) and the amyloid beta (Aβ)
peptide, a system of interest due to transthyretin’s proposed
role in the clearance of Aβ peptide, whose accumulation in the
brain is associated with Alzheimer’s disease. Soaking TTR crystals
separately with Aβ fragments results in distinct reductions
of the crystal void volume and highlights the key sequence residues
involved in binding to TTR. Our findings indicate that the middle
Aβ12–28 fragment interacts more strongly with
TTR than the N- and C-terminals. Analysis of the crystal packing and
solvent content indicates an equimolar interaction between transthyretin
and the Aβ12–28 peptide. This interaction
likely involves surface-exposed regions of TTR, such as the thyroxine
binding pocket or the dimer pocket.