Atypical β-strand insertion mediates the noncovalent cross-linking in amyloid aggregates.

β-Strand motifs are essential recognition modules in protein-protein interactions (PPIs), which govern cellular signaling networks and regulate molecular pathway dynamics. Herein we present an unexpected discovery of a previously uncharacterized β-strand insertion mechanism termed as cross-β-strand linking, wherein β-strands within the β-sheet-rich aggregates form inter-β-sheet connections through insertion into adjacent β-sheets. These cross-β-strand linkers comprise <15% of the total β-strands in the amyloidogenic aggregates, but they can mediate a significant proportion of intermolecular interactions, operating as dynamic molecular adapters that regulate the inter-β-sheet packing geometry. Crucially, these linkers exist as conformational ensembles of heterogeneous substates, bestowing remarkable structural diversity to the aggregates. Through promiscuous engagement with multiple conformational substates, cross-β-strand linkers enable the aggregates to balance order and disorder. In this work, we provide a perspective on how low-abundance structural elements can orchestrate complex molecular architectures in assembly systems.