Peptide TZ1C2 can populate two distinct orientations: a staggered (out-of-register) fibril and an aligned (in-register) coiled-coil trimer. The coordination of two cadmium ions induces a registry shift that results in a reversible transition between these structural forms. This process recapitulates the self-assembly mechanism of native protein fibrils in which a ligand binding event gates a reversible conformational transition between alternate forms of a folded peptide structure.