Human metallothioneins 2 and 3 differentially affect amyloid-beta binding by transthyretin

Transthyretin (TTR), an amyloid-beta (A beta) scavenger protein, and metallothioneins 2 and 3 (MT2 and MT3), low molecular weight metal-binding proteins, have recognized impacts in A beta metabolism. Because TTR binds MT2, an ubiquitous isoform of the MTs, we investigated whether it also interacts with MT3, an isoform of the MTs predominantly expressed in the brain, and studied the role of MT2 and MT3 in human TTR-A beta binding. The TTR-MT3 interaction was characterized by yeast two-hybrid assays, saturation-binding assays, co-immunolocalization and co-immunoprecipitation. The effect of MT2 and MT3 on TTR-A beta binding was assessed by competition-binding assays. The results obtained clearly demonstrate that TTR interacts with MT3 with a K(d) of 373.7 +/- 60.2 nm. Competition-binding assays demonstrated that MT2 diminishes TTR-A beta binding, whereas MT3 has the opposite effect. In addition to identifying a novel ligand for TTR that improves human TTR-A beta binding, the present study highlights the need to clarify whether the effects of MT2 and MT3 in human TTR-A beta binding observed in vitro have a relevant impact on A beta deposition in animal models of Alzheimer's disease.