Metal ions and redox balance regulate distinct amyloid-like aggregation pathways of GAPR-1

Members of the CAP superfamily (Cysteine-rich secretory proteins, Antigen 5, and Pathogenesis-Related 1 proteins) are characterized by the presence of a structurally conserved CAP domain. The common structure-function relationship of this domain is still poorly understood. In this study, we unravel specific molecular mechanisms modulating the quaternary structure of the mammalian CAP protein GAPR-1 (Golgi-Associated plant Pathogenesis-Related protein 1). Copper ions are shown to induce a distinct amyloid-like aggregation pathway of GAPR-1 in the presence of heparin. This involves an immediate shift from native multimers to monomers which are prone to form amyloid-like fibrils. The Cu(2+)-induced aggregation pathway is independent of a conserved metal-binding site and involves the formation of disulfide bonds during the nucleation process. The elongation process occurs independently of the presence of Cu(2+) ions, and amyloid-like aggregation can proceed under oxidative conditions. In contrast, the Zn(2+)-dependent aggregation pathway was found to be independent of cysteines and was reversible upon removal of Zn(2+) ions. Together, our results provide insight into the regulation of the quaternary structure of GAPR-1 by metal ions and redox homeostasis with potential implications for regulatory mechanisms of other CAP proteins.