Cytosolic proteins as principal copper buffer in an early response to copper by yeast cells

We compared relative copper-binding capacities of cellular components and determined the substance that dominantly sequesters excess copper incorporated into bakers' yeast cells before the copper homeostasis systems start to operate. The fluorescent chelator calcein, whose fluorescence is quenched by its binding with copper, was used for estimation of the copper-binding capacities. The copper-binding capacity was assessed by the degree of competition of a sample against calcein for copper. The result indicated that GSH and a whole of the other low-molecular-weight substances had comparable copper-binding capacities, which were a little higher than that of cytosolic proteins as a whole, when compared at their relative cellular concentrations. However, a centrifugal ultrafiltration study showed that essentially all the copper ions incorporated in cells after treatment with 50 mu m CuCl2 for 5 min were not filterable, indicating their binding to high-molecular-weight substances, namely cytosolic proteins. In the direct fluorescence measurement with calcein-loaded cells, no labile copper was detected in either wild-type yeast or GSH-deficient mutant upon treatment with 50 mu MCuCl2, supporting the notion of the copper buffering by the cytosolic proteins under the in vivo conditions. This property of the proteins may not only play a part in cell's resistance against copper but also it may necessitate the role of copper chaperones in the intracellular trafficking of copper to target proteins.