Copper chaperone for the superoxide dismutases CSD1, CSD2 and CSD3. Binds copper ions and delivers them specifically to CSDs. Is required for assistance in CSDs disulfide bond formation and thereby activation of CSDs. May be involved in the negative regulation of heat stress-responsive genes and thermotolerance. Binds 2 copper ions per subunit. Interacts with CSD1. It is unclear whether the chloroplastic and cytosolic proteins result from 2 different transcripts or from alternative translation initiation from the same transcript. Expressed in roots, shoots, stems and flowers, and at lower levels in rosette and cauline leaves. By copper and senescence. Down-regulated by heat stress. No visible phenotype under normal growth conditions, but mutant plants have increased tolerance to heat stress. In the C-terminal section; belongs to the Cu-Zn superoxide dismutase family. Truncated N-terminus.