Yeast copper thionein: solving a small but long-standing puzzle

In these postgenomic days the Protein Data Bank is featuring protein structures of increasingly large size, thanks to the progress in the power of the X-ray sources and in the computer programs to solve X-ray structures. Yet, there are small proteins whose structures remain elusive. One of the most striking examples is a stable and well-folded protein whose existence has been known for nearly thirty years, whose central importance in eukaryotes metabolism is undisputed, whose size is ridiculously small (Mr 5655), but whose detailed structure is still unknown. Such protein is yeast copper thionein (Cu-MT) Besides the different structure of the metal cluster, the main differences lie in the cysteine topology and in the conformation of some portions of the backbone. The present structure suggests that Cu-MT, in addition to its role as a safe depository for copper ions in the cell, may play an active role in the delivery of copper to metal-free chaperones.