In vivo-folded metal–metallothionein 3 complexes revealthe Cu–thionein rather than Zn–thionein character of thisbrain-specific mammalian metallothionein

摘要

Metallothionein-3 (MT3) is one of the four mammalian metallothioneins(MT), and is constitutively synthesized in the brain. MT3 acts both intracellularlyand extracellularly in this organ, performing functions related toneuronal growth and physiological metal (Zn and Cu) handling. It appearsto be involved in the prevention of neurodegenerative disorders caused byinsoluble Cu–peptide aggregates, as it triggers a Zn–Cu swap that maycounteract the deleterious presence of copper in neural tissues. The literaturedata on MT3 coordination come from studies either on apo-MT3reconstitution or the reaction of Zn–MT3 with Cu~(2+), an ion that is hardlypresent inside cells. To ascertain the MT3 metal-binding features in a scenariocloser to the reductive cell cytoplasm, a study of the recombinantZn~(2+), Cd~(2+) and Cu+ complexes of MT3, bMT3, and aMT3, as well asthe in vitro Zn~(2+)–Cd~(2+) and Zn~(2+)–Cu~+ replacement processes, is presentedhere. We conclude that MT3 has a Cu–thionein character that isstronger than that of the MT1 and MT2 isoforms – also present in themammalian brain – which is mainly contributed by its b domain. In contrast,the a domain retains a high capacity to bind Zn~(2+) ions, and, consequently,the entire MT3 peptide shows a peculiar dual ability to handleboth metal ions. The nature of the formed Cu+–MT3 complexes oscillatesfrom heterometallic Cu6Zn4–MT3 to homometallic Cu10–MT3 major species,in a narrow Cu concentration range. Therefore, the entire MT3 peptideshows a high capacity to bind Cu~+, provided that this occurs in anonoxidative milieux. This reflects a peculiar property of this MT isoform,which accurately senses different Cu contents in the environment in whichit is synthesized.