Separation and Characterization of the Metal‐Thiolate‐Cluster Domains of Recombinant Sea Urchin Metallothionein

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Abstract
Partial metal depletion and 113Cd‐NMR studies have suggested that the recombinant Cd‐containing metallothionein of the sea urchin Strongylocentrutus purpurutus (Cd7‐MTA) binds its metal ions in a four‐metal (Cd4Cys11) and a three‐metal (Cd3Cys9) cluster associated with the N‐terminal and C‐terminal halves of the protein, respectively [Wang, Y., Mackay, E. A., Zerbe, O., Hess, D., Hunziker, P. E., Vasak, M. & Kägi, J. H. R. (1995)Biochemistry 34, 7460–7467]. This partitioning has now been confirmed by bisecting native Cd7‐MTA with subtilisin into products bearing only a single metalthiolate cluster. Their separation by reverse‐phase HPLC and on‐line electrospray mass spectrometry in combination with sequence analysis revealed selective cleavage of the protein into a set of N‐terminal polypeptides containing 37–39 residues with four Cd ions and a set of C‐terminal polypeptides containing 24 and 25 residues with three Cd ions. Thus, sea urchin MTA like its mammalian counterparts is made up of two separate cluster‐harboring domains. The fragmentation pattern indicated that the sites of cleavage are located in the peptide loop interspaced between the first two metal‐bound cysteine residues of the C‐terminal domain. Accordingly, with cleavage, one of the putative nine thiolate ligands of the three‐metal cluster was lost to the N‐terminal fragment. The coordinational consequences of this repartition were reflected in massive chiroptical changes accompanying the cleavage process. While the liberated N‐terminal domain retained the CD profile of the four‐metal cluster in the parent protein and thereby indicated preservation of its structure, the CD features attributable to the intact three‐metal cluster were largely lost on cleavage. The vanished features bear strong resemblance to the large biphasic ellipticity signal at 250 nm which dominates the CD spectrum of native Cd7‐MTA, and allow us thus to attribute this signal to excitonic coupling interactions of Cd‐thiolate chromophores in the three‐metal cluster.