PURIFICATION AND CHARACTERIZATION OF RECOMBINANT SEA-URCHIN METALLOTHIONEIN EXPRESSED IN ESCHERICHIA-COLI

Metallothioneins (MT) are metalloproteins expressed tissue specificall y during the development of the sea urchin, Strongylocentrotus pururat us. To explore their structural and functional features and to compare them with those of the evolutionary distant mammalian MTs, one isofor m (MTA) was obtained as the cadmium-containing form, from synthetic cD NA heterologously expressed in Escherichia coli. The purified protein was identified as the desired product by a combination of peptide-map analysis, amino acid sequence analysis and ion-spray mass spectroscopy . The existence of seven Cd-113 NMR resonances revealed that the recom binant protein binds seven Cd ions/molecule. The position of the NMR r esonances (605-695 ppm) and the electronic absorption features suggest that the sea urchin MTA, like the mammalian MTs, possesses tetrahedra lly coordinated cadmium-thiolate clusters. With its lame Stokes' radiu s, sea urchin MTA resembles the mammalian forms, suggesting a comparab le elongated molecular shape. Measurements by spectrophotometric pH ti tration of cadmium binding by the recombinant protein suggest that it possesses two metal-thiolate clusters of distinctly different stabilit y. At pH 7 the average apparent association constant for Cd2+ in the c lusters is about 20-times weaker in sea urchin MTA than in rabbit MT-2 .