Primary structure and tissue-specific expression of blue crab (Callinectes sapidus) metallothionein isoforms

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Abstract
In aquatic animals, synthesis of the metal-binding protein metallothionein (MT) can be induced through exposure to elevated levels of metals in food or water. Whether the different routes of exposure lead to expression of different metallothionein isoforms in different tissues in unknown. In this study we examined the induction of metallothionein isoforms in the hepatopancreas and gills of the blue crab Callinectes sapidus. When blue crabs are exposed to cadmium in their diet, the metal accumulates in the hepatopancreas. Size-exclusion and anion-exchange chromatography show the presence of five low-molecular-mass cadmium-binding proteins. All of the observed cadmium-binding proteins belong to the class I MT family. They are designated as MT-Ia, MT-Ib, MT-Ic, MT-IIa and MT-IIb. All purified proteins run as single peaks upon rechromatography on anion-exchange HPLC, except for MT-Ic, which segregates into two peaks corresponding to MT-Ia and MT-Ic. The amino acid sequence of MT-Ia and MT-Ic is identical. MT-Ib differs from MT-Ia and MT-Ic only in having an extra N-terminal methionine. The 18 cysteine residues in MT-Ia and MT-IIa occur in identical positions; however, of the remaining 40 amino acids, 15 are found to be different. MT-IIb is identical with MT-IIa, except for an extra methionine residue at its N-terminal position. It appears therefore that, of the five observed CdMTs, only two are the products of distinct genes. CdMT-Ia and -IIa are posttranslationally modified forms of Ib and IIb, respectively, and CdMT-Ia and -Ic appear to be conformational isomers. Cadmium-induced expression of the two genes is tissue-specific. When crabs are exposed to cadmium in water, the metal accumulates in the gills, where it is bound to MT-II. MT-I is virtually absent.