PRIMARY STRUCTURE AND TISSUE-SPECIFIC EXPRESSION OF BLUE-CRAB (CALLINECTES-SAPIDUS) METALLOTHIONEIN ISOFORMS

In aquatic animals, synthesis of the metal-binding protein metallothio nein (MT) can be induced through exposure to elevated levels of metals in food or water. Whether the different routes of exposure lead to ex pression of different metallothionein isoforms in different tissues is unknown. In this study we examined the induction of metallothionein i soforms in the hepatopancreas and gills of the blue crab Callinectes s apidus. When blue crabs are exposed to cadmium in their diet, the meta l accumulates in the hepatopancreas. Size-exclusion and anion-exchange chromatography show the presence of five low-molecular-mass cadmium-b inding proteins. All of the observed cadmium-binding proteins belong t o the class I MT family. They are designated as MT-Ia, MT-Ib, MT-Ic, M T-IIa and MT-IIb. All purified proteins run as single peaks upon rechr omatography on anion-exchange HPLC, except for MT-Ic, which segregates into two peaks corresponding to MT-Ia and MT-Ic. The amino acid seque nce of MT-Ia and MT-Ic is identical. MT-Ib differs from MT-Ia and MT-I c only in having an extra N-terminal methionine. The 18 cysteine resid ues 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 iden tical with MT-IIa, except for an extra methionine residue at its N-ter minal position. It appears therefore that, of the five observed CdMTs, only two are the products of distinct genes. CdMT-Ia and -IIa are pos t-translationally modified forms of Ib and IIb, respectively, and CdMT -Ia and -Ic appear to be conformational isomers. Cadmium-induced expre ssion of the two genes is tissue-specific. When crabs are exposed to c admium in the water, the metal accumulates in the gills, where it is b ound to MT-II. MT-I is virtually absent.