A cadmium-binding protein in a cadmium tolerant strain of Chlorella pyrenoidosa

Abstract

The presence of a Cd-binding component with a molecular weight of 12,800 daltons was detected in lysates of Chlorella pyrenoidosa cultures which had grown in the presence of 0.5 μg Cd/ml for 5 days. Based upon its molecular size and ability to co-incorporate 35S along with 115Cd, we have concluded that this component is a protein. Its similarity to metallothionein isolated from mammalian cells was suggested by its molecular size, heat stability, and its absence in cells that had never been exposed to Cd. When the gel chromatographic separation was conducted at pH 7.5, the Cd-binding protein was partially degraded into a component with a molecular weight of 6500 daltons, suggesting that the Chlorella protein may be a dimer. The synthesis of this protein was not stimulated by Zn. Cultures grown in the presence of 10 μg Mn/ml and 0.5 μg Cd/ml also failed to synthesize the Cd-binding protein; no cadmium was accumulated under these conditions. These studies suggest that the mechanism whereby C. pyrenoidosa accumulates Cd and tolerates high intracellular Cd levels is related to its ability to synthesize a metallothionein-like protein.