Role of a copper-specific metallothionein of the blue crab, Callinectes sapidus, in copper metabolism associated with degradation and synthesis of hemocyanin

Abstract

We have identified three MT encoding genes in the blue crab: MT-I, inducible by cadmium, zinc and copper; MT-II, inducible by cadmium and zinc; and MT-III, inducible by copper only [Syring et al., Comp. Biochem. Physiol. C, 125 (2000) 325-332]. To examine the role of the CuMT-I and CuMT-III isoforms in copper metabolism associated with the synthesis and degradation of the oxygen-binding copper protein, hemocyanin, we (1) cloned and sequenced hemocyanin cDNA, (2) examined interaction of the CuMTs with endoplasmic reticulum (ER) vesicles and (3) measured changes in levels of hemocyanin, MT-I, MT-III protein and mRNA that occur in crabs during different stages of the molt cycle. The cDNA-derived hemocyanin amino-acid sequence revealed the presence of a leader peptide indicating that hemocyanin is a secretory protein that is synthesized on the ER. Copper uptake studies show that ER vesicles take up both Cu1+ and Cu2+ in an ATP-independent process. The copper transporter has a Km of 10.8+/-2.4 microM copper and a Vmax of 6.1+/-0.5 nmol Cu/mg protein/10 min. ER vesicles contain hemocyanin, and bind CuMT-I and, preferentially, CuMT-III. However, binding does not result in copper transfer to the ER. There are statistically significant changes in hepatopancreas MT-III and hemocyanin mRNA, and in hemolymph hemocyanin concentrations during the molt cycle. MT-I mRNA remains constant. Changes in MT-III mRNA are positively correlated with changes in hemocyanin mRNA and hemocyanin protein, which points to coordinate control of MT-III and hemocyanin transcription. No CuMT-III protein is observed in hepatopancreas of intermolt crabs when levels of both MT-III and hemocyanin mRNA are high, suggesting rapid utilization of copper bound to MT-III when cells are actively synthesizing hemocyanin. CuMT-III is present in premolt and softshell crabs, and its emergence appears to coincide with a decrease in hemocyanin synthesis and increase in hemocyanin degradation. These results support the hypothesis that the copper-specific metallothionein is intricately involved in copper homeostasis associated with both the synthesis and degradation of hemocyanin.