Cadmium transport activity of four mercury transporters (MerC, MerE, MerF, and MerT) and effects of the periplasmic mercury-binding protein MerP on Mer-dependent cadmium uptake.

Mercury superfamily proteins, i.e. inner membrane-spanning proteins (MerC, MerE, MerF, and MerT) and a periplasmic mercury-binding protein (MerP), transport mercury into the cytoplasm. A previous study demonstrated that a Mer transporter homolog exhibits cadmium transport activity; based on this, the present study aimed to evaluate the cadmium transport activity of MerC, MerE, MerF, and MerT and the effects of MerP co-expression in Escherichia coli. Bacteria expressing MerC, MerE, MerF, or MerT without MerP were more sensitive to cadmium and significantly absorbed more cadmium than did the control strain. Expression of MerP in combination with MerC, MerE, MerF, or MerT increased the bacterial sensitivity to cadmium and cadmium accumulation compared to a single expression of MerC, MerE, MerF, or MerT. Cadmium uptake mediated by MerC, MerE, MerF, or MerT was inhibited under cold or acidic conditions. These findings suggest that MerC, MerE, MerF, and MerT are broad-spectrum heavy metal transporters that mediate both mercury and cadmium transport into cells and that MerP accelerates the cadmium transport ability of MerC, MerE, MerF, and MerT.