Cyclic-nucleotide- and Ca2+/calmodulin-regulated channels in plants: targets for manipulating heavy-metal tolerance, and possible physiological roles

Recently we discovered a tobacco protein (designated NtCBP4) that modulates heavy-metal tolerance in transgenic plants. Structurally, NtCBP4 is simila r to mammalian cyclic-nucleotide-gated non-selective cation channels contai ning six putative transmembrane domains, a predicted pore region, a conserv ed cyclic-nucleotide-binding domain, and a high-affinity calmodulin-binding site that coincides with its cyclic-nucleotide-binding domain. Transgenic tobacco expressing the plasma-membrane-localized NtCBP4 exhibit improved to lerance to Ni2+ and hypersensitivity to Pb2+, which are associated with a d ecreased accumulation of Ni2+ and a enhanced accumulation of Pb2+ respectiv ely. Transgenic plants expressing a truncated version of NtCBP4, from which regulatory domains had been removed, have a different phenotype. Here we d escribe our approach to studying the involvement of NtCBP4 in heavy-metal t olerance and to elucidate its physiological role.