A tobacco plasma membrane calmodulin-binding transporter confers Ni2+ tolerance and Pb2+ hypersensitivity in transgenic plants

All organisms require a minimal amount of metal ions to sustain their metab olism, growth and development. At the same time, their intrinsic metal-upta ke systems render them vulnerable to toxic levels of metals in the biospher e. Using radiolabeled recombinant calmodulin as a probe to screen a tobacco cDNA library, we have discovered a protein designated NtCBP4 (Nicotiana ta bacum calmodulin-binding protein) that can modulate plant tolerance to heav y metals. Structurally, NtCBP4 is similar to vertebrate and invertebrate K and to nonselective cation channels, as well as to recently reported prote ins from barley and Arabidopsis. Here we report on the subcellular localiza tion of NtCBP4 and the phenotype of transgenic plants overexpressing this p rotein. The localization of NtCBP4 in the plasma membrane was manifested by fractionating tobacco membranes on sucrose gradients or by aqueous two-pha se partitioning, and subsequently using immunodetection. Several independen t transgenic lines expressing NtCBP4 had higher than normal levels of NtCBP 4. These transgenic lines were indistinguishable from wild type under norma l growth conditions. However, they exhibited improved tolerance to Ni2+ and hypersensitivity to Pb2+ which are associated with reduced Ni2+ accumulati on and enhanced Pb2+ accumulation. respectively. To our knowledge this is t he first report of a plant protein that modulates plant tolerance or accumu lation of Pb2+. We propose that NtCBP4 is involved in metal uptake across t he plant plasma membrane. This gene may prove useful for implementing selec tive ion tolerance in crops and improving phytoremediation strategies.