Figure 2. Involement of copper ions in ethylene-mediated response and ethylene signal transduction. Copper ion is hypothesized to be transported into the cell and brought to the RAN1 protein. The RAN1 protein then delivers copper ion to the ETR1 apoprotein, which then becomes functional and is possibly transported to the plasma membrane. In the absence of ethylene, ETR1 functions to activate CTR1, which regulates the ethylene response pathway (Woeste and Kieber, 2000).