PROBLEM TO BE SOLVED: To provide a transgenic plant effective as a means which economically and efficiently produces a required protein multimer, isolates a prescribed ligand such as a metal ion from a fluid like gas or liquid and/or concentrates the same. SOLUTION: The transgenic plant has somatic and germ cells containing at least two mammalian genes which codes for polypeptides capable of autogenously associating with each other to form a biologically active multimer. COPYRIGHT: (C)2005,JPO&NCIPI