Identification and purification of a self-associating copper-binding protein from mouse hepatic cytosols

The proteins which bind copper when it first enters cells and deliver that copper to the cellular sites where copper is incorporated into copper enzymes are unknown. When radiolabeled (64Cu(II)) cytosol proteins were fractionated on Superose columns, two large copper-binding protein fractions were detected which together bound 63% of the total 64Cu recovered from the column. A self-associating, 50-kDa copper-binding protein was identified whose tetramer and dimer appeared to elute in one of the major copper-binding fractions and the monomer eluted in the other. This protein was purified to homogeneity by successive Mono Q, chelating Superose, and phenyl-Superose columns. The concentrated, purified protein showed high amounts of tetramer and monomer plus some dimer, each of which bound copper. Rechromatography of the tetramer-dimer fraction on Superose gave rise to approximately equal amounts of tetramer, dimer, and monomer. Similarly, rechromatography of the monomer fraction gave rise to dimer and tetramer. Thus, the results with the purified protein confirmed that the 50-kDa protein exists as a tetramer-dimer-monomer equilibrium. The 50-kDa protein apparently makes a major contribution to the total copper-binding activity of the mouse hepatic cytosol and may play a significant role in intracellular copper metabolism.