Evidence for a 40,000 dalton species of plastocyanin as the active component

Evidence is presented for a molecular weight of 40,000 daltons for plastocyanin from spinach and Scenedesmus employing SDS polyacrylamide gel analysis. We propose that this component is the tetramer of a 10,000 dalton species, the component found after reduction with dithiothreitol. The tetramer is virtually absent in the apoprotein but reappears on reconstitution by copper sulfate titration. Tetramer, but neither dimer nor monomer produced by various procedures given here, demonstrates activity and EPR-active copper strongly suggesting it represents the native state of the protein.