Requirement of the delta subunit of chloroplast coupling factor 1 for photophosphorylation.

1. Chloroplast coupling factor (CF1) can be prepared by a rapid and simple procedure after release of the protein from the membrane by brief exposure to chloroform. It is suitable for large scale preparation of the coupling factor. 2. The protein contains five subunits (alpha, beta, gamma, delta, and epsilon in order of decreasing molecular weight) and serves as a coupling factor for photophosphorylation. However, when this preparation of CF1 was passed through a DEAE-Sephadex A-50 column, a protein with four subunits, missing the delta subunit, was obtained. The four-subunit protein did not serve as a coupling factor. 3. Photophosphorylation was restored to CF4-depleted chloroplast by addition of the four-subunit protein together with a partially purified preparation of the delta subunit. The latter subunit, when added alone, not only did not stimulate photophosphorylation but consistently diminished the residual activity. 4. The delta subunit is required for the binding of CF1 and may represent the stalk seen in electron micrographs as a link between the protein and the membrane.