Isolation and Properties of Chloroplast Coupling Factor from Wheat

1 Wheat chloroplast coupling factor (CF 1 ) was estracted with a modification of the chloroform extraction method of Younis et al. ( J. Biol. Chem. 252 , 1814‐1818, 1977). A one‐step purification on an 8‐25% sucrose gradient yielded a CF 1 which was at least 98% pure as judged by sodium dodecyl sulfate/polyacrylamide gel eletrophoresis. 2 Inclusion of proteolysis inhibitors during extraction and purification consistently gave a CF 1 containing all five subunits. Selective loss of the δ and ɛ subunits was observed when proteolysis inhibitors were omitted. 3 Proteolysis inhibitors prevented the release of wheat CF 1 from the thylakoid by the low‐ionic‐strength wash method of Strotmann et al. ( Biochim. Biophys. Acta, 314 , 202‐210, 1973). The enzyme extracted with chloroform and low ionic strength were compared by electrophoresis and no evidence of a difference in molecular weight of any subunit was observed. This suggests that a proteolytic event is not required for release of wheat CF 1 by the low‐ionic‐strength method, even though release is inhibited by proteolysis inhibitors. 4 The γ subunit of wheat CF 1 probably contains at least one internal disulfide bridge, as the electrophoretic mobility of this subunit is lower in the presence of reducing agent than in its absence. 5 Wheat CF 1 was viewed by the electron microscope after negative staining. Discrete particles, many appearing hexagonal, were observed at high magnifications. Markham rotational analysis confirmed that the enzyme has sixfold symmetry in at least one of its orientations.