Identification of a Novel Isoform of the Chloroplast-Coupling Factor α-Subunit

Studies were conducted to identify a 64-kD thylakoid membrane protein of unknown function. The protein was extracted from chloroplast thylakoids under low ionic strength conditions and purified to homogeneity by preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Four peptides generated from the proteolytic cleavage of the wheat 64-kD protein were sequenced and found to be identical to internal sequences of the chloroplast-coupling factor (CF(1)) α-subunit. Antibodies for the 64-kD protein also recognized the α-subunit of CF(1). Both the 64-kD protein and the 61-kD CF(1) α-subunit were present in the monocots barley (Hordeum vulgare), maize (Zea mays), oat (Avena sativa), and wheat (Triticum aestivum); but the dicots pea (Pisum sativum), soybean (Glycine max Merr.), and spinach (Spinacia oleracea) contained only a single polypeptide corresponding to the CF(1) α-subunit. The 64-kD protein accumulated in response to high irradiance (1000 μmol photons m(−2) s(−1)) and declined in response to low irradiance (80 μmol photons m(−2) s(−1)) treatments. Thus, the 64-kD protein was identified as an irradiance-dependent isoform of the CF(1) α-subunit found only in monocots. Analysis of purified CF(1) complexes showed that the 64-kD protein represented up to 15% of the total CF(1) α-subunit.