IDENTIFICATION OF A NOVEL ISOFORM OF THE CHLOROPLAST-COUPLING FACTOR ALPHA-SUBUNIT

Studies were conducted to identify a 64-kD thylakoid membrane protein of unknown function. The protein was extracted from chloroplast thylak oids under low ionic strength conditions and purified to homogeneity b y preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresi s. Four peptides generated from the proteolytic cleavage of the wheat 64-kD protein were sequenced and found to be identical to internal seq uences of the chloroplast-coupling factor (CF1) alpha-subunit. Antibod ies for the 64-kD protein also recognized the alpha-subunit of CF1. Bo th the 64-kD protein and the 61-kD CF1 alpha-subunit were present in t he monocots barley (Hordeum vulgare), maize (Zea mays), oat (Avena sat iva), and wheat (Triticum aestivum); but the dicots pea (Pisum sativum ), soybean (Glycine max Merr.), and spinach (Spinacia oleracea) contai ned only a single polypeptide corresponding to the CF1 alpha-subunit. The 64-kD protein accumulated in response to high irradiance (1000 mu mol photons m(-2) s(-1)) and declined in response to low irradiance (8 0 mu mol photons m(-2) s(-1)) treatments. Thus, the 64-kD protein was identified as an irradiance-dependent isoform of the CF1 alpha-subunit found only in monocots. Analysis of purified CF1 complexes showed tha t the 64-kD protein represented up to 15% of the total CF1 alpha-subun it.