Identification of a Novel Isoform of the Chloroplast-Coupling Factor a-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 sulfatepolyacrylamide 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 (CF1) a-subunit. Antibodies for the 64-kD protein also recognized the a-subunit of CF1. Both the 64-kD protein and the 61-kD CF1 a-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 CF1 a-subunit. The 64-kD protein accumulated in response to high irradiance (1000 mmol photons m s) and declined in response to low irradiance (80 mmol photons m s) treatments. Thus, the 64-kD protein was identified as an irradiance-dependent isoform of the CF1 a-subunit found only in monocots. Analysis of purified CF1 complexes showed that the 64-kD protein represented up to 15% of the total CF1 a-subunit.