Submitted to: Journal of Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/1/1997
Publication Date: N/A
Interpretive Summary: Photophosphorylation is the final step in photosynthesis that converts light energy into the chemical energy stored in ATP. Plants then use the ATP as energy source to form sugar from carbon dioxide. A chloroplast enzyme called coupling factor is responsible for synthesis of this ATP. A collaboration between USDA-ARS scientists at N.C. State University and scientists at West Georgia College has identified a new form of chloroplas coupling factor that is distinct from the common form of the enzyme. The new form is found in grasses (e.g. wheat) but not in broad leaf crop plants (e.g. soybean) whereas common form is present in all plants. The two forms of coupling factor also respond to light differently. The new form is an abundant enzyme in plants grown at high irradiance but is almost undetectable in plants grown under low irradiance whereas the common form is an abundant enzyme under both conditions. The functional significance of fthe new form of coupling factor is not known at present, but its discovery could provide new insights into the regulation of photosynthetic ATP production.
Technical Abstract: Studies were conducted to identify a 64-kD thylakoid membrane protein of unknown function that was previously shown to be regulated by growth irradiance (Burkey, K.O.  Plant Physiol. 98:1211-1213). The protein was extracted from chloroplast thylakoids under low ionic strength conditions and purified to hemogeneity by preparative SDS-PAGE. Four peptides generated from the proteolytic cleavage of the wheat 64-kD protei 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 CF1 a-subunit were present in representative monocots (barley, maize, oats, and wheat) but representative dicots (pea, soybean, and spinach) contained only a single polypeptide corresponding to the CF1 a-subunit. The 64-kD protein was a significant fraction of the total CF1 a-subunit at high irradiance (600 to 1000 umol photons m-2s-1) but was present at extremely low levels under low irradiance conditions (80 umol photons m-2s-1). Light acclimation experiments showed that the 64-kD protein was synthesized in response to high irradiance and degraded in response to low irradiance. Thus, the 64-kD protein was identified as an irradiance- dependent isoform of the CF1 a-submit found only in monocots.