P700 Chlorophyll a-Protein

Abstract

The P(700) chlorophyll alpha-protein was purified by preparative sodium dodecyl sulfate (SDS) gel electrophoresis from SDS-solubilized barley (Hordeum vulgare L., cv Himalaya) chloroplast membranes. After elution from the gel in the presence of 0.05 to 0.1% Triton X-100, the recovered protein had a chlorophyll/P(700) ratio of 50 to 60/1 and contained no chlorophyll b or cytochromes. Analysis of the polypeptide composition of the chlorophyll-protein revealed a 58 to 62 kilodalton (kD) polypeptide component but no lower molecular weight polypeptides. The 58 to 62 kD component was further resolved into two distinct polypeptide bands which were subsequently mapped by partial cyanogen bromide digestion and Staphylococcus aureus proteolysis. Based on results from the mapping experiments and other data, we suggest that the two components are conformational variants of a single polypeptide. Measurement of the chlorophyll to protein ratio by quantitative amino acid analysis and consideration of the yield of P(700) in the protein isolate suggest that, contrary to previous models (Bengis and Nelson, 1975, 1977), P(700)in vivo is associated with a minimum of four subunits of approximately 60 kD.Antibodies raised against the photochemically active chlorophyll-protein complex from barley reacted specifically with the 58 to 62 kD apoprotein. The same preparative electrophoresis procedure was used to isolate photochemically active P(700) chlorophyll a-protein from soybean (Glycine max L.), tobacco (Nicotiana tobacum L.), petunia (Petunia x hybrida), tomato (Lycopersicum esculentum), and Chlamydomonas reinhardti. The isolated complex from all species exhibited identical polypeptide compositions and chlorophyll/P(700) ratios. Antibodies to the barley protein cross reacted with all species tested demonstrating the highly conserved structure of the apoprotein.