Comparative studies on the polypeptide composition of chloroplast lamellae and lamellar fractions

The polypeptide composition of spinach chloroplast membranes and membrane fractions has been examined by the technique of sodium dodecylsulfate-polyacrylamide gel electrophoresis. Chloroplasts were fragmented into grana (Photosystem II enriched) and stroma lamellae (Photosystem I in character) by the French press technique. The grana lamellae were further fractionated by the use of digitonin into two fractions, one enriched in Photosystem II and the other enriched in Photosystem I. These membranes are composed of at least 15 polypeptides two of which, with approximate weights of 39 and 50 kdaltons, are observed only in granal fractions. Quantitatively the primarily Photosystem II fractions are enriched in polypeptides in the 30-23 kdalton range whereas the Photosystem I (or Photosystem I-enriched) fractions are enriched in polypeptides in the 60-54 kdalton region. The experiments reported show that contamination by soluble proteins or other membranes is negligible. The results indicate that subtle differences in composition account for the large differences in structure and function within the chloroplast membrane system.