Changes in Plastid Proteins during Ripening of Tomato Fruits

Abstract

Summary Plastids and plastid fragments were recovered from homogenates of the pericarp tissue of mature-green and ripening tomato fruits by differential centrifugation followed by sucrose density gradient centrifugation. The fractions were defined by their positions in the gradient, by their chlorophyll, carotenoid and protein contents, and by the spectra of membrane proteins revealed by polyacrylamide gel electrophoresis. The light harvesting complex and photosystem proteins and the ±- and ²-subunits of coupling factor (CFI) were identified by immunological methods. The proteins of the light harvesting complex and the photosystems were the major membrane proteins in the chloroplasts recovered from mature-green fruit but were less prominent in the plastids from pink fruit and absent in the chromoplasts or ripe fruit. By contrast, the coupling factor subunits were prominent in the membrane of chloroplasts and chromoplasts. The proteins most prominent in chromoplast membranes were also detected in chloroplast fragments, which were less dense than the whole chloroplasts, and which lacked the photosystem proteins. Chloroplast membrane proteins represented about 12 per cent of the protein in mature-green tissue; about 8 per cent of the protein of ripe fruit was recovered as chromoplast membrane protein. Ribulose bisphosphate carboxylase decreased through ripening but even in the maturegreen tissue it represented only 0.3 per cent of total protein indicating that fruit chloroplasts have a very high ratio of photosystem protein to ribulose bisphosphate carboxylase relative to leaf chloroplasts.