Morphological and chemical studies on the crystalloid-forming ‘succulent protein’ from normal and ribosome-deficient Aeonium domesticum plastids

Abstract

Aeonium domesticum cv. variegatum is a mesochimera of the constitution green/white/green with normal proplastids and chloroplasts in the unaffected tissues and ribosome-deficient colourless mutant plastids in the white leaf tissues. All the different plastid types contain ‘succulent protein crystalloids’ (SPC). For more detailed characterization, the SPC elements were freed from the plastids and purified by gel filtration. Electron microscopy of different fractions revealed five levels of structural organization. Beginning with the most complex state, the levels are designated as ‘succulent protein (SP) organizational state’ V (hexagonally arranged and closely packed tubules in the stroma of intact plastids) to I (globular protomers of 5 nm diameter as the basic structure of SPCs). Highly purified SP-fractions were shown by means of sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) to consist of two or three proteins of M_r 56 kdalton, 58 kdalton and 60 kdalton, depending on the buffer medium used for SP isolation and the duration of storage of leaves in the frozen state. In the urea/SDS-PAGE system, these proteins show similar mobilities to α- and β-tubulin, but no immunoreaction against antitubulin. The proteolytic cleavage pattern of tubulin subunits and SP proteins are different. Their locations on two-dimensional isoelectric focusing-SDS gels show some overlappings because of microheterogeneities in both proteins in the pH gradient from pH 4.5 to 6.5. Malatedehydrogenase activity could not be detected in the purified SP fractions.