Isolation and characterization of envelope membranes of cyanelles from Cyanophora paradoxa

Abstract

Cyanelles of C. paradoxa are considered to be the most primitive chloroplasts among eukaryotic algae since they preserve many characteristics related to cyanobacteria, such as rudimentary peptide glycan wall between inner and outer envelopes, carboxysome-like structure, and physobilisomes. Thus, cyanelles are the most suitable material to investigate the process of domestication of cyanobacteria to chloroplasts after symbiosis. In the present study, we have isolated and characterized envelope membranes of cyanelles to compare with those of cyanobacteria and chloroplasts of higher plants. Envelope membranes were separated by density gradient floatation centrifugation from the homogenate of cyanelles. Two yellow bands were separated from thylakoid membranes. The buoyant density of one of the yellow fractions coincided with that of inner envelope membranes of spinach or plasma membranes of cyanobacteria. The density of another fraction was very low and the band was migrated to 0% sucrose-containing layer. It was found by pigment analysis that the heavier yellow fraction was enriched in zeaxanthin while the light fraction was rich in ß-carotene, and that both fractions contained practically no chlorophylls. The heavier yellow fraction had a membranous structure, while the light fraction had a particular structure observed by electron microscopy. Thus, we tentatively assigned the heavier yellow fraction as an inner envelope membrane fraction of cyanelles.