Fractionation and analysis of polypeptides of Euglena gracilis chloroplasts

Intact Euglena gracilis chloroplasts, purified on gradients of silica sol, were lysed osmotically and fractionated by centrifugation on discontinuous gradients of sucrose into their soluble, envelope membrane, and thylakoid membrane components. The proteins of the different subchloroplast fractions, as well as those of whole chloroplasts, were analyzed by electrophoresis on sodium dodecyl sulfate polyacrylamide gels. The polypeptide profile of each fraction was distinctive and was in general similar to the profile obtained for analogous fractions of the chloroplasts of higher plants. The envelope membranes were separated into two fractions in the gradients according to their banding densities. Electron micrographs showed that the light envelope fraction consisted mostly of single-membrane vesicles, whereas the heavy envelope fraction consisted of multiple layers of folded membranes. Both envelope fractions were ultrastructurally distinct from the thylakoid membranes.