Isolation of chloroplast envelopes from Chlamydomonas. Lipid and polypeptide composition

A membrane fraction enriched in chloroplast envelopes was isolated from the alga Chlamydomonas reinhardii. The envelope preparations contained the envelope marker enzyme, galactosyltransferase, with a specific activity about 26- and 37-fold greater than that of the intact chloroplasts and of the starting cells, respectively. Both the envelope fraction and intact chloroplasts incorporated labelled galactose mostly into digalactosyldiglyceride (DGDG), with little incorporation into monogalactosyldiglyceride (MGDG), in marked contrast to what had been observed in higher plants. The envelope preparation was characterized in terms of its composition in polar lipids and polypeptides, and certain features were found in common with the corresponding membranes of higher plants. The ratio of galactolipids MGDG/DGDG was typically lower in the envelopes, compared to the thylakoid fraction. Analysis of the proteins by electrophoresis on sodium dodecyl sulfate (SDS)-polyacrylamide gels revealed a polypeptide pattern that was distinct from those of the thylakoids or of the soluble fraction of the chloroplasts. The major component of the envelope fraction was a 29-hilodalton polypeptide, similar to the main envelope polypeptide found in higher plants and believed to be the phosphate translocator.