Comparison of Envelope Membranes From Higher Plants and Algae Plastids and of Outer Membranes From Cyanobacteria (Blue-Green Algae)

Abstract

The plastid envelope is a fascinating membrane system. In higher plants and in several algae, the envelope consists of a pair of membranes although it is a more complex structure in numerous algae such as Euglena (Lefort-Tran, 1981). Envelope membranes provide a flexible boundary between the plastid and the surrounding cytosol (Douce et al, 1984). Therefore, a major function of envelope membranes is the selective control of metabolite transport, owing to specific translocators located on the inner membrane (Heldt and Heber, 1981). Besides this role in photosynthesis regulation, the envelope membranes are involved in plastid biogenesis. They regulate the uptake and transport of plastid proteins that are synthesized on cytoribosomes (Chua and Schmidt, 1979). In addition, they can catalyze the synthesis of characteristic plastid glycerolipid and prenylquinones (Douce et al, 1984); these studies were made possible owing to the development of efficient methods to purify large amounts of envelope membranes from higher plant chloroplasts. With the exception of Euglena, few studies were done with algae plastid envelopes (Blee, 1981; Brandt, 1982/83), mostly due to the difficulty to prepare intact chloroplasts. However, this has recently been achieved for Chlamydomonas (Mendiola-Morgenthaler et al, 1985) and it is therefore possible to compare some properties (lipid content and role in glycerolipid biosynthesis) of envelope membranes from higher plants and algae; this is the purpose of this article. In addition, further comparison with the outer membranes of cyanobacteria is most interesting since it is generally assumed, without experimental evidences, that the inner envelope membrane from chloroplasts could derive from the plasma membrane of the prokaryotic ancestor whereas the outer envelope membrane of chloroplasts could derive from the endomembrane system of the protoeukaryote that engulfed the free living prokaryote.