Chloroplasts, membrane protein assembly

Chloroplasts in higher plants have three membranes the outer and inner envelope membranes and the thylakoid membrane. Very little is known about membrane protein assembly into the two envelope membranes (Soil and Tien, 1998). The thylakoid has been better studied and in fact appears to use mechanisms very similar to those found in E. coli for membrane protein insertion (Dalbey and Robinson, 1999). Thus, SRP, SecA, SecYEG, YidC, and Tat homologues are all present in the thylakoid membrane or in the stroma (the Tat system was first identified in thylakoids, in fact). In contrast to E. coli, however, there are thylakoid proteins that appear to insert spontaneously into the membrane, insofar as no requirement for any of the known translocation machineries has been detected (Mant et al, 2001). 

The photosynthesis machinery in the chloroplasts is in a system of thylakoid membranes, which are stacked in arrays called grana. The grana look like a stack of coins. The thylakoid membranes contain enzymes and protein assemblies that contain the chlorophyll. The two kinds of protein assemblies are called photosystem I and photosystem II. These assemblies absorb energy from light. 

Are any or all of the proteins of photosynthetically active chloroplast thylakoids present in etioplasts This comes to the heart of the question of whether these membranes are assembled stepwise. It is the kind of information needed to study membrane development and its control. 

Cohen, Y., S. Yalovsky, and R. Nechushtai. 1995. Integration and assembly of photosynthetic protein complexes in chloroplast thylakoid membranes. Biochim. Biophys. Acta 1241 1-30. 

Kapazoglou A, Mould RM, Gray JC (2000) Assembly of the Rieske iron-sulphur protein into the cytochrome bf complex in thylakoid membranes of isolated pea chloroplasts. Eur J Biochem 267 352-360... 

We know that chloroplasts and mitochondria are never synthesized de novo their membranes grow by insertion of new elements into pre-existing membranes. How is the insertion of these new elements controlled Are pre-existing elements inserted into the membrane or are newly synthesized elements assembled into the membrane Is the insertion of these new elements controlled post-translationally or on the level of gene expression Are all of the different complexes coordinately controlled and assembled at the same rate Is assembly of a single protein complex the result of an ordered or a concerted mechanism ... 

MOLECULAR CHAPERONES Molecular chaperones apparently assist unfolded proteins in two ways. First, during a finite time between synthesis and folding, proteins must be protected from inappropriate protein-protein interactions. Some proteins must remain unfolded until they are inserted in an organelle membrane, for example, certain mitochondrial and chloroplast proteins. Second, proteins must fold rapidly and precisely into their correct conformations. Some must be assembled into multisubunit complexes. Investigations of protein folding in a variety of organisms reveal that two major molecular chaperone classes are involved in protein folding. 

The features shared by all members of the plastid family (proplastids, chromoplasts, leuco-plasts, amyloplasts, etioplasts, and chloroplasts) are a p r of outer membranes known as the envelope, a system of internal membranes, and genetic system including histone-free DNA, DNA polymerase, RNA polymerase, and ribosomes capable of assembling proteins. 

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