Thylakoid proteins

Peltier, J. B., Friso, G., Kalume, D. E., Roepstorff, P., Nilsson, F., Adamska, I., and van Wijk, K. J. (2000). Proteomics of the chloroplast systematic identification and targeting analysis of lumenal and peripheral thylakoid proteins. Plant Cell 12, 303-304. 

All thylakoidal proteins seem to be first translocated into the stroma through the previously mentioned general import pathway all of them have a cleavable N-terminal transit peptide. However, there are at least four different pathways into the thylakoid membrane (Robinson et al, 1998 Schnell, 1998). Most of them are reminiscent of the pathways of bacteria, described in Section II,B,1. It is not surprising because chloroplasts are most likely evolved from a prokaryotic endosymbioint, but there are certain differences. 

Brink, S., Bogsch, E., Edwards, W., Hynds, P., and Robinson, C. (1998). Targeting of thylakoid proteins by the ApH-driven twin-arginine translocation pathway requires a specific signal in the hydrophobic domain in conjunction with the two-arginine motif. FEBS Lett. 434, 425—430. 

Chaddock, A., Mant, A., Karnauchov, I., Brink, S., Herrmann, R., Klosgen, R., and Robinson, C. (1995). A new type of signal peptide central role of a twin-arginine motif in transfer signals for the ApH-dependent thylakoidal protein translocase. EMBO J. 

Robinson, C., Hynds, P., Robinson, D., and Mant, A. (1998). Multiple pathways for the targeting of thylakoid proteins in chloroplasts. Plant Mol. Biol. 38, 209-221. 

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). 

Most of the thylakoid proteins are organized into four intrinsic protein complexes PS II complex, Cyt b/f complex, PS I complex and ATP synthetase (Fig. 1). The electron transport complexes are linked by mobile electron transport carriers, plastoquinone, plastocyanin and ferredoxin (see Chapter 10). Furthermore, chloroplasts that possess Chi b have the major light-harvesting Chi a/h-proteins of PS II (LHC II) that may represent over 50% of the thylakoid protein [13], as well... 

The excitement generated by these hydropathy index plots, which predict that most of the thylakoid proteins indeed span the membrane, may have led to a view that it is unnecessary to experimentally demonstrate that the proteins have segments exposed at the outer and inner surfaces. Nevertheless, biochemical evidence such as antibody labelling, protease studies and chemical modification is needed to prove all predicted structures. Already it has become evident that caution is needed in the interpretation of the Hydropathy index plots. This need is demonstrated by the hydropathy index plots of the D1 (herbicide-binding) and D2... 

Finally, thylakoid proteins that bind metal-contalning cofactors follow another pathway Into the thylakoid lumen (Figure 16-31, ApFI pathway). The unfolded precursors of these proteins are first targeted to the stroma, where the N-... 

Gal A, Zer H and Ohad I (1997) Redox-controlled thylakoid protein phosphorylation. News and views. Physiol Plant 100 869-885... 

Tremmel IG, KirchhofF H, Weis E et al. Dependence of plastoquinol diffusion on the shape, size, and density of integral thylakoid proteins. Biochim Biophys Acta 2003 1607 97-109. 

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