Thylakoid membrane proteins, integral

The biogenesis of thylakoid proteins is a complex issue since these proteins are synthesised by two distinct genetic systems. Of particular complexity and interest is the biogenesis of cytoplasmically-synthesised thylakoid lumen proteins, since these proteins must cross three membranes to reach their sites of function. Previous work on one such protein, plastocyanin, indicated that the import of this protein can be divided into two phases. Plastocyanin is initially synthesised as a precursor, which is post-translationally imported across the double-membrane envelope and processed to an intermediate form by a stromal processing peptidase, SPP. The intermediate is then transferred across the thylakoid membrane and processed by a second thylakoidal processing peptidase, TPP (1-3). TPP is an integral thylakoid membrane protein with the active site on the lumenal face of the membrane the enzyme is located exclusively in non-appressed, "stromal" lamellae (4.5). 

Detergent treatment of a suspension of thylakoids dissolves the membranes, releasing complexes containing both chlorophyll and protein. These chlorophyll-protein complexes represent integral components of the thylakoid membrane, and their organization reflects their roles as either light-harvesting com-... 

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. 

Protein Release. Biomembranes consist of lipids and proteins. The latter may be subdivided into so-called intrinsic and extrinsic proteins (49). Intrinsic proteins supposedly are integrated into the membrane phase primarily by the hydrophobic interaction with lipids. Extrinsic proteins are attached to the membranes. Ionic interactions are believed to be important in the binding of extrinsic proteins. When these proteins dissociate from the membrane, they may be sufficiently hydrophilic to be soluble in the aqueous phase. When freeze-aggregated thylakoids are sedimented, a number of membrane proteins are found in the supernatant fluid. Among them are catalytic proteins involved in energy conservation and electron transport (42,48). The total amount of proteins released depends on freezing conditions and the solute environment, but may be as much as 5% of the total membrane protein (48). When frozen in the presence of a cryoprotective solute, at a sufficient concentration, thylakoids remain functional and do not release proteins in significant amounts. Protein release thus accompanies membrane injury and, in fact, is an indication of such injury. 

Photosystem II is the multi-enzymatic chlorophyll-protein complex (water-plastoquinone oxido-reductase) located in the thylakoid membrane of algae, cyanobacteria and higher plants. It is an integral part of the electron transport chain that catalyses primary charge separation. This protein complex consists of over 25 polypeptides, which make up a light-harvesting chlorophyll protein... 

Photosystem 1 (PSl) of all oxygenic photoautotrophs is an integral pigment-protein complex of thylakoid membranes, the main parts of which are well known already the core... 

To check whether the PPase found in the thylakoids is peripherally or integrally bound to the thylakoid membranes or simply adsorbed stromal PPase, thylakoids were washed at different pH. As the extrinsic proteins are easily released at alkaline pH (10), our osmotically shocked thylakoids were washed several times in buffer with pH 6.5 and 8.5. The total PPase activity was then determined in all subsequent supernatants and in the last pellet. As shown in Table 2 the washings release PPase into the aqueous solutions, but the extent of release of PPase from the membrane is about the same whether the washings are performed at pH 6.5 or pH 8.5. About 2% of the PPase activity remained in the final thylakoid pellet. 

Terpenoid indole alkaloid biosynthetic enzymes are associated with at least three different cell types in C. roseus TDC and STR are localized to the epidermis of aerial organs and the apical meristem of roots, D4H and DAT are restricted to the laticifers and idio-blasts of leaves and stems, and GlOH is found in internal parenchyma of aerial organs (St-Pierre et al. 1999 Buriat et al. 2004) thus, vindoline pathway intermediates must be translocated between cell types. Moreover, enzymes involved in terpenoid indole alkaloid biosynthesis in C. roseus are also localized to at least five subcellular compartments TDC, D4H and DAT are in the cytosol, STR and the peroxidase that couples catharanthine to vinblastine are localized to the vacuole indicating transport of tryptamine across the tono-plast, SGD is a soluble enzyme associated with the cytoplasmic face of the endoplasmic reticulum, the P450-dependent monooxygenases are integral endomembrane proteins, and the N-methyltransferase involved in vindoline biosynthesis is localized to thylakoid membranes (De Luca and St-Pierre 2000). 

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