Membrane structure/function thylakoid membranes

Anderson, J.M. (1986). Photoregulation of the composition, function, and structure of thylakoid membranes. Annual Review of Plant Physiology, 37, 93-136. 

Understanding mechanisms controlling metabolon localization in plastids of different membrane architectures Little is known about metabolon structure, assembly, and membrane targeting. The carotenoid biosynthetic pathway exists on plastid membranes. However, plastids have different membrane architectures and therefore tissue- and plastid-specific differences in membrane targeting of the biosynthetic metabolon can be expected. Localization in chloroplasts that harbor both thylakoid and envelope membranes differs from the envelope membranes in endosperm amy-loplasts. In fact, localization on both thylakoid and envelope membranes implies that the carotenoid pathway is really not a single pathway, but a duplicated pathway that may very well have membrane-specific roles with regard to functions in primary and secondary metabolism. 

In PS I, as in PS II, there are a number of Chi alb protein complexes having lightharvesting and energy-transfer functions. Such complexes most probably exist in direct contact with the RC (part of the core complex), and certainly exist as peripheral LHC I antenna complexes further removed from the RC. A native PS I complex (80-180 Chi per RC, 100 kDa) with at least 6 polypeptides was isolated by solubilization of the thylakoid membrane with nonionic detergents (for example, Triton X-100) [177,178]. With further detergent treatment, the PS I complex dissociated into the core complex (CC I with the RC) and the peripheral antenna complex (LHC I) (spinach, barley, pea, Chlamydomonas reinhardtii [179-183]. The peripheral antenna complex (pea, spinach Chi alb ratio 4.0 1, typical fluorescence at 730 nm) contains 3-4 antenna polypeptides (19-25 kDa) [181,184,185]. This complex was also dissociated into two different antenna complexes - LHC la (2 polypeptides of 22 and 23 kDa) and LHC Ib (1 polypeptide, 20 kDa) - which differ in their fluorescence characteristics (680 nm and 730 nm) [184]. No structural data on these polypeptides are available at present. It was postulated that in C. reinhardtii, in addition to the peripheral antenna complex, an antenna system (with 4 polypeptides) exists, which connects the peripheral antenna energetically with the core complex CC 1 [183]. 

Biophysical chemical studies have suggested that the thylakoid membrane of the chloroplast consists of an ultrathln layer of lipids (presumably a lipid bllayer) with sorbed proteins nd pigments organized in a lamellar structure approximately 100 A thick (Figure 4). Although the precise functions of the thylakoid membrane are still obscure, it is believed that the membrane is the locus of the primary photophyslcal and photochemical processes. 

The thylakoids and stroma are the sites of the so-called light and dark reactions of photosynthesis, respectively. This compartmentalization of photosynthetic functions was recognized by Park and Pon when they broke open the chloroplasts, separated the contents into thylakoid and stroma fractions and examined their properties. The specific activities of the thylakoids include photochemical reactions, electron transport, oxygen evolution, ATP synthesis and NADP reduction, while the stroma contains enzymes for CO2 fixation driven by ATP and NADPH and other biochemical reactions in the dark. Our understanding and appreciation of the detailed structure and organization of the thylakoid membranes has increased tremendously in recent years. Further discussion of thylakoid structure will be continued in section VII on page 26. 

We now examine some additional results also obtained by freeze-fracture electron microscopy using thylakoid membrane fragments. The membrane fragments were obtained either by detergent fragmentation and subsequent separation by differential centrifugation or by mechanical fragmentation followed by separation by aqueous-polymer two-phase partition. These studies yielded useful and complementary information on the thylakoid-membrane structure itself as well as the functional properties associated with the particular structure. 

In this section we will discuss the three major classes of the various constituents of the thylakoid membrane, namely lipids, proteins and electron carriers. All the essential constituents are listed in Tables 1 to 5. Since most ofthese constituents or complexes will be discussed in more detail in later chapters, the tables simply serve as a convenient listing ofthese components for easy reference. A new volume by Siegenthaler and Murata" should be consulted for the structure and function of lipids. 

Besides the major role ofharvesting light, LHC II also has two other important functions, one in the regulation of thylakoid membrane structure, i.e., grana stacking, and tbe other in a related phenomenon, the regulation of the distribution of excitation energy between the two photosystems. 

Attention was subsequently directed to the structure of PS I and characterization of the protein structure of photosystem I was initiated hy using a so-called native PS-I complex in which the in vivo structural, functional and spectroscopic characteristics of photosystem I are retained. Such a native PS-I complex was first prepared by Mullet, Burke and Arntzen, who solubilized the thylakoid membrane with a low concentration of Triton XlOO in the absence of salts. The complex was free of cytochrome bffsmd, judged by fluorescence measurements, was also free of certain chlorophyll proteins while retaining the native character of photosystem I. 

Fig. 2. (A) A model for b/in chloroplast thylakoid membrane (B) Topological arrangement of the four subunits of the purified Cyt b/complex (C) A densitometric scan of an SDS-PAGE gel for b/ (B) from Hauska, Schiitz and Biittner (1996) The cytochrome b/ complex - composition, structure and function. In DR Ort and CF Yocum (eds) Oxygenic Photosynthesis - The Light Reactions, p 384. Kluwer (C) from Black, Widger and Cramer (1987) Large-scale purification of active cytochrome b f complex from spinach chloroplasts. Arch Biochem Biophys 252 657.

Fig. 5. Cytochrome t. (A) Its location in the thylakoid membrane (B) Amino-acid sequences for turnip Cyt f(Brassica campestris, B.C.), spinach Spinacia oleracea, S.o.) and Nostoc PCC 7906 (7906) (C) Hydropathy plot and (D) Topological model. (B) from Gray (1992) Cytochrome f Structure, function and biosynthesis. Photosynthesis Res 34 361 (C) from Widger, Cramer, Herrmann and Trebst (1985) Topography and function of thylakoid membrane proteins. Trends Biochem Sci 10 126.

Photophosphoryiation can be blocked by venturi-cidin and similar agents that inhibit the formation of ATP from ADP and P, by the mitochondrial ATP synthase (Table 19-4). (4) ATP synthesis is catalyzed by FqFi complexes, located on the outer surface of the thylakoid membranes, that are very similar in structure and function to the FqFi complexes of mitochondria. 

Carotenoids are non-covalently bound to complexes in the thylakoid membrane and are fundamentally important functional and structural components of the photosynthetic apparatus (Siefermann Harms, 1985, 1987 Peter and Thornber, 1991 Demmig Adams and Adams, 1992 Dreyfuss and Thomber, 1994a, 1994b Horton et al 1994 Grossman et al,... 

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