Figure 4.1 Major plant membranes, their roles, major transport proteins and connectivity. —, connection through each membrane; ---, connection between indicated compartments.
(PM; sometimes referred to as the plasmalemma) forms the boundary around a cell. However, the connectivity of intracellular membrane systems, through plasmodes-mata, to adjacent cells (see also Section 8.5.1) has led some to question this classical notion, viewing plants instead as supracellular organisms (Baluska et al., 2004). Regardless of views on this matter, as membrane cannot form de novo, the trafficking and connectivity between membrane systems facilitates growth, maintains and changes their composition and affords plant 'cells' a dynamic and responsive network primed for survival. Although membrane systems are heterogeneous in their exact constituents, the mass of the general building blocks - lipids, proteins and carbohydrates - is maintained at a ratio of approximately 40:40:20 (Staehe-lin and Newcomb, 2000). However, it is the specific properties of these individual components that allow different membranes to perform their specialised functions.
Glycerophospholipids (e.g. phosphatidylethanolamine [PE], phosphatidylser-ine [PS], phosphatidylcholine [PC], phosphatidylinositol [PI], phosphatidylglycerol [PG] and cardiolipin [CL]; see www.lipidlibrary.co.uk) constitute the most common class of lipids in the PM and mitochondria and also form their major structural components. These glycolipids consist of two hydrophobic hydrocarbon (fatty acid) tails (14-24 C), with at least one tail having one or more cis double bonds. The degree to which tails are saturated affects lipid packing within, and consequently the shape of, the membrane. Esterified to the fatty acid tails are charged polar (hydrophilic) head groups. The high percentage of lipids present with anionic head groups (e.g. PE, PI, PC) gives the PM a relatively high negative surface charge compared to other membranes, a charge that can be used to aid its isolation (see Section 4.5.1). Chloroplast membranes, in contrast to mitochondrial and plasmalemmal membranes, contain glycoglycerol lipids [e.g. mono- (MGDG) and di-galactosylglycerides (DGDG)], rare in most non-photosynthetic membranes, as their major structural lipid components. PS and CL are the distinctive lipids of the mitochondria but in phosphate-deprived conditions, presumably as a phosphate-conservation mechanism, DGDG content increases through direct transfer from chloroplasts (Jouhet et al., 2004). It has been hypothesised that it is the asymmetrical arrangement of MGDG and DGDG, on the inner and outer leaflets respectively, of the thylakoid membrane that allows it to become highly folded and tightly packed, maximising photosynthetic efficiency (Murphy, 1982). PG is also present in anomalously high proportions within the thylakoid membranes where it has been shown to be essential for chloroplast differentiation and autotrophic growth (Hagio et al., 2002).
Other classes of membrane lipids include sterols (e.g. sitosterol and 24-methylcholesterol) and glycosphingolipids (e.g. glycosylceramide). Whereas sterols have been implicated in the regulation of membrane fluidity and glycosphingolipids are thought to have roles in cell signalling (such as abscisic acid [ABA] signalling; Ng et al., 2001), it is likely that the roles of these two lipid classes may be intimately linked (see Sections 4.2.2 and 14.12). Phosphatidic acid, also involved in ABA signalling, is produced by hydrolysis of membrane lipids by phospholipase D and has been implicated in important signalling pathways such as root growth and programmed cell death (Wang, 2005).
The amphipathic (amphiphilic) nature of lipid molecules, which concomitantly form continuous bilayers, generates a selectively permeable barrier around anything membrane-bound and facilitates the potential formation of large solute concentration gradients across the bilayer. Only highly lipid-soluble (e.g. ethanol, glycerol), small non-polar (e.g. O2, CO2) and some small polar (e.g. H2O, urea) molecules are able to traverse the lipid bilayer passively and directly (Chapter 5). Proteins embedded within lipid bilayers can create additional transport pathways for lipid-impermeable substances or augment transport of those that are lipid-permeable (e.g. aquaporins; Luu and Maurel, 2005). Whilst integral membrane proteins are irreversibly bound and their presence controlled by cytotic events, both peripheral (linked by salt bridges to other proteins or lipids) and lipid-linked proteins (e.g. fatty-acid-, prenyl-group-and sterol-linked) can form reversible associations with the membrane. Proteins can form a direct transport corridor across membranes through pumps, channels, carriers (see Section 5.1.2) or plamodesmata (see also Section 8.5) and control vesicle trafficking or regulate such processes indirectly. The protein constituents of the various membranes within plants can also be distinctive (Figure 4.1) and are therefore useful attribute for identifying particular tissue fractions (see Section 4.3.2).
Membrane composition (and also structure) varies depending on species, cell type and plant physiological address (i.e. the plant's current status as a result of its physiological and developmental history). For instance, the protein complement and transport properties (and functions) of the PM of xylem parenchyma cells differ from that of the guard cell (e.g. Gilliham and Tester, 2005). Moreover, both the protein and lipid composition of a given membrane can alter with changes in physiological conditions. The fluidity of lipid bilayers is naturally temperature-dependent (they will undergo a liquid-crystal to gel-like phase transition as temperature increases). Upon changes in temperature, to keep membranes at an acceptable fluidity for optimal physiological function, the plant can adapt the lipid composition of its membranes. For example, to increase fluidity of membranes upon cold stress, plants can increase the percentage of unsaturated phospholipids and decrease the percentage of sphingolipids (Uemura et al., 2006; see below).
The four-dimensional membrane structure is dynamic and influenced by interactions between lipids, proteins, the cytoskeleton and the cell wall (e.g. McMahon and Gallop, 2005). Insights gained through recent technological advances have found the well-documented fluid-mosaic model of a biological membrane, developed by Singer and Nicholson (1972) (see Figure 1.9; Staehelin and Newcomb, 2000), to be a useful but underdeveloped generalisation of a biological membrane (Engelman, 2005). Drawing from biological membrane studies in other organisms, together with those in plants, evidence is emerging that plant PMs, and potentially other endomem-branes, resemble a mosaic of microdomains with a particular molecular composition. This is in contrast with the traditional view that membranes are 'liquid-disordered', with most molecules being able to freely diffuse within the membrane plane. Interactions between areas of the membrane rich in sterols (in both lipid leaflets), and sphingolipids (solely in the outer leaflet), form 'liquid-ordered' microdomains (Martin etal., 2005). Sphingolipids have long acyl chains that form strong and tightly packed associations, thus endowing these domains with high-melting points. As a consequence, an increase in the proportion of these 'liquid-ordered' over 'liquid-disordered' domains decreases the fluidity of the membrane. Generically referred to as 'lipid rafts', these detergent-resistant membrane fractions are often enriched in glycosylphosphatidylinositol-anchored polypeptides (Bhat and Panstruga, 2005). Associations of these and other proteins, promoted by sphingolipids, are believed to form the lipid raft into a functional unit with specialised biochemical and signalling roles such as the induction of cell polarity (Fischer et al., 2004).
As revealed through X-ray crystallography, it has been shown that transport proteins are more-often-than-not multimeric (e.g. Tornroth-Horsefield et al., 2006). Through other studies it has been demonstrated that they can in fact form functional heteromers (e.g. Dreyer et al., 1997). Regardless of their potential presence within lipid rafts, transport protein multimers are also often clustered. Furthermore, they may be in close vicinity to, or loosely associated with, other proteins within the bilayer or with those in the apoplast or symplast which may in turn affect or regulate protein-mediated solute transport activities. In addition, proteins that have large ectodomains, numerous transmembrane spanning regions or are anchored by single helices or lipidic anchors will cover significant areas of the bilayer surface and will therefore influence its structural properties. It has been suggested that lipid bilayers will also vary their thickness to accommodate protein structures (McMahon and Gallop, 2005). Specific interactions occur between lipids and proteins, with lipids acting either as co-factors or to ensure correct protein folding to guarantee membrane functionality (Valiyaveetil et al., 2002). It is therefore not surprising that changes in lipid composition and/or membrane fluidity can affect transport-protein function (e.g. the sterol-induced up-regulation of H+-ATPase; Opekarova and Tanner, 2003).
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