Molecular components and biochemistry of electron transport in plant plasma membranes (review)

Plasma membrane lipid-protein interactions affect signaling processes in sterol-biosynthesis mutants in Arabidopsis thaliana

The plasma membrane is an important organelle providing structure, signaling and transport as major biological functions. Being composed of lipids and proteins with different physicochemical properties, the biological functions of membranes depend on specific protein-protein and protein-lipid interactions. Interactions of proteins with their specific sterol and lipid environment were shown to be important factors for protein recruitment into sub-compartmental structures of the plasma membrane. System-wide implications of altered endogenous sterol levels for membrane functions in living cells were not studied in higher plant cells. In particular, little is known how alterations in membrane sterol composition affect protein and lipid organization and interaction within membranes. Here, we conducted a comparative analysis of the plasma membrane protein and lipid composition in Arabidopsis sterol-biosynthesis mutants smt1 and ugt80A2;B1. smt1 shows general alterations in sterol composition while ugt80A2;B1 is significantly impaired in sterol glycosylation. By systematically analyzing different cellular fractions and combining proteomic with lipidomic data we were able to reveal contrasting alterations in lipid-protein interactions in both mutants, with resulting differential changes in plasma membrane signaling status.

Plasma membrane lipid-protein interactions affect signaling processes in sterol-biosynthesis mutants in Arabidopsis thaliana

The plasma membrane is an important organelle providing structure, signaling and transport as major biological functions. Being composed of lipids and proteins with different physicochemical properties, the biological functions of membranes depend on specific protein-protein and protein-lipid interactions. Interactions of proteins with their specific sterol and lipid environment were shown to be important factors for protein recruitment into sub-compartmental structures of the plasma membrane. System-wide implications of altered endogenous sterol levels for membrane functions in living cells were not studied in higher plant cells. In particular, little is known how alterations in membrane sterol composition affect protein and lipid organization and interaction within membranes. Here, we conducted a comparative analysis of the plasma membrane protein and lipid composition in Arabidopsis sterol-biosynthesis mutants smt1 and ugt80A2;B1. smt1 shows general alterations in sterol composition while ugt80A2;B1 is significantly impaired in sterol glycosylation. By systematically analyzing different cellular fractions and combining proteomic with lipidomic data we were able to reveal contrasting alterations in lipid-protein interactions in both mutants, with resulting differential changes in plasma membrane signaling status.

Non-mitochondrial coenzyme Q

The key role of coenzyme Q (ubiquinone or Q) is in mitochondrial and prokaryotic energetics. Less well investigated is the basis for its presence in eukaryotic membrane locations other than mitochondria and in plasma where both antioxidant and potentially more targeted roles are indicated. Included in the latter is that of a lipid-soluble electron transfer intermediate that serves as the transmembrane component of plasma membrane and Golgi apparatus electron transport, which regulates cytosolic NAD(+) /NADH ratios and is involved in vectorial membrane displacements and in the regulation of cell growth. Important protective effects on circulating lipoproteins and in the prevention of coronary artery disease ensue not only from the antioxidant role of CoQ(10) but also from its ability to directly block protein oxidation and superoxide generation of the TM-9 family of membrane proteins known as age-related NADH oxidase or arNOX (ENOX3) and their shed forms that appear after age 30 and some of which associate specifically with low-density lipoprotein particles to catalyze protein oxidation and crosslinking.

The coexistence of the oxidative and reductive systems in roots: the role of plasma membranes

Different components of the plasma membrane bound and associated redox system, which participate in the energy transfer from the predominantly reducing intercellular environment to the extracellular oxidizing environment, are reviewed. Special attention is given to plant root cells. An analysis of the plasma membrane-associated redox components, such as the cytochromes, quinones, and different types of oxidoreductases (dehydrogenases, oxidases, peroxidases, and superoxide dismutases), is made, as well as their coupling with naturally occurring extracellular substrates, such as oxygen and its reactive forms, phenols, ascorbate, nitrate, ferric ion, and organic acids. The participation of different free radical species in most of the plasma membrane-bound redox reactions is documented.

Transmembrane electron transport in sealed and NAD(P)H-loaded right-side-out plasma membrane vesicles isolated from maize (Zea mays L.) roots

Electron transport across plasma membranes has been observed in vivo in several plant species and tissues after the application of ferricyanide (hexacyanoferrate III, HCF III). In the present work, a transmembrane electron flow was demonstrated in sealed and NAD(P)H-loaded right-side-out (apoplastic-side-out) plasma membrane vesicles isolated from maize (Zea mays L.) roots. HCF III was reduced at a rate of up to 126 nmol min(-1) mg(-1) protein by NADPH-loaded vesicles, while reduction rates with NADH-loaded vesicles were several-fold lower. Coincident with the reduction of HCF III, NAD(P)H oxidation was observed inside the vesicles. The dependence of reduction on K+ indicated an electrogenic transmembrane electron flow. Application of 100 microM calcium decreased HCF III reduction up to 66%, while pre-incubation with 200 microM warfarin or diphenylene iodonium inhibited transmembrane electron transport only weakly. Fe(3+)-EDTA was not reduced significantly by NADPH-loaded plasma membrane vesicles, whereas XTT was reduced at a rate of 765 pmol min(-1) mg(-1) protein. The results suggested a major function for NADPH in transmembrane electron flow and were discussed in conjunction with in vivo experiments.

Phylloquinone, what can we learn from plants?

The plant plasma membrane contains redox proteins able to mediate a trans-membrane electron flow. This electron flow might be responsible for the generation of the active oxygen species observed as a reaction to pathogen attack or stress. Vitamin K1 could be identified as a possible lipid soluble electron carrier in plant plasma membrane preparations. Such a function would be analogous to coenzyme Q in animal plasma membranes. What we are going to outline in this contribution is a concept of how the electron transport system of the plant plasma membrane could interact with quinones, thus contributing to the metabolism of free radicals in plants.

Interaction between electron transport at the plasma membrane and nitrate uptake by maize (Zea mays L.) roots

In the present study nitrate uptake by maize (Zea mays L.) roots was investigated in the presence or absence of ferricyanide (hexacyanoferrate III) or dicumarol. Nitrate uptake caused an alkalization of the medium. Nitrate uptake of intact maize seedlings was inhibited by ferricyanide while the effect of dicumarol was not very pronounced. Nitrite was not detected in the incubation medium, neither with dicumarol-treated nor with control plants after application of 100 microM nitrate to the incubation solution. In a second set of experiments interactions between nitrate and ferricyanide were investigated in vivo and in vitro. Nitrate (1 or 3 mM) did neither influence ferricyanide reductase activity of intact maize roots nor NADH-ferricyanide oxidoreductase activity of isolated plasma membranes. Nitrate reductase activity of plasma-membrane-enriched fractions was slightly stimulated by 25 microM dicumarol but was not altered by 100 microM dicumarol, while NADH-ferricyanide oxidoreductase activity was inhibited in the presence of dicumarol. These data suggest that plasma-membrane-bound standard-ferricyanide reductase and nitrate reductase activities of maize roots may be different. A possible regulation of nitrate uptake by plasmalemma redox activity, as proposed by other groups, is discussed.

Inhibition of trans-membrane hexacyanoferrate III reductase activity and proton secretion of maize (Zea mays L.) roots by thenoyltrifluoroacetone

Intact plants can reduce external oxidants by an appearingly trans-membrane electron transport. In vivo an increase in net medium acidification accompanies the reduction of the apoplastic substrate. Up to now, several NAD(P)H dehydrogenases, b-type cytochromes, and a phylloquinone have been identified and partially purified from plant plasma membranes. The occurrence of a quinone in the plasma membrane of maize roots supports the hypothetical model of a proton-transferring redox system, i.e., an electron transport chain with a quinone as mobile electron and proton carrier. In the present study the trans-membrane electron transport system of intact maize (Zea mays L.) roots was investigated. Flow-through and ionostat systems have been used to estimate the electron and proton transport activity of this material. Application of 4,4,4-trifluoro-1-(2-thienyl)-butane-1,3-dione (thenoyltrifluoroacetone) inhibited the reduction of ferricyanide in the incubation solution of intact maize roots up to 70%. This inhibition could not be washed off by rinsing the roots with fresh incubation medium. The acidification of the medium induced after ferricyanide application was inhibited to about 62%. The effects of thenoyltrifluoroacetone on proton fluxes in the absence of ferricyanide have been characterized in a pH-stat system. The net medium acidification by maize roots was inhibited up to 75% by thenoyltrifluoroacetone in the absence of ferricyanide, while dicumarol inhibited net acidification completely. The inhibition of H(+)-ATPase activity was estimated with plasma membrane vesicles isolated by phase partitioning and treated with 0.05% (w/v) Brij 58. ATP-dependent proton gradients and Pi release were measured after preincubation with the effectors. The proton pumping activity by those plasma membrane vesicles was inhibited by dicumarol (53.6%) and thenoyltrifluoroacctone (77.8%), while the release of Pi was unaffected by both inhibitors.

Oxidoreductases in plant plasma membranes

Electron transporting oxidoreductases at biological membranes mediate several physiological processes. While such activities are well known and widely accepted as physiologically significant for other biological membranes, oxidoreductase activities found at the plasma membrane of plants are still being neglected. The ubiquity of the oxidoreductases in the plasma membrane suggests that the activity observed is of major importance in fact up to now no plant without redox activity at the plasmalemma is known. Involvement in proton pumping, membrane energization, ion channel regulation, iron reduction, nutrient uptake, signal transduction, and growth regulation has been proposed. However, positive proof for one of the numerous theories about the physiological function of the system is still missing. Evidence for an involvement in signalling and regulation of growth and transport activities at the plasma membrane is strong, but the high activity of the system displayed in some experiments also suggests function in defense against pathogens.