Yusuke Kakizaki

Functional Coexpression of the Mitochondrial Alternative Oxidase and Uncoupling Protein Underlies Thermoregulation in the Thermogenic Florets of Skunk Cabbage

Two distinct mitochondrial energy dissipating systems, alternative oxidase (AOX) and uncoupling protein (UCP), have been implicated as crucial components of thermogenesis in plants and animals, respectively. To further clarify the physiological roles of AOX and UCP during homeothermic heat production in the thermogenic skunk cabbage (Symplocarpus renifolius), we identified the thermogenic cells and performed expression and functional analyses of these genes in this organism. Thermographic analysis combined with in situ hybridization revealed that the putative thermogenic cells surround the stamens in the florets of skunk cabbage and coexpress transcripts for SrAOX, encoding Symplocarpus AOX, and SrUCPb, encoding a novel UCP that lacks a fifth transmembrane segment. Mitochondria isolated from the thermogenic florets exhibited substantial linoleic acid (LA)-inducible uncoupling activities. Moreover, our results demonstrate that LA is capable of inhibiting the mitochondrial AOX pathway, whereas the proportion of pyruvate-stimulated AOX capacity was not significantly affected by LA. Intriguingly, the protein expression levels for SrAOX and SrUCPb were unaffected even when the ambient air temperatures increased from 10.3°C to 23.1°C or from 8.3°C to 24.9°C. Thus, our results suggest that functional coexpression of AOX and UCP underlies the molecular basis of heat production, and that posttranslational modifications of these proteins play a crucial role in regulating homeothermic heat production under conditions of natural ambient temperature fluctuations in skunk cabbage.

Two Cys or Not Two Cys? That Is the Question; Alternative Oxidase in the Thermogenic Plant Sacred Lotus

Sacred lotus (Nelumbo nucifera) regulates temperature in its floral chamber to 32°C to 35°C across ambient temperatures of 8°C to 40°C with heating achieved through high alternative pathway fluxes. In most alternative oxidase (AOX) isoforms, two cysteine residues, Cys1 and Cys2, are highly conserved and play a role in posttranslational regulation of AOX. Further control occurs via interaction of reduced Cys1 with α-keto acids, such as pyruvate. Here, we report on the in vitro regulation of AOX isolated from thermogenic receptacle tissues of sacred lotus. AOX protein was mostly present in the reduced form, and only a small fraction could be oxidized with diamide. Cyanide-resistant respiration in isolated mitochondria was stimulated 4-fold by succinate but not pyruvate or glyoxylate. Insensitivity of the alternative pathway of respiration to pyruvate and the inability of AOX protein to be oxidized by diamide suggested that AOX in these tissues may lack Cys1. Subsequently, we isolated two novel cDNAs for AOX from thermogenic tissues of sacred lotus, designated as NnAOX1a and NnAOX1b. Deduced amino acid sequences of both confirmed that Cys1 had been replaced by serine; however, Cys2 was present. This contrasts with AOXs from thermogenic Aroids, which contain both Cys1 and Cys2. An additional cysteine was present at position 193 in NnAOX1b. The significance of the sequence data for regulation of the AOX protein in thermogenic sacred lotus is discussed and compared with AOXs from other thermogenic and nonthermogenic species.

Identification of a gene for pyruvate-insensitive mitochondrial alternative oxidase expressed in the thermogenic appendices in Arum maculatum

Heat production in thermogenic plants has been attributed to a large increase in the expression of the alternative oxidase (AOX). AOX acts as an alternative terminal oxidase in the mitochondrial respiratory chain, where it reduces molecular oxygen to water. In contrast to the mitochondrial terminal oxidase, cytochrome c oxidase, AOX is nonprotonmotive and thus allows the dramatic drop in free energy between ubiquinol and oxygen to be dissipated as heat. Using reverse transcription-polymerase chain reaction-based cloning, we reveal that, although at least seven cDNAs for AOX exist (AmAOX1a, -1b, -1c, -1d, -1e, -1f, and -1g) in Arum maculatum, the organ and developmental regulation for each is distinct. In particular, the expression of AmAOX1e transcripts appears to predominate in thermogenic appendices among the seven AmAOXs. Interestingly, the amino acid sequence of AmAOX1e indicates that the ENV element found in almost all other AOX sequences, including AmAOX1a, -1b, -1c, -1d, and -1f, is substituted by QNT. The existence of a QNT motif in AmAOX1e was confirmed by nano-liquid chromatography-tandem mass spectrometry analysis of mitochondrial proteins from thermogenic appendices. Further functional analyses with mitochondria prepared using a yeast heterologous expression system demonstrated that AmAOX1e is insensitive to stimulation by pyruvate. These data suggest that a QNT type of pyruvate-insensitive AOX, AmAOX1e, plays a crucial role in stage- and organ-specific heat production in the appendices of A. maculatum.

Different molecular bases underlie the mitochondrial respiratory activity in the homoeothermic spadices of Symplocarpus renifolius and the transiently thermogenic appendices of Arum maculatum

Symplocarpus renifolius and Arum maculatum are known to produce significant heat during the course of their floral development, but they use different regulatory mechanisms, i.e. homoeothermic compared with transient thermogenesis. To further clarify the molecular basis of species-specific thermogenesis in plants, in the present study we have analysed the native structures and expression patterns of the mitochondrial respiratory components in S. renifolius and A. maculatum. Our comparative analysis using Blue native PAGE combined with nano LC (liquid chromatography)-MS/MS (tandem MS) has revealed that the constituents of the respiratory complexes in both plants were basically similar, but that several mitochondrial components appeared to be differently expressed in their thermogenic organs. Namely, complex II in S. renifolius was detected as a 340 kDa product, suggesting an oligomeric or supramolecular structure in vivo. Moreover, the expression of an external NAD(P)H dehydrogenase was found to be higher in A. maculatum than in S. renifolius, whereas an internal NAD(P)H dehydrogenase was expressed at a similar level in both species. Alternative oxidase was detected as smear-like signals that were elongated on the first dimension with a peak at around 200 kDa in both species. The significance and implication of these data are discussed in terms of thermoregulation in plants.

A novel functional element in the N-terminal region of Arum concinnatum alternative oxidase is indispensable for catalytic activity of the enzyme in HeLa cells.

Alternative oxidase (AOX) is a quinol-oxygen oxidoreductase, which is known to possess a dicarboxylate diiron reaction center held in structurally postulated alpha-helical bundle. However, little is known about the structural or functional features of its N-terminal region in any organism, with the exception of a regulatory cysteine residue (CysI) in angiosperm plants. Here, we show that transcripts of two AOX1 isozymes (AcoAOX1a and AcoAOX1b) are coexpressed in thermogenic appendices of Arum concinnatum, while their enzymatic activities seem to be distinct. Namely, AcoAOX1a, an abundantly expressed transcript in vivo, shows an apparent cyanide-insensitive and n-propyl gallate-sensitive respiration during ectopic expression of the protein in HeLa cells, whereas AcoAOX1b exhibits a lower transcript expression, and appears to be totally inactive as AOX at the protein level. Our functional analyses further reveal that an E83K substitution in AcoAOX1b, which is located far upstream of CysI in the N-terminal region, is the cause of this loss of function. These results suggest the presence of a naturally occurring inactive AOX homologue in thermogenic plants. Accordingly, our results further imply that the N-terminal region of the AOX protein functionally contributes to the dynamic activities of respiratory control within the mitochondria.

In vivo redox state of the ubiquinone pool in the spadices of the thermogenic skunk cabbage, Symplocarpus renifolius

In vivo ubiquinone (UQ) reduction levels were determined in thermogenic stigma and post-thermogenic male stages of spadices of the skunk cabbage, Symplocarpus renifolius. In contrast to Arum maculatum, in which the UQ pool is almost fully reduced during thermogenesis, the reduction levels of UQ9 and UQ10 were not affected by the thermogenic status or developmental stage of individual S. renifolius spadices. Moreover, these levels were controlled within the ranges 40–75% and 35–60%, respectively. These results suggest that the reduction state of the UQ pool per se is not primarily involved in thermoregulation in S. renifolius.

Engineering Plant Alternative Oxidase Function in Mammalian Cells: Substitution of the Motif-like Sequence ENV for QDT Diminishes Catalytic Activity of Arum concinnatum AOX1a Expressed in HeLa Cells

Alternative oxidase (AOX) is a nonproton motive quinol–oxygen oxidoreductase which is a component of the mitochondrial respiratory chain in higher plants. In this study, we have characterized the catalytic activity and regulatory behaviors of Arum concinnatum AOX isoforms, namely AcoAOX1a and AcoAOX1b, and their artificial mutants in HeLa cells. We demonstrated that substitution of the motif-like sequence ENV on the C-terminal half of AcoAOX1a for QDT diminishes its activity and proposed that the innate inactivity of AcoAOX1b in HeLa cells is, at least in part, attributable to its QDT motif. Furthermore, we show that introduction of F130L in the hydrophilic N-terminal extension of AcoAOX1a resulted in greater activity in the presence of pyruvate. This result indicates that functional significance of the N-terminal extension is not particular to the conventional regulatory cysteine. On the basis of these findings, we discuss new insights into the structural integrity of AOX in HeLa cells and the applicability of mammalian cells for functional analysis of this enzyme.