Expression of seven carbonic anhydrases in red alga Gracilariopsis chorda and their subcellular localization in a heterologous system, Arabidopsis thaliana

Expression and localization of two β-carbonic anhydrases in Bienertia, a single-cell C4 plant

Carbonic anhydrases (CAs) are ubiquitous enzymes that catalyze reversibly both the hydration and dehydration reactions of CO2 and HCO3-, respectively. Higher plants contain many different isoforms of CAs that can be classified into α-, β- and γ-type subfamilies. β-type CAs play a key role in the CO2-concentrating mechanism, thereby contributing to efficient photosynthesis in the C4 plants in addition to many other biochemical reactions in plant metabolism. Here, we characterized at the molecular, cellular and biochemical levels two β-type CAs in Bienertia sinuspersici, a plant that operates a C4 carbon concentrating mechanism within individual cells without the Kranz anatomy. These two β-type CAs (BsCAβs), named BsCAβ1 and BsCAβ2, in Bienertia were strongly induced along with maturation of leaves. Both BsCAβ1 and BsCAβ2 existed as a dimeric form in vivo but showed differential localization. BsCAβ2 was localized exclusively to the plasma membrane in Bienertia and when expressed heterologously in the C3 Arabidopsis. In contrast, BsCAβ1 largely localized to the cytosol together with a portion to the plasma membrane (PM) in both plants. BsCAβ2 had two cysteine residues at the N-terminal region for palmitoylation and their substitution with serine residues led to a change in the localization from the plasma membrane (PM) to the cytosol. Thus, we propose that BsCAβ2 localizes to the PM using a lipid moiety added posttranslationally plays a role in conversion of cytosolic CO2 into HCO3- as part of the CO2-concentrating mechanism, thereby contributing to the single-cell C4 photosynthesis in Bienertia.

Diurnal Rhythms in the Red Seaweed Gracilariopsis chorda are Characterized by Unique Regulatory Networks of Carbon Metabolism

Cellular and physiological cycles are driven by endogenous pacemakers, the diurnal and circadian rhythms. Key functions such as cell cycle progression and cellular metabolism are under rhythmic regulation, thereby maintaining physiological homeostasis. The photoreceptors phytochrome and cryptochrome, in response to light cues, are central input pathways for physiological cycles in most photosynthetic organisms. However, among Archaeplastida, red algae are the only taxa that lack phytochromes. Current knowledge about oscillatory rhythms is primarily derived from model species such as Arabidopsis thaliana and Chlamydomonas reinhardtii in the Viridiplantae, whereas little is known about these processes in other clades of the Archaeplastida, such as the red algae (Rhodophyta). We used genome-wide expression profiling of the red seaweed Gracilariopsis chorda and identified 3,098 rhythmic genes. Here, we characterized possible cryptochrome-based regulation and photosynthetic/cytosolic carbon metabolism in this species. We found a large family of cryptochrome genes in G. chorda that display rhythmic expression over the diurnal cycle and may compensate for the lack of phytochromes in this species. The input pathway gates regulatory networks of carbon metabolism which results in a compact and efficient energy metabolism during daylight hours. The system in G. chorda is distinct from energy metabolism in most plants, which activates in the dark. The green lineage, in particular, land plants, balance water loss and CO2 capture in terrestrial environments. In contrast, red seaweeds maintain a reduced set of photoreceptors and a compact cytosolic carbon metabolism to thrive in the harsh abiotic conditions typical of intertidal zones.

α-CAs from Photosynthetic Organisms

Carbonic anhydrases (CAs) are ubiquitous enzymes that catalyze the reversible carbon dioxide hydration reaction. Among the eight different CA classes existing in nature, the α-class is the largest one being present in animals, bacteria, protozoa, fungi, and photosynthetic organisms. Although many studies have been reported on these enzymes, few functional, biochemical, and structural data are currently available on α-CAs isolated from photosynthetic organisms. Here, we give an overview of the most recent literature on the topic. In higher plants, these enzymes are engaged in both supplying CO₂ at the Rubisco and determining proton concentration in PSII membranes, while in algae and cyanobacteria they are involved in carbon-concentrating mechanism (CCM), photosynthetic reactions and in detecting or signaling changes in the CO₂ level in the environment. Crystal structures are only available for three algal α-CAs, thus not allowing to associate specific structural features to cellular localizations or physiological roles. Therefore, further studies on α-CAs from photosynthetic organisms are strongly needed to provide insights into their structure–function relationship.

Identification and molecular characterization of the alternative spliced variants of beta carbonic anhydrase 1 (βCA1) from Arabidopsis thaliana

Carbonic anhydrases (CAs) are ubiquitous zinc metalloenzymes that catalyze the interconversion of carbon dioxide and bicarbonate. Higher plants mainly contain the three evolutionarily distinct CA families αCA, βCA, and γCA, with each represented by multiple isoforms. Alternative splicing (AS) of the CA transcripts is common. However, there is little information on the spliced variants of individual CA isoforms. In this study, we focused on the characterization of spliced variants of βCA1 from Arabidopsis. The expression patterns and subcellular localization of the individual spliced variants of βCA1 were examined. The results showed that the spliced variants of βCA1 possessed different subcellular and tissue distributions and responded differently to environmental stimuli. Additionally, we addressed the physiological role of βCA1 in heat stress response and its protein-protein interaction (PPI) network. Our results showed that βCA1 was regulated by heat stresses, and βca1 mutant was hypersensitive to heat stress, indicating a role for βCA1 in heat stress response. Furthermore, PPI network analysis revealed that βCA1 interacts with multiple proteins involved in several processes, including photosynthesis, metabolism, and the stress response, and these will provide new avenues for future investigations of βCA1.

Gamma carbonic anhydrases are subunits of the mitochondrial complex I of diatoms

Diatoms are unicellular organisms containing red algal-derived plastids that probably originated as result of serial endosymbioses between an ancestral heterotrophic organism and a red alga or cryptophyta algae from which has only the chloroplast left. Diatom mitochondria are thus believed to derive from the exosymbiont. Unlike animals and fungi, diatoms seem to contain ancestral respiratory chains. In support of this, genes encoding gamma type carbonic anhydrases (CAs) whose products were shown to be intrinsic complex I subunits in plants, Euglena and Acanthamoeba were found in diatoms, a representative of Stramenopiles. In this work, we experimentally show that mitochondrial complex I in diatoms is a large complex containing gamma type CA subunits, supporting an ancestral origin. By using a bioinformatic approach, a complex I integrated CA domain with heterotrimeric subunit composition is proposed.

Identification and Characterization of a Periplasmic α-Carbonic Anhydrase (CA) in the Gametophytes of Saccharina japonica (Phaeophyceae)

Periplasmic or external carbonic anhydrases (CAs) have been well accepted as playing a crucial role in the acquisition of dissolved inorganic carbon; however, no cytological evidence or molecular information on these enzymes has been reported in seaweeds to date. In this study, the full-length cDNA sequence coding for a putative periplasmic Sjα-CA2 was cloned from the gametophytes of Saccharina japonica, an industrial brown seaweed. It was 1,728 bp in length and included a 263-bp 5'-untranslated region (UTR), a 577-bp 3'-UTR, and an 888-bp open reading frame encoding a protein precursor consisting of 295 amino acids. The mature protein, after removal of a predicted 28-residue signal peptide, was composed of 267 amino acids with a relative molecular weight of 29.27 kDa. Multisequence alignment and phylogenetic analysis indicated that it was a member of the α-CA family. Enzyme activity assays showed that the recombinant Sjα-CA2 in Escherichia coli possessed CO2 hydration and esterase activities, thus identifying this gene Sjα-CA2 in function. Immunogold electron microscopic observations with the prepared anti-Sjα-CA2 polyclonal antibody illustrated that Sjα-CA2 was located in periplasmic space of the kelp gametophyte cells. Quantitative real-time PCR results revealed that the transcription of Sjα-CA2 was induced by elevated HCO 3 - levels, but it was little changed while the kelp gametophytes were subjected to elevated CO2 concentrations. This study suggests that the periplasmic Sjα-CA2 might play a role in adapting to elevated environmental levels of HCO 3 - by dehydration of HCO 3 - to generate CO2 , which could be readily taken up by S. japonica gametophytes.

Overexpression and Purification of Gracilariopsis chorda Carbonic Anhydrase (GcCAα3) in Nicotiana benthamiana, and Its Immobilization and Use in CO2 Hydration Reactions

Carbonic anhydrase (CA; EC 4.2.2.1) is a Zn-binding metalloenzyme that catalyzes the reversible hydration of CO2. Recently, CAs have gained a great deal of attention as biocatalysts for capturing CO2 from industrial flue gases owing to their extremely fast reaction rates and simple reaction mechanism. However, their general application for this purpose requires improvements to stability at high temperature and under in vitro conditions, and reductions in production and scale-up costs. In the present study, we developed a strategy for producing GcCAα3, a CA isoform from the red alga Gracilariopsis chorda, in Nicotiana benthamiana. To achieve high-level expression and facile purification of GcCAα3, we designed various constructs by incorporating various domains such as translation-enhancing M domain, SUMO domain and cellulose-binding domain CBM3. Of these constructs, MC-GcCAα3 that had the M and CBM3 domains was expressed at high levels in N. benthamiana via agroinfiltration with a yield of 1.0 g/kg fresh weight. The recombinant protein was targeted to the endoplasmic reticulum (ER) for high-level accumulation in plants. Specific and tight CBM3-mediated binding of recombinant GcCAα3 proteins to microcrystalline cellulose beads served as a means for both protein purification from total plant extracts and protein immobilization to a solid surface for increased stability, facilitating multiple rounds of use in CO2 hydration reactions.