Phylogenetic and biochemical evidence supports the recruitment of an ADP-glucose translocator for the export of photosynthate during plastid endosymbiosis

Genomics-Informed Insights into Endosymbiotic Organelle Evolution in Photosynthetic Eukaryotes

The conversion of free-living cyanobacteria to photosynthetic organelles of eukaryotic cells through endosymbiosis transformed the biosphere and eventually provided the basis for life on land. Despite the presumable advantage conferred by the acquisition of photoautotrophy through endosymbiosis, only two independent cases of primary endosymbiosis have been documented: one that gave rise to the Archaeplastida, and the other to photosynthetic species of the thecate, filose amoeba Paulinella. Here, we review recent genomics-informed insights into the primary endosymbiotic origins of cyanobacteria-derived organelles. Furthermore, we discuss the preconditions for the evolution of nitrogen-fixing organelles. Recent genomic data on previously undersampled cyanobacterial and protist taxa provide new clues to the origins of the host cell and endosymbiont, and proteomic approaches allow insights into the rearrangement of the endosymbiont proteome during organellogenesis. We conclude that in addition to endosymbiotic gene transfers, horizontal gene acquisitions from a broad variety of prokaryotic taxa were crucial to organelle evolution.

Parameters of Starch Granule Genesis in Chloroplasts of Arabidopsis thaliana

Starch is the primary storage carbohydrate in most photosynthetic organisms and allows the accumulation of carbon and energy in form of an insoluble and semi-crystalline particle. In the last decades large progress, especially in the model plant Arabidopsis thaliana, was made in understanding the structure and metabolism of starch and its conjunction. The process underlying the initiation of starch granules remains obscure, although this is a fundamental process and seems to be strongly regulated, as in Arabidopsis leaves the starch granule number per chloroplast is fixed with 5-7. Several single, double, and triple mutants were reported in the last years that showed massively alterations in the starch granule number per chloroplast and allowed further insights in this important process. This mini review provides an overview of the current knowledge of processes involved in the initiation and formation of starch granules. We discuss the central role of starch synthase 4 and further proteins for starch genesis and affecting metabolic factors.

Biotic Host-Pathogen Interactions As Major Drivers of Plastid Endosymbiosis

The plastid originated 1.5 billion years ago through a primary endosymbiosis involving a heterotrophic eukaryote and an ancient cyanobacterium. Phylogenetic and biochemical evidence suggests that the incipient endosymbiont interacted with an obligate intracellular chlamydial pathogen that housed it in an inclusion. This aspect of the ménage-à-trois hypothesis (MATH) posits that Chlamydiales provided critical novel transporters and enzymes secreted by the pathogens in the host cytosol. This initiated the efflux of photosynthate to both the inclusion lumen and host cytosol. Here we review the experimental evidence supporting the MATH and focus on chlamydial genes that replaced existing cyanobacterial functions. The picture emerging from these studies underlines the importance of chlamydial host-pathogen interactions in the metabolic integration of the primary plastid.

The inside and outside: topological issues in plant cell wall biosynthesis and the roles of nucleotide sugar transporters

The cell wall is a complex extracellular matrix composed primarily of polysaccharides. Noncellulosic polysaccharides, glycoproteins and proteoglycans are synthesized in the Golgi apparatus by glycosyltransferases (GTs), which use nucleotide sugars as donors to glycosylate nascent glycan and glycoprotein acceptors that are subsequently exported to the extracellular space. Many nucleotide sugars are synthesized in the cytosol, leading to a topological issue because the active sites of most GTs are located in the Golgi lumen. Nucleotide sugar transporters (NSTs) overcome this problem by translocating nucleoside diphosphate sugars from the cytosol into the lumen of the organelle. The structures of the cell wall components synthesized in the Golgi are diverse and complex; therefore, transporter activities are necessary so that the nucleotide sugars can provide substrates for the GTs. In this review, we describe the topology of reactions involved in polysaccharide biosynthesis in the Golgi and focus on the roles of NSTs as well as their impacts on cell wall structure when they are altered.

The Arabidopsis Golgi-localized GDP-L-fucose transporter is required for plant development

Nucleotide sugar transport across Golgi membranes is essential for the luminal biosynthesis of glycan structures. Here we identify GDP-fucose transporter 1 (GFT1), an Arabidopsis nucleotide sugar transporter that translocates GDP-L-fucose into the Golgi lumen. Using proteo-liposome-based transport assays, we show that GFT preferentially transports GDP-L-fucose over other nucleotide sugars in vitro, while GFT1-silenced plants are almost devoid of L-fucose in cell wall-derived xyloglucan and rhamnogalacturonan II. Furthermore, these lines display reduced L-fucose content in N-glycan structures accompanied by severe developmental growth defects. We conclude that GFT1 is the major nucleotide sugar transporter for import of GDP-L-fucose into the Golgi and is required for proper plant growth and development.

Nucleotide sugars are transported from the cytoplasm to the Golgi lumen where they are incorporated into cell wall polysaccharides and used for glycosylation of proteins and lipids. Here the authors identify GFT1, an Arabidopsis Golgi-localized GDP-fucose transporter that is required for plant growth and development

Was the Chlamydial Adaptative Strategy to Tryptophan Starvation an Early Determinant of Plastid Endosymbiosis?

Chlamydiales were recently proposed to have sheltered the future cyanobacterial ancestor of plastids in a common inclusion. The intracellular pathogens are thought to have donated those critical transporters that triggered the efflux of photosynthetic carbon and the consequent onset of symbiosis. Chlamydiales are also suspected to have encoded glycogen metabolism TTS (Type Three Secretion) effectors responsible for photosynthetic carbon assimilation in the eukaryotic cytosol. We now review the reasons underlying other chlamydial lateral gene transfers evidenced in the descendants of plastid endosymbiosis. In particular we show that half of the genes encoding enzymes of tryptophan synthesis in Archaeplastida are of chlamydial origin. Tryptophan concentration is an essential cue triggering two alternative modes of replication in Chlamydiales. In addition, sophisticated tryptophan starvation mechanisms are known to act as antibacterial defenses in animal hosts. We propose that Chlamydiales have donated their tryptophan operon to the emerging plastid to ensure increased synthesis of tryptophan by the plastid ancestor. This would have allowed massive expression of the tryptophan rich chlamydial transporters responsible for symbiosis. It would also have allowed possible export of this valuable amino-acid in the inclusion of the tryptophan hungry pathogens. Free-living single cell cyanobacteria are devoid of proteins able to transport this amino-acid. We therefore investigated the phylogeny of the Tyr/Trp transporters homologous to E. coli TyrP/Mre and found yet another LGT from Chlamydiales to Archaeplastida thereby considerably strengthening our proposal.

Evolution of photorespiration from cyanobacteria to land plants, considering protein phylogenies and acquisition of carbon concentrating mechanisms

Photorespiration and oxygenic photosynthesis are intimately linked processes. It has been shown that under the present day atmospheric conditions cyanobacteria and all eukaryotic phototrophs need functional photorespiration to grow autotrophically. The question arises as to when this essential partnership evolved, i.e. can we assume a coevolution of both processes from the beginning or did photorespiration evolve later to compensate for the generation of 2-phosphoglycolate (2PG) due to Rubisco's oxygenase reaction? This question is mainly discussed here using phylogenetic analysis of proteins involved in the 2PG metabolism and the acquisition of different carbon concentrating mechanisms (CCMs). The phylogenies revealed that the enzymes involved in the photorespiration of vascular plants have diverse origins, with some proteins acquired from cyanobacteria as ancestors of the chloroplasts and others from heterotrophic bacteria as ancestors of mitochondria in the plant cell. Only phosphoglycolate phosphatase was found to originate from Archaea. Notably glaucophyte algae, the earliest branching lineage of Archaeplastida, contain more photorespiratory enzymes of cyanobacterial origin than other algal lineages or land plants indicating a larger initial contribution of cyanobacterial-derived proteins to eukaryotic photorespiration. The acquisition of CCMs is discussed as a proxy for assessing the timing of periods when photorespiratory activity may have been enhanced. The existence of CCMs also had marked influence on the structure and function of photorespiration. Here, we discuss evidence for an early and continuous coevolution of photorespiration, CCMs and photosynthesis starting from cyanobacteria via algae, to land plants.

Metabolic connectivity as a driver of host and endosymbiont integration

The origin of oxygenic photosynthesis in the Archaeplastida common ancestor was foundational for the evolution of multicellular life. It is very likely that the primary endosymbiosis that explains plastid origin relied initially on the establishment of a metabolic connection between the host cell and captured cyanobacterium. We posit that these connections were derived primarily from existing host-derived components. To test this idea, we used phylogenomic and network analysis to infer the phylogenetic origin and evolutionary history of 37 validated plastid innermost membrane (permeome) metabolite transporters from the model plant Arabidopsis thaliana. Our results show that 57% of these transporter genes are of eukaryotic origin and that the captured cyanobacterium made a relatively minor (albeit important) contribution to the process. We also tested the hypothesis that the bacterium-derived hexose-phosphate transporter UhpC might have been the primordial sugar transporter in the Archaeplastida ancestor. Bioinformatic and protein localization studies demonstrate that this protein in the extremophilic red algae Galdieria sulphuraria and Cyanidioschyzon merolae are plastid targeted. Given this protein is also localized in plastids in the glaucophyte alga Cyanophora paradoxa, we suggest it played a crucial role in early plastid endosymbiosis by connecting the endosymbiont and host carbon storage networks. In summary, our work significantly advances understanding of plastid integration and favors a host-centric view of endosymbiosis. Under this view, nuclear genes of either eukaryotic or bacterial (noncyanobacterial) origin provided key elements of the toolkit needed for establishing metabolic connections in the primordial Archaeplastida lineage.

Toward an understanding of the function of Chlamydiales in plastid endosymbiosis

Plastid endosymbiosis defines a process through which a fully evolved cyanobacterial ancestor has transmitted to a eukaryotic phagotroph the hundreds of genes required to perform oxygenic photosynthesis, together with the membrane structures, and cellular compartment associated with this process. In this review, we will summarize the evidence pointing to an active role of Chlamydiales in metabolic integration of free living cyanobacteria, within the cytosol of the last common plant ancestor.

Identification and Phylogenetic Analysis of a Novel Starch Synthase in Maize

Starch is an important reserve of carbon and energy in plants, providing the majority of calories in the human diet and animal feed. Its synthesis is orchestrated by several key enzymes, and the amount and structure of starch, affecting crop yield and quality, are determined mainly by starch synthase (SS) activity. To date, five SS isoforms, including SSI-IV and Granule Bound Starch Synthase (GBSS) have been identified and their physiological functions have been well characterized. Here, we report the identification of a new SS isoform in maize, designated SSV. By searching sequenced genomes, SSV has been found in all green plants with conserved sequences and gene structures. Our phylogenetic analysis based on 780 base pairs has suggested that SSIV and SSV resulted from a gene duplication event, which may have occurred before the algae formation. An expression profile analysis of SSV in maize has indicated that ZmSSV is mainly transcribed in the kernel and ear leaf during the grain filling stage, which is partly similar to other SS isoforms. Therefore, it is likely that SSV may play an important role in starch biosynthesis. Subsequent analysis of SSV function may facilitate understanding the mechanism of starch granules formation, number and structure.

Primary endosymbiosis and the evolution of light and oxygen sensing in photosynthetic eukaryotes

The origin of the photosynthetic organelle in eukaryotes, the plastid, changed forever the evolutionary trajectory of life on our planet. Plastids are highly specialized compartments derived from a putative single cyanobacterial primary endosymbiosis that occurred in the common ancestor of the supergroup Archaeplastida that comprises the Viridiplantae (green algae and plants), red algae, and glaucophyte algae. These lineages include critical primary producers of freshwater and terrestrial ecosystems, progenitors of which provided plastids through secondary endosymbiosis to other algae such as diatoms and dinoflagellates that are critical to marine ecosystems. Despite its broad importance and the success of algal and plant lineages, the phagotrophic origin of the plastid imposed an interesting challenge on the predatory eukaryotic ancestor of the Archaeplastida. By engulfing an oxygenic photosynthetic cell, the host lineage imposed an oxidative stress upon itself in the presence of light. Adaptations to meet this challenge were thus likely to have occurred early on during the transition from a predatory phagotroph to an obligate phototroph (or mixotroph). Modern algae have recently been shown to employ linear tetrapyrroles (bilins) to respond to oxidative stress under high light. Here we explore the early events in plastid evolution and the possible ancient roles of bilins in responding to light and oxygen.

Transcriptomic analysis reveals evidence for a cryptic plastid in the colpodellid Voromonas pontica, a close relative of chromerids and apicomplexan parasites

Colpodellids are free-living, predatory flagellates, but their close relationship to photosynthetic chromerids and plastid-bearing apicomplexan parasites suggests they were ancestrally photosynthetic. Colpodellids may therefore retain a cryptic plastid, or they may have lost their plastids entirely, like the apicomplexan Cryptosporidium. To find out, we generated transcriptomic data from Voromonas pontica ATCC 50640 and searched for homologs of genes encoding proteins known to function in the apicoplast, the non-photosynthetic plastid of apicomplexans. We found candidate genes from multiple plastid-associated pathways including iron-sulfur cluster assembly, isoprenoid biosynthesis, and tetrapyrrole biosynthesis, along with a plastid-type phosphate transporter gene. Four of these sequences include the 5' end of the coding region and are predicted to encode a signal peptide and a transit peptide-like region. This is highly suggestive of targeting to a cryptic plastid. We also performed a taxon-rich phylogenetic analysis of small subunit ribosomal RNA sequences from colpodellids and their relatives, which suggests that photosynthesis was lost more than once in colpodellids, and independently in V. pontica and apicomplexans. Colpodellids therefore represent a valuable source of comparative data for understanding the process of plastid reduction in humanity's most deadly parasite.

Origin and evolution of plastids and photosynthesis in eukaryotes

Recent progress in understanding the origins of plastids from endosymbiotic cyanobacteria is reviewed. Establishing when during geological time the endosymbiosis occurred remains elusive, but progress has been made in defining the cyanobacterial lineage most closely related to plastids, and some mechanistic insight into the possible existence of cryptic endosymbioses perhaps involving Chlamydia-like infections of the host have also been presented. The phylogenetic affinities of the host remain obscure. The existence of a second lineage of primary plastids in euglyphid amoebae has now been confirmed, but the quasipermanent acquisition of plastids by animals has been shown to be more ephemeral than initially suspected. A new understanding of how plastids have been integrated into their hosts by transfer of photosynthate, by endosymbiotic gene transfer and repatriation of gene products back to the endosymbiont, and by regulation of endosymbiont division is presented in context.

Chlamydia, cyanobiont, or host: who was on top in the ménage à trois?

The endosymbiont hypothesis proposes that photosynthate from the cyanobiont was exported to the cytosol of the eukaryote host and polymerized from ADP-glucose into glycogen. Chlamydia-like pathogens are the second major source of foreign genes in Archaeplastida, suggesting that these obligate intracellular pathogens had a significant role during the establishment of endosymbiosis, likely through facilitating the metabolic integration between the endosymbiont and the eukaryotic host. In this opinion article, we propose that a hexose phosphate transporter of chlamydial origin was the first transporter responsible for exporting photosynthate out of the cyanobiont. This connection pre-dates the recruitment of the host-derived carbon translocators on the plastid inner membranes of green and red algae, land plants, and photosynthetic organisms of higher order endosymbiotic origin.

Abnormal glycosphingolipid mannosylation triggers salicylic acid-mediated responses in Arabidopsis

The Arabidopsis thaliana protein GOLGI-LOCALIZED NUCLEOTIDE SUGAR TRANSPORTER (GONST1) has been previously identified as a GDP-d-mannose transporter. It has been hypothesized that GONST1 provides precursors for the synthesis of cell wall polysaccharides, such as glucomannan. Here, we show that in vitro GONST1 can transport all four plant GDP-sugars. However, gonst1 mutants have no reduction in glucomannan quantity and show no detectable alterations in other cell wall polysaccharides. By contrast, we show that a class of glycosylated sphingolipids (glycosylinositol phosphoceramides [GIPCs]) contains Man and that this mannosylation is affected in gonst1. GONST1 therefore is a Golgi GDP-sugar transporter that specifically supplies GDP-Man to the Golgi lumen for GIPC synthesis. gonst1 plants have a dwarfed phenotype and a constitutive hypersensitive response with elevated salicylic acid levels. This suggests an unexpected role for GIPC sugar decorations in sphingolipid function and plant defense signaling. Additionally, we discuss these data in the context of substrate channeling within the Golgi.

Proteomic analysis of the Cyanophora paradoxa muroplast provides clues on early events in plastid endosymbiosis

Glaucophytes represent the first lineage of photosynthetic eukaryotes of primary endosymbiotic origin that diverged after plastid establishment. The muroplast of Cyanophora paradoxa represents a primitive plastid that resembles its cyanobacterial ancestor in pigment composition and the presence of a peptidoglycan wall. To attain insights into the evolutionary history of cyanobiont integration and plastid development, it would thus be highly desirable to obtain knowledge on the composition of the glaucophyte plastid proteome. Here, we provide the first proteomic analysis of the muroplast of C. paradoxa. Mass spectrometric analysis of the muroplast proteome identified 510 proteins with high confidence. The protein repertoire of the muroplast revealed novel paths for reduced carbon flow and export to the cytosol through a sugar phosphate transporter of chlamydial origin. We propose that C. paradoxa possesses a primordial plastid mirroring the situation in the early protoalga.

Metabolic effectors secreted by bacterial pathogens: essential facilitators of plastid endosymbiosis?

Under the endosymbiont hypothesis, over a billion years ago a heterotrophic eukaryote entered into a symbiotic relationship with a cyanobacterium (the cyanobiont). This partnership culminated in the plastid that has spread to forms as diverse as plants and diatoms. However, why primary plastid acquisition has not been repeated multiple times remains unclear. Here, we report a possible answer to this question by showing that primary plastid endosymbiosis was likely to have been primed by the secretion in the host cytosol of effector proteins from intracellular Chlamydiales pathogens. We provide evidence suggesting that the cyanobiont might have rescued its afflicted host by feeding photosynthetic carbon into a chlamydia-controlled assimilation pathway.

Non-random sharing of Plantae genes

The power of eukaryote genomics relies strongly on taxon sampling. This point was underlined in a recent analysis of red algal genome evolution in which we tested the Plantae hypothesis that posits the monophyly of red, green (including plants) and glaucophyte algae. The inclusion of novel genome data from two mesophilic red algae enabled us to robustly demonstrate the sisterhood of red and green algae in the tree of life. Perhaps more exciting was the finding that >1,800 putative genes in the unicellular red alga Porphyridium cruentum showed evidence of gene-sharing with diverse lineages of eukaryotes and prokaryotes. Here we assessed the correlation between the putative functions of these shared genes and their susceptibility to transfer. It turns out that genes involved in complex interactive networks such as biological regulation and transcription/translation are less susceptible to endosymbiotic or horizontal gene transfer, when compared to genes with metabolic and transporter functions.

Glucose-1-phosphate transport into protoplasts and chloroplasts from leaves of Arabidopsis

Almost all glucosyl transfer reactions rely on glucose-1-phosphate (Glc-1-P) that either immediately acts as glucosyl donor or as substrate for the synthesis of the more widely used Glc dinucleotides, ADPglucose or UDPglucose. In this communication, we have analyzed two Glc-1-P-related processes: the carbon flux from externally supplied Glc-1-P to starch by either mesophyll protoplasts or intact chloroplasts from Arabidopsis (Arabidopsis thaliana). When intact protoplasts or chloroplasts are incubated with [U-(14)C]Glc-1-P, starch is rapidly labeled. Incorporation into starch is unaffected by the addition of unlabeled Glc-6-P or Glc, indicating a selective flux from Glc-1-P to starch. However, illuminated protoplasts incorporate less (14)C into starch when unlabeled bicarbonate is supplied in addition to the (14)C-labeled Glc-1-P. Mesophyll protoplasts incubated with [U-(14)C]Glc-1-P incorporate (14)C into the plastidial pool of adenosine diphosphoglucose. Protoplasts prepared from leaves of mutants of Arabidopsis that lack either the plastidial phosphorylase or the phosphoglucomutase isozyme incorporate (14)C derived from external Glc-1-P into starch, but incorporation into starch is insignificant when protoplasts from a mutant possessing a highly reduced ADPglucose pyrophosphorylase activity are studied. Thus, the path of assimilatory starch biosynthesis initiated by extraplastidial Glc-1-P leads to the plastidial pool of adenosine diphosphoglucose, and at this intermediate it is fused with the Calvin cycle-driven route. Mutants lacking the plastidial phosphoglucomutase contain a small yet significant amount of transitory starch.

The evolution of glycogen and starch metabolism in eukaryotes gives molecular clues to understand the establishment of plastid endosymbiosis

Solid semi-crystalline starch and hydrosoluble glycogen define two distinct physical states of the same type of storage polysaccharide. Appearance of semi-crystalline storage polysaccharides appears linked to the requirement of unicellular diazotrophic cyanobacteria to fuel nitrogenase and protect it from oxygen through respiration of vast amounts of stored carbon. Starch metabolism itself resulted from the merging of the bacterial and eukaryote pathways of storage polysaccharide metabolism after endosymbiosis of the plastid. This generated the three Archaeplastida lineages: the green algae and land plants (Chloroplastida), the red algae (Rhodophyceae), and the glaucophytes (Glaucophyta). Reconstruction of starch metabolism in the common ancestor of Archaeplastida suggests that polysaccharide synthesis was ancestrally cytosolic. In addition, the synthesis of cytosolic starch from the ADP-glucose exported from the cyanobacterial symbiont possibly defined the original metabolic flux by which the cyanobiont provided photosynthate to its host. Additional evidence supporting this scenario include the monophyletic origin of the major carbon translocators of the inner membrane of eukaryote plastids which are sisters to nucleotide-sugar transporters of the eukaryote endomembrane system. It also includes the extent of enzyme subfunctionalization that came as a consequence of the rewiring of this pathway to the chloroplasts in the green algae. Recent evidence suggests that, at the time of endosymbiosis, obligate intracellular energy parasites related to extant Chlamydia have donated important genes to the ancestral starch metabolism network.

Connecting the plastid: transporters of the plastid envelope and their role in linking plastidial with cytosolic metabolism

Plastids have a multitude of functions in eukaryotic cells, ranging from photosynthesis to storage, and a role in essential biosynthetic pathways. All plastids are of either primary or higher-order endosymbiotic origin. That is, either a photosynthetic cyanobacterium was integrated into a mitochondriate eukaryotic host cell (primary endosymbiosis) or a plastid-bearing eukaryotic cell merged with another eukaryotic cell (secondary or higher-order endosymbioses), thereby passing on the plastid between various eukaryotic lineages. For all of these endosymbioses to become functional, it was essential to establish metabolic connections between organelle and host cell. Here, we review the present understanding of metabolite exchange between plastids and the surrounding cytosol in the context of the endosymbiotic origin of plastids in various eukaryotic lineages. We show that only a small number of transporters that can be traced down to the primary endosymbiotic event are conserved between plastids of diverse origins.

The metabolite transporters of the plastid envelope: an update

The engulfment of a photoautotrophic cyanobacterium by a primitive mitochondria-bearing eukaryote traces back to more than 1.2 billion years ago. This single endosymbiotic event not only provided the early petroalgae with the metabolic capacity to perform oxygenic photosynthesis, but also introduced a plethora of other metabolic routes ranging from fatty acids and amino acids biosynthesis, nitrogen and sulfur assimilation to secondary compounds synthesis. This implicated the integration and coordination of the newly acquired metabolic entity with the host metabolism. The interface between the host cytosol and the plastidic stroma became of crucial importance in sorting precursors and products between the plastid and other cellular compartments. The plastid envelope membranes fulfill different tasks: they perform important metabolic functions, as they are involved in the synthesis of carotenoids, chlorophylls, and galactolipids. In addition, since most genes of cyanobacterial origin have been transferred to the nucleus, plastidial proteins encoded by nuclear genes are post-translationally transported across the envelopes through the TIC-TOC import machinery. Most importantly, chloroplasts supply the photoautotrophic cell with photosynthates in form of reduced carbon. The innermost bilayer of the plastidic envelope represents the permeability barrier for the metabolites involved in the carbon cycle and is literally stuffed with transporter proteins facilitating their transfer. The intracellular metabolite transporters consist of polytopic proteins containing membrane spans usually in the number of four or more α-helices. Phylogenetic analyses revealed that connecting the plastid with the host metabolism was mainly a process driven by the host cell. In Arabidopsis, 58% of the metabolite transporters are of host origin, whereas only 12% are attributable to the cyanobacterial endosymbiont. This review focuses on the metabolite transporters of the inner envelope membrane of plastids, in particular the electrochemical potential-driven class of transporters. Recent advances in elucidating the plastidial complement of metabolite transporters are provided, with an update on phylogenetic relationship of selected proteins.