Phylogenetic variations in a novel family of hyperstable apple snail egg proteins: insights into structural stability and functional trends

The relationship between protein stability and function evolution has not been explored in proteins from natural sources. Here, we investigate the phylogenetic differences of Perivitellin-1 (PV1) a novel family of hyperstable egg carotenoproteins crucial to the reproductive success of Pomacea snails, as they have evolved clade-specific protective functions. We studied P. patula PV1 (PpaPV1) from Flagellata clade eggs, the most basal of Pomacea and compared it with PV1s orthologs from derived clades. PpaPV1 stands as the most stable, with longer unfolding half-life, resistance to detergent unfolding, and therefore higher kinetic stability than PV1s from derived clades. In fact, PpaPV1 is among the most hyperstable proteins described in nature. In addition, its spectral characteristics providing a pale egg coloration, mild lectin activity and glycan specificity are narrower than derived clades. Our results provide evidence indicating large structural and functional changes along the evolution of the genus. Notably, the lectin binding of PpaPV1 is less pronounced, and its glycan specificity is narrower compared to PV1s in the sister Bridgesii clade. Our findings underscore the phylogenetic disparities in terms of structural and kinetic stability, as well as defensive traits like a potent lectin activity affecting the gut morphology of potential predators within the Bridgesii clade or a conspicuous, likely warning coloration, within the Canaliculata clade. This work provides a comprehensive comparison of the structural attributes, stability profiles, and functional roles of apple snail egg PV1s from multiple species within a phylogenetic context. Furthermore, it proposes an evolutionary hypothesis suggesting a trade-off between structural stability and the functional aspects of apple snail's major egg defense protein.

Exaptation of two ancient immune proteins into a new dimeric pore-forming toxin in snails

The Membrane Attack Complex-Perforin (MACPF) family is ubiquitously found in all kingdoms. They have diverse cellular roles, however MACPFs with pore-forming toxic function in venoms and poisons are very rare in animals. Here we present the structure of PmPV2, a MACPF toxin from the poisonous apple snail eggs, that can affect the digestive and nervous systems of potential predators. We report the three-dimensional structure of PmPV2, at 17.2 Å resolution determined by negative-stain electron microscopy and its solution structure by small angle X-ray scattering (SAXS). We found that PV2s differ from nearly all MACPFs in two respects: it is a dimer in solution and protomers combine two immune proteins into an AB toxin. The MACPF chain is linked by a single disulfide bond to a tachylectin chain, and two heterodimers are arranged head-to-tail by non-covalent forces in the native protein. MACPF domain is fused with a putative new Ct-accessory domain exclusive to invertebrates. The tachylectin is a six-bladed β-propeller, similar to animal tectonins. We experimentally validated the predicted functions of both subunits and demonstrated for the first time that PV2s are true pore-forming toxins. The tachylectin "B" delivery subunit would bind to target membranes, and then the MACPF "A" toxic subunit would disrupt lipid bilayers forming large pores altering the plasma membrane conductance. These results indicate that PV2s toxicity evolved by linking two immune proteins where their combined preexisting functions gave rise to a new toxic entity with a novel role in defense against predation. This structure is an unparalleled example of protein exaptation.

Biosynthesis in the Albumen Gland-Capsule Gland Complex Limits Reproductive Effort in the Invasive Apple Snail Pomacea canaliculata

High fecundity often contributes to successful invasives. In molluscs, this may be facilitated by the albumen gland-capsule gland complex, which in gastropods secretes the egg perivitelline fluid that nourishes and protects embryos. The biochemistry of the albumen gland-capsule gland complex and its relationship with fecundity remain largely unknown. We addressed these issues in Pomacea canaliculata (Lamarck, 1822), a highly invasive gastropod whose fecundity and reproductive effort exceed those of ecologically similar gastropods. We evaluated the dynamics of its major secretion compounds (calcium, polysaccharides, and total proteins) as well as the gene expression and stored levels of perivitellins during key moments of the reproductive cycle, that is, before and after first copulation and at low, medium, and high reproductive output. Copulation and first oviposition do not trigger the onset of albumen gland-capsule gland complex biosynthesis. On the contrary, soon after an intermediate reproductive effort, genes encoding perivitellins overexpressed. A high reproductive effort caused a decrease in all albumen gland-capsule gland complex secretion components. Right after a high reproductive output, the albumen gland-capsule gland complex restored the main secretion components, and calcium recovered baseline reserves; but proteins and polysaccharides did not. These metabolic changes in the albumen gland-capsule gland complex after multiple ovipositions were reflected in a reduction in egg mass but did not compromise egg quality. At the end of the cycle, egg dry weight almost doubled the initial albumen gland-capsule gland complex weight. Results indicate that albumen gland-capsule gland complex biosynthesis limits a constantly high reproductive output. Therefore, lowering fecundity by targeting biosynthesis could effectively reduce the rate of this species' spread.

Effect of crude oil petroleum hydrocarbons on protein expression of the prawn Macrobrachium borellii

Hydrocarbon pollution is a major environmental threat to ecosystems in marine and freshwater environments, but its toxicological effect on aquatic organisms remains little studied. A proteomic approach was used to analyze the effect of a freshwater oil spill on the prawn Macrobrachium borellii. To this aim, proteins were extracted from midgut gland (hepatopancreas) of male and female prawns exposed 7 days to a sublethal concentration (0.6 ppm) of water-soluble fraction of crude oil (WSF). Exposure to WSF induced responses at the protein expression level. Two-dimensional gel electrophoresis (2-DE) revealed 10 protein spots that were differentially expressed by WSF exposure. Seven proteins were identified using MS/MS and de novo sequencing. Nm23 oncoprotein, arginine methyltransferase, fatty aldehyde dehydrogenase and glutathione S-transferase were down-regulated, whereas two glyceraldehyde-3-phosphate dehydrogenase isoforms and a lipocalin-like crustacyanin (CTC) were up-regulated after WSF exposure. CTC mRNA levels were further analyzed by quantitative real-time PCR showing an increased expression after WSF exposure. The proteins identified are involved in carbohydrate and amino acid metabolism, detoxification, transport of hydrophobic molecules and cellular homeostasis among others. These results provide evidence for better understanding the toxic mechanisms of hydrocarbons. Moreover, some of these differentially expressed proteins would be employed as potential novel biomarkers.

Egg carotenoproteins in neotropical Ampullariidae (Gastropoda: Arquitaenioglossa)

Carotenoid-binding proteins are commonly found in invertebrates. Their carotenoids form non-covalent complexes with proteins giving tissues a variety of colors. In molluscs they have been described in only a few species. In particular, the egg perivitellin fluid of those Ampullariid species which deposit eggs above the waterline is provided with carotenoproteins playing several roles ranging from photoprotection, antioxidant or antitrypsin actions to nutrient provision for development. These molecules form complex glyco-lipo-carotenoproteins of high molecular weight where either free astaxanthin (3,3'-dihydroxy-beta, beta'-carotene- 4,4'dione) or astaxanthin esterified with fatty acids, occur more frequently. This review compiles the current knowledge on the biochemical composition and biophysical data on the chemical and thermal stability of egg carotenoproteins in ampullariid. In addition, recent data on their metabolism, their cellular site of biosynthesis during perivitellogenesis, as well as their carotenoid binding properties are reviewed, highlighting the physiological significance of carotenoproteins in the context of the reproductive biology of these molluscs.

Pallial oviduct of Pomacea canaliculata (Gastropoda): ultrastructural studies of the parenchymal cellular types involved in the metabolism of perivitellins

Seasonal variations in the morphology of the parenchymal mass and function of the albumen gland/capsule gland complex have been studied in Pomacea canaliculata, together with the cellular types involved in the synthesis and secretion of perivitellin fluid components. The two major parenchymal cell types, albumen secretory cells (AS) and labyrinthic cells (LC), undergo seasonal variations throughout the annual reproductive cycle, which is divided into three periods. Both cellular types show maximal development and structural complexity during the reproductive period (spring and summer). AS cells have a well-developed Golgi complex and rough endoplasmic reticulum and their secretory granules show electron-dense particles of about 20 nm (probably galactogen). These cells are uniquely involved in ovorubin and PV2 perivitellin synthesis and their secretory granules are the single storage site for these two major perivitellins, as revealed by immunoelectron microscopy. AS also possess calcium deposits that infiltrate the cytoplasmic matrix. The luminal surfaces of LC exhibit long cilia intermingled with sparce short microvilli. Basally, the plasma membrane shows deep irregular folds that extend through the cytoplasm up to the subapical region. Calcium deposits infiltrate the cytoplasm and accumulate in the extracellular space of the basal labyrinth. Nerve terminals seem to be involved in the regulation of parenchymal cell secretion. At the post-reproductive period, AS markedly change their aspect following the release of most of the secretory granules into the acinar lumen. LC decrease in volume, the number of their cilia decreases, their cytoplasmic folds are much thinner and their extracellular spaces lack calcium particles. At the pre-reproductive period (winter), AS and LC recover and prepare for the subsequent period.