The zebrafish genome encodes the largest vertebrate repertoire of functional aquaporins with dual paralogy and substrate specificities similar to mammals

Background  

Aquaporins are integral membrane proteins that facilitate the transport of water and small solutes across cell membranes. These proteins are vital for maintaining water homeostasis in living organisms. In mammals, thirteen aquaporins (AQP0-12) have been characterized, but in lower vertebrates, such as fish, the diversity, structure and substrate specificity of these membrane channel proteins are largely unknown.     

Plant aquaporins: Roles in plant physiology

Background

Aquaporins are membrane channels that facilitate the transport of water and small neutral molecules across biological membranes of most living organisms.

Scope of review

Here, we present comprehensive insights made on plant aquaporins in recent years, pointing to their molecular and physiological specificities with respect to animal or microbial counterparts.

Major conclusions

In plants, aquaporins occur as multiple isoforms reflecting a high diversity of cellular localizations and various physiological substrates in addition to water. Of particular relevance for plants is the transport by aquaporins of dissolved gases such as carbon dioxide or metalloids such as boric or silicic acid. The mechanisms that determine the gating and subcellular localization of plant aquaporins are extensively studied. They allow aquaporin regulation in response to multiple environmental and hormonal stimuli. Thus, aquaporins play key roles in hydraulic regulation and nutrient transport in roots and leaves. They contribute to several plant growth and developmental processes such as seed germination or emergence of lateral roots.

General significance

Plants with genetically altered aquaporin functions are now tested for their ability to improve plant resistance to stresses. This article is part of a Special Issue entitled Aquaporins.     

植物水通道蛋白及其活性调节

水通道蛋白是对水专一的通道蛋白,普遍存在于动、植物及微生物中。研究表明高等植物的质膜和液泡膜上存在着丰富的水通道蛋白,其种类繁多,分布广泛,并具有一定的组织特异性。植物水通道蛋白的活性受到严格的调控,其调节方式主要有两种,分别为基因水平的表达调控和翻译后的修饰作用。     

Aquaporins and biological rhythm

Aquaporins are membrane-intrinsic proteins that facilitate membrane transport of water and small solutes or even gases. Aquaporin genes are found in almost all living organisms. In plants the proteins account for water uptake and transport as well as CO₂ availability for photosynthesis. These processes are subjected to diurnal or circadian regimes. Expression and even function of aquaporins also follows day - night rhythms. Significance of aquaporin function in chronobiology has been provided by recent publications, which are summarised here. Examples of the significance of aquaporins in processes related to chronobiology are given for root water transport and leaf movement in several plant species.     

Folding and stability of the aquaglyceroporin GlpF: Implications for human aqua(glycero)porin diseases

Aquaporins are highly selective polytopic transmembrane channel proteins that facilitate the permeation of water across cellular membranes in a large diversity of organisms. Defects in aquaporin function are associated with common diseases, such as nephrogenic diabetes insipidus, congenital cataract and certain types of cancer. In general, aquaporins have a highly conserved structure; from prokaryotes to humans. The conserved structure, together with structural dynamics and the structural framework for substrate selectivity is discussed. The folding pathway of aquaporins has been a topic of several studies in recent years. These studies revealed that a conserved protein structure can be reached by following different folding pathways. Based on the available data, we suggest a complex folding pathway for aquaporins, starting from the insertion of individual helices up to the formation of the tetrameric aquaporin structure. The consequences of some known mutations in human aquaporin-encoding genes, which most likely affect the folding and stability of human aquaporins, are discussed.     

水通道的生物学研究

美国约翰·霍普金斯大学生化教授彼得·埃格瑞博士,他被瑞典皇家科学院授予2003年诺贝尔化学奖,以承认他们的实验室在1991年发现的水通道。水通道或称水孔蛋白的发现为这类蛋白的生化学、生理学和遗传学开创了一个黄金时代。 现在已鉴定了10余种哺乳动物水通道,每种都有其特定的组织分布。在肾脏、肺、眼和脑等组织表达多种水通道,构成水转运的网络。已确定水孔蛋白的基本结构是在细胞膜中以四聚体的形式存在。水通道参与许多临床疾病,包括尿崩症到各种形式水肿的病生理学过程。它们可能是治疗水平衡紊乱性疾病的靶。     

Aquaporin Expression and Freeze Tolerance in Candida albicans

Aquaporins are members of the major intrinsic protein superfamily of integral membrane proteins which enable the transport of water, glycerol, and other solutes across membranes in various organisms. In microorganisms, the physiological role of aquaporins is not yet defined. We found a clear correlation between expression of the Candida albicans aquaporin-encoding gene AQY1 and freeze tolerance. A connection with the function for the aquaporin in the natural environment of C. albicans is, however, not obvious.     

Aquaporins in health and disease

The molecular basis of membrane water-permeability remained elusive until the recent discovery of the aquaporin water-channel proteins. The fundamental importance of these proteins is suggested by their conservation from bacteria through plants to mammals. Ten mammalian aquaporins have thus far been identified, each with a distinct distribution. In the kidney, lung, eye and brain, multiple water-channel homologs are expressed, providing a network for water transport in those locations. It is increasingly clear that alterations in aquaporin expression or function can be rate-limiting for water transport across certain membranes. Aquaporins are likely to prove central to the pathophysiology of a variety of clinical conditions from diabetes insipidus to various forms of edema and, ultimately, they could be a target for therapy in diseases of altered water homeostasis.     

Purification and characterization of two protein kinases acting on the aquaporin SoPIP2;1

Aquaporins are water channel proteins that facilitate the movement of water and other small solutes across biological membranes. Plants usually have large aquaporin families, providing them with many ways to regulate the water transport. Some aquaporins are regulated post-translationally by phosphorylation. We have previously shown that the water channel activity of SoPIP2;1, an aquaporin in the plasma membrane of spinach leaves, was enhanced by phosphorylation at Ser115 and Ser274. These two serine residues are highly conserved in all plasma membrane aquaporins of the PIP2 subgroup. In this study we have purified and characterized two protein kinases phosphorylating Ser115 and Ser274 in SoPIP2;1. By anion exchange chromatography, the Ser115 kinase was purified from the soluble protein fraction isolated from spinach leaves. The Ca²⁺-dependent Ser274 kinase was purified by peptide affinity chromatography using plasma membranes isolated from spinach leaves. When characterized, the Ser115 kinase was Mg²⁺-dependent, Ca²⁺-independent and had a pH-optimum at 6.5. In accordance with previous studies using the oocyte expression system, site-directed mutagenesis and kinase and phosphatase inhibitors, the phosphorylation of Ser274, but not of Ser115, was increased in the presence of phosphatase inhibitors while kinase inhibitors decreased the phosphorylation of both Ser274 and Ser115. The molecular weight of the Ser274 kinase was approximately 50 kDa. The identification and characterization of these two protein kinases is an important step towards elucidating the signal transduction pathway for gating of the aquaporin SoPIP2;1.     

Aquaporin expression and freeze tolerance in Candida albicans

Aquaporins are members of the major intrinsic protein superfamily of integral membrane proteins which enable the transport of water, glycerol, and other solutes across membranes in various organisms. In microorganisms, the physiological role of aquaporins is not yet defined. We found a clear correlation between expression of the Candida albicans aquaporin-encoding gene AQY1 and freeze tolerance. A connection with the function for the aquaporin in the natural environment of C. albicans is, however, not obvious.     

Biological significance and topological basis of aquaporin-partnering protein-protein interactions

Aquaporins (AQPs) are intramolecular channels essential for transport of H₂O, CO₂, and other small substrates across membranes. Through this function, AQPs can modulate CO₂ uptake and assimilation in plants and regulate water relations and many other physiological processes in all living organisms. To execute their physiological roles, AQPs may experience 3 types of hetero-molecular interaction, between AQPs and their kinases; between AQP isoforms; and between AQPs and other proteins that are neither AQPs nor kinases. Interacting with non-AQP non-kinase proteins may enable AQPs to extend their functions beyond substrate transport, and most fascinatingly, to serve as a gateway control for translocation of virulence effectors from pathogenic bacteria into the cytosol of eukaryotic cells. In this mini review, we will summarize the latter 2 types of interaction and discuss the physiological and/or pathological significance. We will also discuss a research angle to elucidate the structural basis of AQP-partnering protein interactions.     

Aquaporins of the PIP2 class are required for efficient anther dehiscence in tobacco

Several processes during sexual reproduction in higher plants involve the movement of water between cells or tissues. Before flower anthesis, anther and pollen dehydration takes place before the release of mature pollen at dehiscence. Aquaporins represent a class of proteins that mediates the movement of water over cellular membranes. Aquaporins of the plasmamembrane PIP2 family are expressed in tobacco (Nicotiana tabacum) anthers and may therefore be involved in the movement of water in this organ. To gain more insight into the role these proteins may play in this process, we have analyzed their localization using immunolocalizations and generated plants displaying RNA interference of PIP2 aquaporins. Our results indicate that PIP2 protein expression is modulated during anther development. Furthermore, in tobacco PIP2 RNA interference plants, anther dehydration was slower, and dehiscence occurred later when compared with control plants. Together, our results suggest that aquaporins of the PIP2 class are required for efficient anther dehydration prior to dehiscence.     

Aquaporin-facilitated transmembrane diffusion of hydrogen peroxide

Background

Hydrogen peroxide (H₂O₂) is an important signaling compound that has recently been identified as a new substrate for several members of the aquaporin superfamily in various organisms. Evidence is emerging about the physiological significance of aquaporin-facilitated H₂O₂ diffusion.

Scope of review

This review summarizes current knowledge about aquaporin-facilitated H₂O₂ diffusion across cellular membranes. It focuses on physicochemical and experimental evidence demonstrating the involvement of aquaporins in the transport of this redox signaling compound and discusses the regulation and structural prerequisites of these channels to transmit this signal. It also provides perspectives about the potential importance of aquaporin-facilitated H₂O₂ diffusion processes and places this knowledge in the context of the current understanding of transmembrane redox signaling processes.

Major conclusions

Specific aquaporin isoforms facilitate the passive diffusion of H₂O₂ across biological membranes and control H₂O₂ membrane permeability and signaling in living organisms.

General significance

Redox signaling is a very important process regulating the physiology of cells and organisms in a similar way to the well-characterized hormonal and calcium signaling pathways. Efficient transmembrane diffusion of H₂O₂, a key molecule in the redox signaling network, requires aquaporins and makes these channels important players in this signaling process. Channel-mediated membrane transport allows the fine adjustment of H₂O₂ levels in the cytoplasm, intracellular organelles, the apoplast, and the extracellular space, which are essential for it to function as a signal molecule. This article is part of a Special Issue entitled Aquaporins.     

The role of aquaporins in root water uptake

The capacity of roots to take up water is determined in part by the resistance of living tissues to radial water flow. Both the apoplastic and cell-to-cell paths mediate water transport in these tissues but the contribution of cell membranes to the latter path has long been difficult to estimate. Aquaporins are water channel proteins that are expressed in various membrane compartments of plant cells, including the plasma and vacuolar membranes. Plant aquaporins are encoded by a large multigene family, with 35 members in Arabidopsis thaliana, and many of these aquaporins show a cell-specific expression pattern in the root. Mercury acts as an efficient blocker of most aquaporins and has been used to demonstrate the significant contribution of water channels to overall root water transport. Aquaporin-rich membranes may be needed to facilitate intense water flow across root tissues and may represent critical points where an efficient and spatially restricted control of water uptake can be exerted. Roots, in particular, show a remarkable capacity to alter their water permeability over the short term (i.e. in a few hours to less than 2-3 d) in response to many stimuli, such as day/night cycles, nutrient deficiency or stress. Recent data suggest that these rapid changes can be mostly accounted for by changes in cell membrane permeability and are mediated by aquaporins. Although the processes that allow perception of environmental changes by root cells and subsequent aquaporin regulation are nearly unknown, the study of root aquaporins provides an interesting model to understand the regulation of water transport in plants and sheds light on the basic mechanisms of water uptake by roots.     

The role of aquaporins in cellular and whole plant water balance

Aquaporins are water channel proteins belonging to the major intrinsic protein (MIP) superfamily of membrane proteins. More than 150 MIPs have been identified in organisms ranging from bacteria to animals and plants. In plants, aquaporins are present in the plasma membrane and in the vacuolar membrane where they are abundant constituents. Functional studies of aquaporins have hitherto mainly been performed by heterologous expression in Xenopus oocytes. A main issue is now to understand their role in the plant, where they are likely to be important both at the cellular and at the whole plant level. Plants contain a large number of aquaporin isoforms with distinct cell type- and tissue-specific expression patterns. Some of these are constitutively expressed, whereas the expression of others is regulated in response to environmental factors, such as drought and salinity. At the protein level, regulation of water transport activity by phosphorylation has been reported for some aquaporins.     

Molecular Cloning,Overexpression and Characterization of a Novel Water Channel Protein from Rhodobacter sphaeroides

Aquaporins are highly selective water channel proteins integrated into plasma membranes of single cell organisms; plant roots and stromae; eye lenses, renal and red blood cells in vertebrates. To date, only a few microbial aquaporins have been characterized and their physiological importance is not well understood. Here we report on the cloning, expression and characterization of a novel aquaporin, RsAqpZ, from a purple photosynthetic bacterium, Rhodobacter sphaeroides ATCC 17023. The protein was expressed homologously at a high yield (∼20 mg/L culture) under anaerobic photoheterotrophic growth conditions. Stopped-flow light scattering experiments demonstrated its high water permeability (0.17±0.05 cm/s) and low energy of activation for water transport (2.93±0.60 kcal/mol) in reconstituted proteoliposomes at a protein to lipid ratio (w/w) of 0.04. We developed a fluorescence correlation spectroscopy based technique and utilized a fluorescent protein fusion of RsAqpZ, to estimate the single channel water permeability of RsAqpZ as 1.24 (±0.41) x 10⁻¹² cm³/s or 4.17 (±1.38)×10¹⁰ H₂O molecules/s, which is among the highest single channel permeability reported for aquaporins. Towards application to water purification technologies, we also demonstrated functional incorporation of RsAqpZ in amphiphilic block copolymer membranes.     

Switch from an aquaporin to a glycerol channel by two amino acids substitution

The MIP (major intrinsic protein) proteins constitute a channel family of currently 150 members that have been identified in cell membranes of organisms ranging from bacteria to man. Among these proteins, two functionally distinct subgroups are characterized: aquaporins that allow specific water transfer and glycerol channels that are involved in glycerol and small neutral solutes transport. Since the flow of small molecules across cell membranes is vital for every living organism, the study of such proteins is of particular interest. For instance, aquaporins located in kidney cell membranes are responsible for reabsorption of 150 liters of water/day in adult human. To understand the molecular mechanisms of solute transport specificity, we analyzed mutant aquaporins in which highly conserved residues have been substituted by amino acids located at the same positions in glycerol channels. Here, we show that substitution of a tyrosine and a tryptophan by a proline and a leucine, respectively, in the sixth transmembrane helix of an aquaporin leads to a switch in the selectivity of the channel, from water to glycerol.     

Multifaceted roles of aquaporins as molecular conduits in plant responses to abiotic stresses

Abiotic stress has become a challenge to food security due to occurrences of climate change and environmental degradation. Plants initiate molecular, cellular and physiological changes to respond and adapt to various types of abiotic stress. Understanding of plant response mechanisms will aid in strategies aimed at improving stress tolerance in crop plants. One of the most common and early symptoms associated with these stresses is the disturbance in plant–water homeostasis, which is regulated by a group of proteins called “aquaporins”. Aquaporins constitute a small family of proteins which are classified further on the basis of their localization, such as plasma membrane intrinsic proteins, tonoplast intrinsic proteins, nodulin26-like intrinsic proteins (initially identified in symbiosomes of legumes but also found in the plasma membrane and endoplasmic reticulum), small basic intrinsic proteins localized in ER (endoplasmic reticulum) and X intrinsic proteins present in plasma membrane. Apart from water, aquaporins are also known to transport CO₂, H₂O₂, urea, ammonia, silicic acid, arsenite and wide range of small uncharged solutes. Besides, aquaporins also function to modulate abiotic stress-induced signaling. Such kind of versatile functions has made aquaporins a suitable candidate for development of transgenic plants with increased tolerance toward different abiotic stress. Toward this endeavor, the present review describes the versatile functions of aquaporins in water uptake, nutrient balancing, long-distance signal transfer, nutrient/heavy metal acquisition and seed development. Various functional genomic studies showing the potential of specific aquaporin isoforms for enhancing plant abiotic stress tolerance are summarized and future research directions are given to design stress-tolerant crops.