The yeast osmosensitive mutantfps1Δ transformed by the cauliflower BobTIP1;1 aquaporin withstand a hypo-osmotic shock

OzTracs: Optical Osmolality Reporters Engineered from Mechanosensitive Ion Channels

Interactions between physical forces and membrane proteins underpin many forms of environmental sensation and acclimation. Microbes survive osmotic stresses with the help of mechanically gated ion channels and osmolyte transporters. Plant mechanosensitive ion channels have been shown to function in defense signaling. Here, we engineered genetically encoded osmolality sensors (OzTracs) by fusing fluorescent protein spectral variants to the mechanosensitive ion channels MscL from E. coli or MSL10 from A. thaliana. When expressed in yeast cells, the OzTrac sensors reported osmolality changes as a proportional change in the emission ratio of the two fluorescent protein domains. Live-cell imaging revealed an accumulation of fluorescent sensors in internal aggregates, presumably derived from the endomembrane system. Thus, OzTrac sensors serve as osmolality-dependent reporters through an indirect mechanism, such as effects on molecular crowding or fluorophore solvation.

Water Transport in Yeasts

Water moves across membranes through the lipid bilayer and through aquaporins, in this case in a regulated manner. Aquaporins belong to the MIP superfamily and two subfamilies are represented in yeasts: orthodox aquaporins considered to be specific water channels and aquaglyceroporins (heterodox aquaporins). In Saccharomyces cerevisiae genome, four aquaporin isoforms were identified, two of which are genetically close to orthodox aquaporins (ScAqy1 and ScAqy2) and the other two are more closely related to the aquaglyceroporins (ScFps1 and ScAqy3). Advances in the establishment of water channels structure are reviewed in this chapter in relation with the mechanisms of selectivity, conductance and gating. Aquaporins are important for key aspects of yeast physiology. They have been shown to be involved in sporulation, rapid freeze-thaw tolerance, osmo-sensitivity, and modulation of cell surface properties and colony morphology, although the underlying exact mechanisms are still unknown.

First cloning and characterization of two functional aquaporin genes from an arbuscular mycorrhizal fungus Glomus intraradices

Arbuscular mycorrhizal (AM) symbiosis is known to stimulate plant drought tolerance. However, the molecular basis for the direct involvement of AM fungi (AMF) in plant water relations has not been established. Two full-length aquaporin genes, namely GintAQPF1 and GintAQPF2, were cloned by rapid amplification of cDNA 5'- and 3'-ends from an AMF, Glomus intraradices. Aquaporin localization, activities and water permeability were examined by heterologous expression in yeast. Gene expression during symbiosis was also analyzed by quantitative real-time polymerase chain reaction. GintAQPF1 was localized to the plasma membrane of yeast, whereas GintAQPF2 was localized to both plasma and intracellular membranes. Transformed yeast cells exhibited a significant decrease in cell volume on hyperosmotic shock and faster protoplast bursting on hypo-osmotic shock. Polyethylene glycol (PEG) stimulated, but glycerol inhibited, the aquaporin activities. Furthermore, the expression of the two genes in arbuscule-enriched cortical cells and extraradical mycelia of maize roots was also enhanced significantly under drought stress. GintAQPF1 and GintAQPF2 are the first two functional aquaporin genes from AMF reported to date. Our data strongly support potential water transport via AMF to host plants, which leads to a better understanding of the important role of AMF in plant drought tolerance.

Characterization of Leishmania donovani aquaporins shows presence of subcellular aquaporins similar to tonoplast intrinsic proteins of plants

Leishmania donovani, a protozoan parasite, resides in the macrophages of the mammalian host. The aquaporin family of proteins form important components of the parasite-host interface. The parasite-host interface could be a potential target for chemotherapy. Analysis of L. major and L. infantum genomes showed the presence of five aquaporins (AQPs) annotated as AQP9 (230aa), AQP putative (294aa), AQP-like protein (279aa), AQP1 (314aa) and AQP-like protein (596aa). We report here the structural modeling, localization and functional characterization of the AQPs from L. donovani. LdAQP1, LdAQP9, LdAQP2860 and LdAQP2870 have the canonical NPA-NPA motifs, whereas LdAQP putative has a non-canonical NPM-NPA motif. In the carboxyl terminal to the second NPA box of all AQPs except AQP1, a valine/alanine residue was found instead of the arginine. In that respect these four AQPs are similar to tonoplast intrinsic proteins in plants, which are localized to intracellular organelles. Confocal microscopy of L. donovani expressing GFP-tagged AQPs showed an intracellular localization of LdAQP9 and LdAQP2870. Real-time PCR assays showed expression of all aquaporins except LdAQP2860, whose level was undetectable. Three-dimensional homology modeling of the AQPs showed that LdAQP1 structure bears greater topological similarity to the aquaglyceroporin than to aquaporin of E. coli. The pore of LdAQP1 was very different from the rest in shape and size. The cavity of LdAQP2860 was highly irregular and undefined in geometry. For functional characterization, four AQP proteins were heterologously expressed in yeast. In the fps1Δ yeast cells, which lacked the key aquaglyceroporin, LdAQP1 alone displayed an osmosensitive phenotype indicating glycerol transport activity. However, expression of LdAQP1 and LdAQP putative in a yeast gpd1Δ strain, deleted for glycerol production, conferred osmosensitive phenotype indicating water transport activity or aquaporin function. Our analysis for the first time shows the presence of subcellular aquaporins and provides structural and functional characterization of aquaporins in Leishmania donovani.

Yeast water channels: an overview of orthodox aquaporins

In yeast, the presence of orthodox aquaporins has been first recognized in Saccharomyces cerevisiae, in which two genes (AQY1 and AQY2) were shown to be related to mammal and plant water channels. The present review summarizes the putative orthodox aquaporin protein sequences found in available genomes of yeast and filamentous fungi. Among the 28 yeast genomes sequenced, most species present only one orthodox aquaporin, and no aquaporins were found in eight yeast species. Alignment of amino acid sequences reveals a very diverse group. Similarity values vary from 99% among species within the Saccharomyces genus to 34% between ScAqy1 and the aquaporin from Debaryomyces hansenii. All of the fungal aquaporins possess the known characteristic sequences, and residues involved in the water channel pore are highly conserved. Advances in the establishment of the structure are reviewed in relation to the mechanisms of selectivity, conductance and gating. In particular, the involvement of the protein cytosolic N-terminus as a channel blocker preventing water flow is addressed. Methodologies used in the evaluation of aquaporin activity frequently involve the measurement of fast volume changes. Particular attention is paid to data analysis to obtain accurate membrane water permeability parameters. Although the presence of aquaporins clearly enhances membrane water permeability, the relevance of these ubiquitous water channels in yeast performance remains obscure.

Ectopic Expression of a Foreign Aquaporin Disrupts the Natural Expression Patterns of Endogenous Aquaporin Genes and Alters Plant Responses to Different Stress Conditions

Although the number of reports demonstrating the roles of individual aquaporins in plants under diverse physiological conditions is expanding, the importance of interactions between different aquaporin isoforms and their integrated functions under stress conditions remain unclear. Here, we expressed one cucumber aquaporin gene, designated CsPIP1;1, and one figleaf gourd aquaporin gene, designated CfPIP2;1, in Arabidopsis thaliana, and investigated the effect of its expression on the natural expression patterns of endogenous PIP genes under stress conditions. The transcript levels of endogenous Arabidopsis PIP members were altered differently depending on stress conditions by the expression of CsPIP1;1 or CfPIP2;1. The transgenic Arabidopsis plants that constitutively express CfPIP2;1 displayed better growth compared with the wild-type plants under dehydration stress conditions, whereas CsPIP1;1 expression exerted a negative effect on the growth of Arabidopsis under dehydration stress conditions. CsPIP1;1 or CfPIP2;1 expression facilitated seed germination under high salt stress conditions, but had no influence on the growth of Arabidopsis under cold stress conditions. Our results indicate that the ectopic expression of a foreign aquaporin gene perturbs differently the natural expression patterns of endogenous aquaporin genes depending on particular stress conditions, and thereby influences the responses of plants to different stress conditions. This implies that the up- and/or down-regulation of aquaporins and their integrated functions are crucial to the maintenance of proper water balance under stress conditions.

Characterization of plant aquaporins

Plants have been reported to contain a large set of aquaporins (38 for Arabidopsis), which has been divided into four subfamilies on the basis of similarities in their amino acid sequences. They belong to the large superfamily of major intrinsic proteins (MIP), which was the basis for the nomenclature PIP, TIP, and NIP, also indicating the subcellular localization plasma membrane, tonoplast, and nodule of the respective founding member. The fourth subfamily of small and basic intrinsic proteins is not well characterized so far. The increasing number of reports dealing with various aspects of plant aquaporins is starting to advance our understanding of aquaporin biology in plants. Fundamental questions include: what is the basic function of the different plant aquaporins, what is their primary substrate, and what is the consequence of function/malfunction of a particular aquaporin for the overall function of the plant? Biochemical and biophysical techniques can be employed to get information on the basic functional characteristics of plant aquaporins. An impressive set of techniques has been used to study aquaporin function on molecular, subcellular, and cellular levels in plants, as well as in heterologous expression systems. The physiological role of aquaporins in plants is much less well understood, but reports unraveling the physiological role of aquaporins, mainly employing genetic techniques and functional measurement on the whole plant level, are emerging. The goal of this chapter is to give an overview on the applied methods, together with some exemplary findings.