Functional properties of soybean nodulin 26 from a comparative three-dimensional model

Plant aquaporins: A frontward to make crop plants drought resistant

Drought stress alters gene expression and causes cellular damage in crop plants. Drought inhibits photosynthesis by reducing the content and the activity of the photosynthetic carbon reduction cycle, ultimately decreasing the crop yield. The role of aquaporins (AQP) in improving the growth and adaptation of crop plants under drought stress is of importance. AQP form channels and control water transport in and out of the cells and are associated with drought tolerance mechanisms. The current review addresses: (1) the evolution of AQPs in plants, (2) the classification of plant AQPs, (3) the role of AQPs in drought alleviation in crop plants, and (4) the phytohormone crosstalk with AQPs in crops exposed to drought stress.

Phylogeny and evolution of the major intrinsic protein family

BACKGROUND INFORMATION

MIPs (major intrinsic proteins) form channels across biological membranes that control recruitment of water and small solutes such as glycerol and urea in all living organisms. Because of their widespread occurrence and large number, MIPs are a sound model system to understand evolutionary mechanisms underlying the generation of protein structural and functional diversity. With the recent increase in genomic projects, there is a considerable increase in the quantity and taxonomic range of MIPs in molecular databases.

RESULTS

In the present study, I compiled more than 450 non-redundant amino acid sequences of MIPs from NCBI databases. Phylogenetic analyses using Bayesian inference reconstructed a statistically robust tree that allowed the classification of members of the family into two main evolutionary groups, the GLPs (glycerol-uptake facilitators or aquaglyceroporins) and the water transport channels or AQPs (aquaporins). Separate phylogenetic analyses of each of the MIP subfamilies were performed to determine the main groups of orthology. In addition, comparative sequence analyses were conducted to identify conserved signatures in the MIP molecule.

CONCLUSIONS

The earliest and major gene duplication event in the history of the MIP family led to its main functional split into GLPs and AQPs. GLPs show typically one single copy in microbes (eubacteria, archaea and fungi), up to four paralogues in vertebrates and they are absent from plants. AQPs are usually single in microbes and show their greatest numbers and diversity in angiosperms and vertebrates. Functional recruitment of NOD26-like intrinsic proteins to glycerol transport due to the absence of GLPs in plants was highly supported. Acquisition of other MIP functions such as permeability to ammonia, arsenite or CO2 is restricted to particular MIP paralogues. Up to eight fairly conserved boxes were inferred in the primary sequence of the MIP molecule. All of them mapped on to one side of the channel except the conserved glycine residues from helices 2 and 5 that were found in the opposite side.

Mercury inhibits the L170C mutant of aquaporin Z by making waters clog the water channel

We conduct in silico experiments of the L170C mutant of the Escherichia coli aquaporin Z (AQPZ) with and without mercury bonded to residue Cys 170. We find that bonding mercury to Cys 170 does not induce consequential structural changes to the protein. We further find that mercury does not stick in the middle of the water channel to simply occlude water permeation, but resides on the wall of the water pore. However, we observe that the water permeation coefficient of L170C-Hg(+) (with one mercury ion bonded to Cys 170) is approximately half of that of the mercury-free L170C. We examine the interactions between the mercury ion and the waters in its vicinity and find that five to six waters are strongly attracted by the mercury ion, occluding the space of the water channel. Therefore we conclude that mercury, at low concentration, inhibits AQPZ-L170C mutant by making water molecules clog the water channel.

The conjugation of an AQP1-directed immunotoxin in the study of site-directed therapy within the CNS

PURPOSE

The water channel, aquaporin (AQP)1, is highly specific to the choroid plexus (CP) epithelium within the brain. It is therefore a potential target through which therapeutic agents could be selectively directed to the CP. Here we describe the conjugation of a monoclonal antibody (mAb), raised against an extra-cellular domain of AQP1, to the A chain of ricin (RCA). This reagent should allow study of a highly specific chemical lesion of the CP.

METHODS

A published method was used to couple the anti-AQP1 mAb to the A chain of ricin using an SPDP cross-linker. The conjugate was purified by Superdex S-200 chromatography. Column fractions were analysed by SDS-PAGE and Western blotting. Fractions containing conjugate were assayed for cytotoxicity on rat CP cells in culture, using a WST-1 viability assay to assess cell death.

RESULTS

SDS-PAGE and Western blotting demonstrated separation of conjugated anti-AQP1 mAb from unconjugated ricin by the Superdex column. Cultured rat CP cells were killed with an IC(50) of 1.81 μg/ml when exposed to the anti-AQP1 mAb-RCA conjugation reaction products. Selectivity of this reagent was demonstrated by the higher IC(50) exhibited by non-AQP1-expressing primary fibroblast cultures and cultures exposed to the reaction products from the non-specific IgG-RCA conjugation attempt.

CONCLUSION

We have been able to demonstrate selective cytotoxicity in vitro of an anti-AQP1 mAb-RCA conjugate. We hope that this work will generate further interest in the use of this site-specific conjugate to target the CP in conditions such as hydrocephalus, tumours that arise from here, and other AQP1-expressing tumours.

Aquaporins in plants

Although very often exposed to a rapid changing environment, plants are in general unable to evade from unfavourable conditions. Therefore, a fine tuned adaptation of physiology including the water balance appears to be of crucial importance. As a consequence a relatively large number of aquaporin genes are present in plant genomes. So far aquaporins in plants were shown to be involved in root water uptake, reproduction or photosynthesis. Accordingly, plant aquaporin classification as simple water pores has changed corresponding to their molecular function into channels permeable for water, small solutes and/or gases. An adjustment of the respective physiological process could be achieved by regulation mechanisms, which range from post-translational modification, molecular trafficking to heteromerization of aquaporin isoforms. Here the function of the four plant aquaporin family subclasses with regard to substrate specificity, regulation and physiological relevance is described.