Plant aquaporins with non-aqua functions: deciphering the signature sequences

Abstract

Research in recent years on plant Major Intrinsic Proteins (MIPs), commonly referred to as 'aquaporins', has seen a vast expansion in the substrates found to be transported via these membrane channels. The diversity in sizes, chemical nature and physiological significance of these substrates has meant a need to critically analyse the possible structural and biochemical properties of MIPs that transport these, in order to understand their roles. In this work we have undertaken a comprehensive analysis of all plant MIPs, coming from different families, that have been proven to transport ammonia, boron, carbon dioxide, hydrogen peroxide, silicon and urea. The sequences were analysed for all primary selectivity-related motifs (NPA motifs, ar/R filter, P1-P5 residues). In addition, the putative regulatory phosphorylation and glycosylation sites and mechanistic regulators such as loop lengths have been analysed. Further, nine specificity-determining positions (SDPs) were predicted for each group. The results show the ar/R filter residues, P2-P4 positions and some of the SDPs are characteristic for certain groups, and O-glycosylation sites are unique to a subgroup while N-glycosylation was characteristic of the other MIPs. Certain residues, especially in loop C, and structural parameters such as loop lengths also contribute to the uniqueness of groups. The comprehensive analysis makes significant inroads into appraising the intriguing diversity of plant MIPs and their roles in fundamental life processes, and provides tools for plant selections, protein engineering and transgenics.

Pollen-Specific Aquaporins NIP4;1 and NIP4;2 Are Required for Pollen Development and Pollination in Arabidopsis thaliana

“…These genes are arranged in tandem, separated only by 2453 bp (Figure 1). AQP solute selectivity and permeability is mainly applied by the ar/R (aromatic/arginine) constriction, the narrowest constriction located close to the extracellular vestibule (Hove and Bhave, 2011). There is also evidence that the ar/R region creates a repulsive barrier against protons at the channel entrance (Kreida and Törnroth-Horsefield, 2015).…”

Rapid Ammonia Gas Transport Accounts for Futile Transmembrane Cycling under NH3/NH4 + Toxicity in Plant Roots

“…4), which have high transport capacity and are known to conduct NH 3 fluxes (Jahn et al, 2004;Holm et al, 2005;Loqué et al, 2005;Saparov et al, 2007;Hove and Bhave, 2011). Kozono et al (2002) estimated the rate of water transport through AQP1 to be 3 3 10 9 molecules per subunit per second, roughly 30-fold higher than K + transport via the potassium crystallographically-sited activation (KcsA) channel, which is among the fastest ion channels (Morais-Cabral et al, 2001).…”

“…Consequently, the fluxes of NH 3 have largely been considered negligible (Britto et al, 2001a;Loqué and von Wirén, 2004), in contrast to NH 4 + fluxes, which are well characterized physiologically (Lee and Ayling, 1993;Wang et al, 1993aWang et al, , 1993bKronzucker et al, 1996) and at the molecular level (Rawat et al, 1999;von Wirén et al, 2000;Ludewig et al, 2007), at least at lower concentrations. However, the transport of NH 3 across membranes has received new attention in the light of evidence that some members of the aquaporin (AQP) family of transporters, a diverse and ubiquitous class of major intrinsic proteins (Maurel et al, 2008;Hove and Bhave, 2011), can mediate NH 3 fluxes in singlecell systems (Jahn et al, 2004;Holm et al, 2005;Loqué et al, 2005;Saparov et al, 2007). However, a convincing demonstration that AQPs transport NH 3 in planta is currently lacking.…”

Identification and Characterization of Plasma Membrane Intrinsic Protein (PIP) Aquaporin Genes in Petals of Opening Carnation Flowers

“…The sequences were H-I-G-R for the ar/R motif and T-S-A-Y-W for P1-P5 residues. These were identical to the residues of AtTIP2;1 and AtTIP2;3 that have transport activities of ammonia and hydrogen peroxide (Hove and Bhave, 2011), suggesting that DcTIP2;2 may be involved in transport of ammonia or hydrogen peroxide. Since water uptake into cells across the plasma membrane is mediated by PIPs, we further focused on PIP isoforms expressed in carnation flowers.…”

“…The number of ESTs suggest that DcPIP1;1, DcPIP2;1, and DcTIP2;2 are expressed abundantly in flowers. The members of the TIP subfamily function in transport of water as well as ammonia, hydrogen peroxide, and urea (Hove and Bhave, 2011;Sade et al, 2009). The motifs related to substrate selectivity of AQPs, ar/R motifs (Wallace and Roberts, 2004) and P1-P5 residues (Froger et al, 1998) were searched for in the sequence of DcTIP2;2.…”