Heteromerization of PIP aquaporins affects their intrinsic permeability

Abstract

The plant aquaporin plasma membrane intrinsic proteins (PIP) subfamily represents one of the main gateways for water exchange at the plasma membrane (PM). A fraction of this subfamily, known as PIP1, does not reach the PM unless they are coexpressed with a PIP2 aquaporin. Although ubiquitous and abundantly expressed, the role and properties of PIP1 aquaporins have therefore remained masked. Here, we unravel how FaPIP1;1, a fruit-specific PIP1 aquaporin from Fragaria x ananassa, contributes to the modulation of membrane water permeability (P f ) and pH aquaporin regulation. Our approach was to combine an experimental and mathematical model design to test its activity without affecting its trafficking dynamics. We demonstrate that FaPIP1;1 has a high water channel activity when coexpressed as well as how PIP1-PIP2 affects gating sensitivity in terms of cytosolic acidification. PIP1-PIP2 random heterotetramerization not only allows FaPIP1;1 to arrive at the PM but also produces an enhancement of FaPIP2;1 activity. In this context, we propose that FaPIP1;1 is a key participant in the regulation of water movement across the membranes of cells expressing both aquaporins.T he plasma membrane (PM) is the first barrier that limits water exchange in plant cells. The rate of its water transport capacity is mainly associated with aquaporins. Among the seven aquaporin subfamilies described in the plant kingdom, only plasma membrane intrinsic proteins (PIP) and some members of the nodulin-26-like intrinsic proteins (NIP) and X intrinsic proteins (XIP) subfamilies have been shown to be preferentially localized at the PM (1, 2). Of these, PIP aquaporins appear to have a large role in controlling membrane water permeability, whereas NIP and XIP have been mainly described as solute transporters (2-4). Plant PIP aquaporins represent a conserved subfamily that has been historically divided into two subgroups due to their differences in primary structure, PIP1 and PIP2. Interestingly, PIP aquaporins compose ∼40% of the total aquaporin set, and the PIP1 and PIP2 ratio among different species is relatively constant (5-12). Fig. S1 shows the distribution of all aquaporin genes present in plants whose genome has been completely sequenced and analyzed. Antisense inhibition experiments on Arabidopsis thaliana PIP1 and PIP2 have suggested that the two subgroups of aquaporins contribute to root or leaf hydraulic conductivity in the same way (13). In several plant species, members of the PIP1 and PIP2 subgroups were shown to be coexpressed in the same cell type (14-17).Although PIP1 are as ubiquitous as PIP2, the functional properties of each type of channel are different. PIP2 are very well described as a homotetramer with high water transport activity (18, 19) and a gating mechanism unequivocally associated with specific and conserved amino acid motifs triggered by cytosolic acidification (20-22), phosphorylation (23, 24), or divalent cation concentration (22). In contrast, PIP1 have shown complex heterogeneity in water and solute transpor...

Aquaporins Contribute to ABA-Triggered Stomatal Closure through OST1-Mediated Phosphorylation

“…Thus, an apparent functional cooperativity of PIPs coexpressed in plants is emerging. The assembly of distinct aquaporin isoforms in heterotetramers may in part contribute to this phenomenon (Zelazny et al, 2007;Yaneff et al, 2014). Oocytes were injected with the indicated amount (nanograms; in parentheses) of cRNAs encoding wild-type (PIP2;1) or mutated (S121A) PIP2;1.…”

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

“…These results were consistent with the EST search results and indicate that DcPIP2;1 and DcPIP1;1 are two major PIP isoforms expressed in carnations. It is likely that these PIP isoforms are involved in the uptake of water into petal cells, since both PIP2 and PIP1 have water transport activity in heterotetramers (Yaneff et al, 2014).…”

“…PIP2 shows high water transport activity while PIP1s show low or no water permeability (Chaumont et al, 2000;Daniels et al, 1994;Fetter et al, 2004;Yamada et al, 1995). However, PIP1 shows water permeability by interacting with PIP2 (Fetter et al, 2004), particularly by forming a heterotetramer with PIP2 (Yaneff et al, 2014). Both PIP1 and PIP2 have important roles in water transport across the plasma membrane.…”

Two aquaporins, multiple ways of assembly

“…However, while most PIP1 protein is retained in the endoplasmic reticulum unless co-expressed with PIP2, PIP2 is effectively transported to the plasma membrane. [3][4][5][6] The routing modification of PIP1 from the endoplasmatic reticulum to the plasma membrane after its co-expression with PIP2 is due to PIP1-PIP2 physical interaction 3,5 and although this interaction is well supported by experimental evidences, the biophysical aspects and the molecular mechanisms of this event have been identified only recently. 7,8 We investigated the full characterization of the biological and biophysical properties of the different hetero-oligomeric configurations formed by red beet PIP1 and PIP2 subunits.…”