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...

Chromatin modifications are critical for the establishment and maintenance of differentiation programs. G9a, the enzyme responsible for histone H3 lysine 9 dimethylation in mammalian euchromatin, exists as two isoforms with differential inclusion of exon 10 (E10) through alternative splicing. We find that the G9a methyltransferase is required for differentiation of the mouse neuronal cell line N2a and that E10 inclusion increases during neuronal differentiation of cultured cells, as well as in the developing mouse brain. Although E10 inclusion greatly stimulates overall H3K9me2 levels, it does not affect G9a catalytic activity. Instead, E10 increases G9a nuclear localization. We show that the G9a E10(+) isoform is necessary for neuron differentiation and regulates the alternative splicing pattern of its own pre-mRNA, enhancing E10 inclusion. Overall, our findings indicate that by regulating its own alternative splicing, G9a promotes neuron differentiation and creates a positive feedback loop that reinforces cellular commitment to differentiation.