3.7. Water transport through aquaporins

1. Directionality of water flow is determined by osmotic and hydraulic forces

2. Membrane permeability to water can be defined with either an osmotic coefficient (P f ) or a diffusional coefficient (P d )

3. The nonequivalence of P f and P d provides evidence for water channels P f involves net flow of water. Each water molecule entering the channel form the left will knock out one molecule on the right. In the diffusion flow case, a molecule of labeled water entering the channel from the left can diffuse back into the solution on the left.

Model for water flow through a single- file, multiple occupancy aquaporin Water movement across biological membranes occurs through both the lipid bilayer and the pores formed by water channels.

4. Aquaporins are members of the major intrinsic protein family, which can form water channels when expressed in heterologous systems characterized by the highly conserved NPA (Asn-Pro-Ala) residue in the N and C terminal. plasma membrane intrinsic protein, PIP in plasma membrane tonoplant intrinsic protein, TIP in vacuole

Structure of an aquaporin showing the six transmembrane helices and two conserved NPA (Asn-Pro-Ala) residue

Aquaporin function can be confirmed by expression of the cDNA in Xenopus oocytes. cDNA expression Hypoosmotic shock Faster swelling (inhibited by Hg2+)

Three-dementional structure of aquaporin-1 from human erythrocytes. Extracellular view of eight asymmetrical subunits that form two tetramers. One of the monomers of the central tetramer is colored gold.

5. Aquaporin activity is regulated transcriptionally and posttranslationally - Each isoform has a tissue specific distribution - There is evidence that some are up-regulated in response to certain environmental stimuli such as blue light, ABA, GA. - Aquaporin activity can be regulated by phosphorylation (CDPK).

H2OH2O Aquaporin TranscriptionPosttranslation Environmental stimuli (blue light, ABA, GA, cold & drought) Phosphorylation by Ca 2+ dependent protein kinase

Figure. Schematic representation of putative mechanisms involved in plant aquaporin regulation. (a) Control of transcription and protein abundance. Drought and salinity, as other environmental stimuli, are known to act on aquaporin gene transcription and possibly interfere with aquaporin translation and degradation, thereby determining protein abundance. (b) Sub-cellular relocalization. The redistribution of a TIP aquaporin, from the tonoplast (TP) to small intracellular vesicles, was demonstrated in Mesembryanthemum crystallinum suspension cells exposed to a hyperosmotic treatment (Vera-Estrella et al. 2004). The occurrence of a similar relocalization mechanism for PIP aquaporins is shown but remains hypothetical.

6. Plasma membrane aquaporins may play a role in facilitating transcellular water flow - in water absorption in root - in water transpiration in leaf

7. Differential water permeabilities of the vacuolar and plasma membranes can prevent large changes in cytoplasmic volume during water stress water permeability of the vacuolar membrane water permeability of the plasma membrane (100-fold) Normal stressed

vacuole Plasma membrane aquaporins may play a role in facilitating transcellular water flow Plant water channel Plasma membrane Tonoplant intrinsic protein(TIP) Plasma membrane intrinsic protein(PIP)

Differential water permeabilities of the vacuolar and plasma membranes can prevent large changes in cytoplasmic volume during water stress The water permeability of the vacuolar membrane The water permeability of the plasma membrane