CO2 Transport by PIP2 Aquaporins of Barley

CO2 permeability of plasma membrane intrinsic protein 2 (PIP2) aquaporins of Hordeum vulgare L. was investigated. Five PIP2 members were heterologously expressed in Xenopus laevis oocytes. CO2 permeability was determined by decrease of cytosolic pH in CO2-enriched buffer using a hydrogen ion-selective microelectrode. HvPIP2; 1, HvPIP2; 2, HvPIP2; 3 and HvPIP2; 5 facilitated CO2 transport across the oocyte cell membrane. However, HvPIP2; 4 that is highly homologous to HvPIP2; 3 did not. The isoleucine residue at position 254 of HvPIP2; 3 was conserved in PIP2 aquaporins of barley, except HvPIP2; 4, which possesses methionine instead. CO2 permeability was lost by the substitution of the Ile254 of HvPIP2; 3 by methionine, while water permeability was not affected. These results suggest that PIP2 aquaporins are permeable to CO2. and the conserved isoleucine at the end of the E-loop is crucial for CO2 selectivity.