CO2 Transport by PIP2 Aquaporins of Barley

Izumi C Mori,Mineo Shibasaka,Jiye Rhee,Tomoaki Horie,Shizuka Sasano,Maki Katsuhara,Toshiyuki Kaneko

doi:10.1093/pcp/pcu003

Abstract

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.

Highlights

It is widely accepted that carbon dioxide is transported across biomembranes through aquaporins in a cell
The CO2 permeability of HvPIP2 cRNA-injected Xenopus laevis oocytes was investigated by measuring the rate of cytosolic acidification in CO2-enriched buffer (Nakhoul et al 1998)
In addition to HvPIP2;1, we examined the CO2 permeability of the other four PIP2 members identified from barley (Horie et al 2011) to gain insight into the CO2 transport of PIP2 aquaporins

Summary

Introduction

It is widely accepted that carbon dioxide is transported across biomembranes through aquaporins in a cell (for a review, see Kaldenhoff, 2012). Ectopic expression of the ice plant PIP1, McMIPB, in tobacco plants increased the CO2 assimilation rate and mesophyll CO2 conductance (Kawase et al 2013). These reports imply a physiological significance of the CO2 permeation of PIP1 aquaporins. On the other hand, Hanba et al (2004) showed that overexpression of the barley PIP2, HvPIP2;1, in rice plants increased their photosynthetic rate and mesophyll CO2 conductance. This suggested that some PIP2 aquaporins are possibly permeable to CO2.

Methods

Results

Conclusion