Molecular Cloning, Water Channel Activity and Tissue Specific Expression of Two Isoforms of Radish Vacuolar Aquaporin1.

A major membrane intrinsic protein (VM23) in vacuoles of radish (Raphanus) tap root was investigated. The cDNAs for two isoforms of VM23, γ- and δ-VM23, encode polypeptides of 253 and 248 amino acids, respectively, γ-and δ-VM23 correspond to the γ- and δ-TIP (tonoplast intrinsic protein) of Arabidopsis. The deduced amino acid sequences of the two VM23 isoforms were 60% identical. The ammo-terminal sequence of γ-VM23 showed agreement with the direct sequence of the purified VM23, suggesting that γ-VM23 is the most abundant molecule among the VM23 isoforms. When mRNAs of γ- and δ-VM23 were injected into Xenopus oocytes, the osmotic water permeability of oocytes increased 6-fold (60 to 200 μM s1) of the control oocytes. The transcripts of both isoforms were detected in a high level in growing hypocotyls and young leaves, but δ-VM23 was not detected in seedling roots. Light illumination enhanced the transcription of two genes of VM23 in cotyledons and roots but suppressed their expression in hypocotyls the growth of which was inhibited by light. These findings suggest that the expression of VM23 is tightly related to cell elongation [ABSTRACT FROM AUTHOR]