Functional and molecular characteristics of rice and barley NIP aquaporins transporting water, hydrogen peroxide and arsenite

Mercury-sensitive water transport activities were detected in seven NIP (Nodulin 26-like intrinsic protein) type aquaporins among eleven NIPs examined. Amino acid substitutions in rice OsNIP3;3 revealed that mercury-sensitivity depended on a histidine (but not on a cysteine) in apoplastic loop C in plant NIP aquaporins, although the cysteine is involved in the mercury-sensitivity of animal aquaporins. Rice OsNIP3;3 was also first identified as a unique aquaporin facilitating all water, hydrogen peroxide and arsenite transports. In rice OsNIP3;2, hydrogen peroxide and arsenite transport activities were detected, but water transport was not. Barley HvNIP1;2- or rice OsNIP2;1-expressing yeast cells showed the arsenite transport activity but not the H₂O₂transport activity. The present work revealed novel molecular mechanisms of water and other low molecular weight compounds transport/selection in barley and rice NIP aquaporins, including the histidine-related mercury-sensitivity in the water transport of aquaporins.