The LxxxA motif in the third transmembrane helix of the maize aquaporin ZmPIP2;5 acts as an ER export signal

The subcellular localization of aquaporins belonging to the plasma membrane intrinsic protein (PIP) subfamily is highly regulated. In maize (Zea mays), ZmPIP1s are retained in the endoplasmic reticulum (ER) whereas ZmPIP2s are able to reach the plasma membrane (PM). We recently identified a new sorting determinant which is buried within the third transmembrane domain (TM3) of ZmPIP2; 5. The Leu127 and Ala131 are required for the localization of ZmPIP2; 5 in the PM and for its exit from the ER. However, when inserted into ZmPIP1; 2, these amino acids were not sufficient to export the protein out of the ER. Here, we show that, when inserted into a truncated version of ZmPIP1; 2 consisting only of its TM3 region, Leu127 and Ala131 of ZmPIP2; 5 are able to partially bring the protein to the PM, demonstrating the active anterograde sorting function of this motif.