Topogenic properties of transmembrane segments of Arabidopsis thaliana NHX1 reveal a common topology model of the Na+/H+ exchanger family

The membrane topology of the Arabidopsis thaliana Na+/H+ exchanger isoform I (AtNHX1) was investigated by examining the topogenic function of transmembrane (TM) segments using a cell-free system. Even though the signal peptide found in the human Na+/H+ exchanger (NHE) family is missing, the N-terminal hydrophobic segment was efficiently inserted into the membrane and had an N-terminus lumen topology depending on the next TM segment. The two N-terminal TM segments had the same topology as those of TM2 and TM3 of human NHE1. In contrast, TM2 and TM3 of human NHE1 did not acquire the correct topology when the signal peptide (denoted as TM1) was deleted. Furthermore, there were three hydrophobic segments with the same topogenic properties as the TM9-H10-TM10 segments of human NHE1, which has one lumenal loop (H10) and two flanking TM segments (TM9 and TM10). These data indicate that the plant NHX isoforms can form the common membrane topology proposed for the human NHE family, even though it does not have a signal peptide.