Na+/H+ Exchanger NHE3 Has 11 Membrane Spanning Domains and a Cleaved Signal Peptide: Topology Analysis Using In Vitro Transcription/Translation

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Abstract
The transmembrane topology of Na+/H+ exchanger NHE3 has been studied using in vitro transcription/translation of two types of fusion vectors designed to test membrane insertion properties of cDNA sequences encoding putative NHE3 membrane spanning domains (msds). These vectors encode N-terminal 101 (HKM0) or 139 (HKM1) amino acids of the H,K-ATPase α-subunit, a linker region and a reporter sequence containing five N-linked glycosylation consensus sites in the C-terminal 177 amino acids of the H,K-ATPase β-subunit. The glycosylation status of the reporter sequence was used as a marker for the analysis of signal anchor and stop transfer properties of each putative msd in both the HKM0 and the HKM1 vectors. The linker region of the vectors was replaced by sequences that contain putative msds of NHE3 individually or in pairs. In vitro transcription/translation was performed using [35S]methionine in a reticulocyte lysate system ± microsomes, and the translation products were identified by autoradiography following separation using SDS−PAGE. We propose a revised NHE3 topology model, which contains a cleaved signal peptide followed by 11 msds, including extracellular orientation of the N-terminus and intracellular orientation of the C-terminus. The presence of a cleavable signal peptide in NHE3 was demonstrated by its cleavage from NHE3 during translational processing of full-length and truncated NHE3 in the presence of microsomes. Of 11 putative msds, six (msds 1, 2, 4, 7, 10, and 11) acted as both signal anchor and stop transfer sequences, while five (msds 3, 5, 6, 8, and 9) had signal anchor activities when tested alone. Of the latter, 3, 5, 6, and 9 were shown to act as stop transfer sequences after C-terminal extension. The actual membrane orientation of each sequential transmembrane segment of NHE3 was deduced from the membrane location of the N- and C-termini of NHE3. The regions between putative msds 8 and 9 and between msds 10 and 11, which correspond to the fourth and fifth extracellular loops, did not act as msds when tested alone. However, the extension of the fifth extracellular loop with adjacent putative msds showed some membrane-associated properties suggesting that the fifth extracellular loop might be acting as a “P-loop”-like structure.