Work

Mechanism of CO 2 and NH 3 transport through human aquaporin 1: Evidence for parallel CO 2 pathways

Year: 2025

Type: article

Abstract: The traditional view had been that dissolved gases cross membranes simply by dissolving in and diffusing through membrane lipid. However, some membranes are impermeable to CO 2 and NH 3 , whereas some aquaporin (AQP) water channels—tetramers with hydrophobic central pores—are permeable to CO 2 , NH 3 , or both. Nevertheless, we understand neither the routes that CO 2 and NH 3 take through AQP tetramers, nor the basis of CO 2 /NH 3 selectivity. Here, we show— for human AQP1 (hAQP1)—that virtually all NH 3 and H 2 O pass through the hydrophilic, monomeric pores. However CO 2 passes both through the monomeric pores and another pathway. We expressed hAQP1 in Xenopus oocytes and used microelectrodes to monitor the maximal surface-pH transient (ΔpH S ) caused by CO 2 or NH 3 influxes. We found that p-chloromercuribenzene sulfonate (pCMBS)—which reacts with C189 in the monomeric pore—eliminates the entire hAQP1-dependent (*) NH 3 signal (ΔpH S *) NH3 , but only half of the signals for CO 2 (ΔpH S *) CO2 or osmotic water permeability P f *. 4,4’-diisothiocyanatostilbene-2,2’-disulfonate (DIDS), eliminates the remaining (ΔpH S *) CO2 but has no effect on (ΔpH S *) NH3 or P f *. Together, the two drugs completely eliminate the CO 2 permeability of hAQP1. When we express hAQP1 in Pichia pastoris , treat spheroplasts with DIDS, and examine hAQP1 by SDS-PAGE, reactivity with an anti-DIDS antibody shows that DIDS crosslinks hAQP1 monomers. Our results provide the first evidence that a molecule can move through an AQP via a route other than the monomeric pore, and raise the possibility that selectivity depends on the extent to which CO 2 /NH 3 move through monomeric pores vs. an alternate pathway (e.g., the central pore). Key Points Some membranes have little or no CO 2 permeability, absent protein channels like aquaporin-1 (AQP1). We confirm that, during CO 2 influx, heterologous expression of human AQP1 (hAQP1) in Xenopus oocytes increases the magnitude of the transient surface-pH increase by an amount (ΔpH S *) CO2 , measured with microelectrodes. During NH 3 influx, hAQP1 expression increases the magnitude of the transient pH S decrease by (ΔpH S *) NH3 . p-chloromercuribenzene sulfonate (pCMBS), which reacts with C189 in the monomeric pore, reduces (ΔpH S *) CO2 by; (ΔpH S *) NH3 , to zero; and AQP1-dependent osmotic water permeability ( P f *), by half. 4,4’-diisothiocyanatostilbene-2,2’-disulfonate (DIDS) reduces (ΔpH S *) CO2 by half, but has no effect on (ΔpH S *) NH3 or P f *. DIDS crosslinks AQP1 monomers expressed in Pichia pastoris . Together, pCMBS+DIDS reduces (ΔpH S *) CO2 to zero. The C189S mutation of AQP1 eliminates the effects of pCMBS, but not DIDS. Our results thus show that CO 2 traverses AQP1 via the monomeric pore plus a novel, DIDS-sensitive route that may be the central pore.

Source: The Journal of Physiology

DOI: 10.1113/jp289556

Authors: Raif Musa-Aziz, R. Ryan Geyer, Seong-Ki Lee, Fraser J. Moss, Walter F. Boron

Institutions: Universidade de São Paulo, Case Western Reserve University

Language: English

Topic: Ion Transport and Channel Regulation

Subfield: Molecular Biology

Field: Biochemistry, Genetics and Molecular Biology

Domain: Life Sciences

Open Access status: hybrid

Estimated APC paid: $5,400