RHBG Membrane Protein Introduction

Introduction of RHBG

Ammonium transporter Rh type B (RHBG) is one member of the Rh subfamily, ammonium transporter family. There are three mammalian members have been found, erythroid Rh A-glycoprotein (RhAG) and nonerythroid Rh B-glycoprotein (RhBG) and Rh C-glycoprotein (RhCG). RhBG is mainly expressed on plasma membranes of various mammalian organs, such as the liver and kidney, especially in the hepatocytes and the collecting duct (CD) of the kidney. There are 5 isoforms of RHBG are produced by alternative splicing. RHBG is predicted to contain 12 membrane-spanning domains and intracytoplasmic N- and C-termini.

Function of RHBG Membrane Protein

The obvious function of RHBG is to transport ammonia to facilitate rapid and low-energy-dependent bi-directional ammonium movement across the plasma membrane. In the kidney basolateral plasma membrane in distal sites, from the distal convoluted tubule (DCT) to the inner medullary collecting duct (IMCD), RHBG is abundantly expressed to mediate an important role in collecting duct ammonia secretion. In human hepatocellular carcinoma, RHBG is overexpressed consistent with the activating β-catenin signaling, suggesting a correlation between Wnt/β-catenin signaling and human RHBG regulation. Similarly, the RHBG expression in SW480 colon cancer cells is also dependent on β-catenin, further supporting the role of β-catenin signaling in RHBG regulation. It has been documented that the targeting and ammonium transport function of RHBG is regulated by phosphorylation and membrane skeleton binding of the C-ter cytoplasmic domain.

Fig.1 A model illustrating how the Rh proteins-Rhag (in the erythrocyte membranes). (Wright, 2009)

Application of RHBG Membrane Protein in Literature

1. Abdulnour-Nakhoul S., et al. Structural determinants of NH3 and NH4⁺ transport by mouse Rhbg, a renal Rh glycoprotein. Am J Physiol Renal Physiol. 2016, 311: F1280-F1293. PubMed ID: 27681563
   
   

   This article reveals that expression of H183A or W230A mutants inhibits NH3/NH4⁺ and methylamine (MA)/methyl ammonium (MA⁺)-induced decrease in pHs to the level observed in H₂O-injected oocytes. H183 and H342 may affect membrane expression of the transporter.

2. Takeda K. and Takemasa T. Expression of ammonia transporters Rhbg and Rhcg in mouse skeletal muscle and the effect of 6-week training on these proteins. Physiol Rep. 2015, 3(10). pii: e12596. PubMed ID: 26471760
   
   

   This article indicates that ammonia transporters Rhbg and Rhcg are more abundant in slow fiber than fast fiber muscles. In addition, the blood ammonia elevation induced by high-intensity exercise with performance improvement is suppressed by 6 weeks of swim training, although the levels of ammonia transporter proteins do not change.

3. Merhi A., et al. Wnt/beta-Catenin Signaling Regulates the Expression of the Ammonium Permease Gene RHBG in Human Cancer Cells. PLoS One. 2015, 10: e0128683. PubMed ID: 26029888
   
   

   The results of this article indicate RHBG expression as a direct target of β-catenin regulation, a pathway frequently deregulated in many cancers and associated with tumorigenesis.

4. Genetet S., et al. Evidence of a structural and functional ammonium transporter RhBG.anion exchanger 1 ankyrin-G complex in kidney epithelial cells. J Biol Chem. 2015, 290: 6925-6936. PubMed ID: 25616663
   
   

   This article firstly describes a structural and functional RhBG·kAE1·ankyrin-G complex at the plasma membrane of kidney epithelial cells, compared with the well-known Rh·eAE1·ankyrin-R complex in the red blood cell membrane. This renal complex could participate in the regulation of acid-base homeostasis.

5. Geyer R.R., et al. Relative CO(2)/NH(3) permeabilities of human RhAG, RhBG and RhCG. J Membr Biol. 2013, 246: 915-926. PubMed ID:24077989
   
   

   This article proposes a mechanism whereby RhBG and RhCG contribute to acid secretion in the collecting duct (CD) by enhancing the transport of not only NH₃ but also CO₂ across the membranes of CD cells.

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Reference

1. Wright PA and Wood CM. (2009). A new paradigm for ammonia excretion in aquatic animals: role of Rhesus (Rh) glycoproteins. 212: 2303-2312.

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