Humans have a small group of genes in the Mep/Amt/Rh family, including one that codes for the Rhesus blood group antigen, RhD (RHD). The RhD protein is best known for causing erythroblastosis fetalis. Women who lack the RhD protein can produce antibodies against it if they give birth to babies that do have it (inherited from the father, see “The Structure and Function of the Rh antigen Complex“). Sometimes those maternal anti-RhD antibodies cross into the fetal circulation and destroy the circulating red blood cells.
In addition to RHD, humans have the structurally-related genes RHCE, RHAG, RHBG and RHCG in the Rh family. Animals have Rh family ammonia transporters while structurally-related transport proteins are found in bacteria (Amt) and yeast (Mep). In 2004, the structure of an Amt ammonia transport protein from a bacterium was described (1) and it was reported that this transport protein forms trimers with a narrow ammonia-selective channel in each monomer.
In 2010, a similar structure was reported for the human RhCG ammonia transport protein (2). The RhCG protein subunits also crystalized as trimers (Figure 1A). RhCG functions in the kidney for ammonia excretion (3). It is thought that functional red blood cell Rh family transport complexes can contain mixtures of RhD, RhCE and RhAG subunits where RhAG forms a functional channel while RhD and RhCE apparently play supporting roles, possibly linking trimeric Rh protein complexes to other proteins. The membrane transport function of RhAG remains controversial (4).
Image credits. The image shows a cartoon representation of a protein complex. Protein Data Bank structure 2NUU, ammonia transport complex (The crystal structure of the Escherichia coli AmtB–GlnK complex reveals how GlnK regulates the ammonia channel)