Structural alignment of internal symmetry. All MIPs consist of 6 transmembrane helices and two half helices, HB and HE, that together form a seventh transmembrane domain, as illustrated by the cartoon representation of the AQP4 structure to the left (PDB ID: 3GD8). Internal sequence similarities and the two-fold quasi symmetry suggest that MIPs have evolved through an internal duplication. Highlighted in green are the structural elements H3 and HB, whereas corresponding parts in the second repeat are coloured in magenta. The close up to the right depicts a structural alignment of these elements showing asparagine and proline of the NPA motif at the beginning of HB and HE as sticks. The side chain of the conserved glutamine in H3 is directed towards the nitrogen of the NPA proline in HB. In almost all MIPs the corresponding interaction in the second half of the protein is provided by a backbone oxygen in H6. This is possible due to a conserved proline hindering an α-helical H-bond within H6. Interestingly, the proline in H6 is not conserved in MIPDs which in general have glutamine or glutamate at this position, suggesting that these MIPs are more symmetrical. This structure might in fact resemble the ancestral form created by the internal duplication.