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I have come across the term "internal symmetry" in the context of membrane proteins, but have never found a satisfactory definition.

I'm struggling to figure out exactly what this term means... What plane is this symmetry seen on? Is it dimers that are symmetrical or can a monomer also be internally symmetrical?

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  • $\begingroup$ Can you please provide the original context? I don't think the concept applies generally...but depending on how weakly one is using the term 'symmetry' it could be in most-any plane and within or across a monomer. $\endgroup$ – Ryan Sep 25 '15 at 18:54
  • $\begingroup$ This review paper; Structural Symmetry in Membrane Proteins, made it's way to me soon after I posted the question. It covers the significance of symmetry in membrane proteins. The term is as vague as @Ryan mentioned it could be! $\endgroup$ – James Sep 29 '15 at 11:15
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Internal symmetry in this case refers to cases where a part of the protein structure can be superimposed (approximately) on another. It's not a strict mathematical symmetry, more a 'resemblance'

For instance in the 12 transmembrane helix transporters, the first six helices are arranged similarly to the second six, such that if one were to cut the protein in half one would see that the two halves (approximately) superimposed.

Fig 3 of this Nature Micro shows it clearly. http://www.nature.com/articles/nmicrobiol20159 Where the blue helices in 3a could be rotated 180 degrees and then look very similar in arrangment to the yellow helices.

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