The G zipper motif is found in transmembrane proteins at an above random frequency and there are models explaining how it might help with multiple transmembrane intra-membrane helix bundle assembly.
However there are a bunch of different "flavours" of these G zipper motifs. Here are the ones I have come across so far:
I don't think I am completely up to date so that is a non-exhaustive list. Have I missed any? Broadly speaking what accounts for their variation?