Transmembrane channels formed strands

Figure 8 The bacterial ferric citrate uptake protein, FecA, without ferric citrate bound (left) and with ferric citrate (right). The 22-stranded j3 barrel (in blue) forms the transmembrane channel the plug domain (in green) blocks passage of non-ferric citrate substrates the switch helix (in orange, not seen in the ligated structure) unwinds to signal binding of ferric citrate. ...

Since the outside of the barrel faces hydrophobic lipids of the membrane and the inside forms the solvent-exposed channel, one would expect the P strands to contain alternating hydrophobic and hydrophilic side chains. This requirement is not strict, however, because internal residues can be hydrophobic if they are in contact with hydrophobic residues from loop regions. The prediction of transmembrane p strands from amino acid sequences is therefore more difficult and less reliable than the prediction of transmembrane a helices. 

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