Quaternary Structure of GlpF and AQPs

Each tetramer comprises four closely associated monomeric channels circled by a hydrophobic surface long enough to span the lipid bilayer (Fig. 4). Toward the cytoplasmic and periplasmic surfaces are layers that include side chains of tyrosine and tryptophan that can productively interact with the polar-nonpolar interface in the lipid head-group region as in other integral membrane proteins (Koeppe and Anderson, 1996). These layers are flanked by two outer layers of charged residues, 35 A apart, that result in net positive charge on the cytoplasmic side. 

Twelve ordered octyl glucoside molecules are associated with the aromatic regions near the external side of each tetramer (Fig. 4). The pyranoside rings interact laterally with the external tryptophan/tyrosine layer, while the alkyl chains extend toward the hydrophobic core. 

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