The structure of interfaces between subunits of dimeric and tetrameric proteins

The structures of the interfaces of nine dimeric and nine tetramericproteins have been analyzed and have been seen to follow generalprinciples. These interfaces are combinations of four structuralmotifs, which resemble features of monomeric proteins. Theseare: (i) extended beta sheet; (ii) helix–helix packing;(iii) sheet–sheet packing; and (iv) loop interactions.Other common structural features in the interfaces studied aretwo-fold symmetry, charged hydrogen bonds and channel formation(found only in tetramers). Monomer–monomer interfacesare intermediate in hydrophobicity and charge between the interfacesbetween secondary structures of monomeric proteins and the exteriorsof monomeric proteins. A typical interface has one of the firstthree of the structural motifs at its centre and loop interactionsaround the outside, where most of the charge resides.