Interaction preferences across protein-protein interfaces of obligatory and non-obligatory components are different

Background: A polypeptide chain of a protein- protein complex is said to be obligatory if it is bound to another chain throughout its functional lifetime. Such a chain might not adopt the native fold in the unbound form. A non- obligatory polypeptide chain associates with another chain and dissociates upon molecular stimulus. Although conformational changes at the interaction interface are expected, the overall 3-D structure of the non-obligatory chain is unaltered. The present study focuses on protein- protein complexes to understand further the differences between obligatory and non-obligatory interfaces. Results: A non-obligatory chain in a complex of known 3-D structure is recognized by its stable existence with same fold in the bound and unbound forms. On the contrary, an obligatory chain is detected by its existence only in the bound form with no evidence for the native-like fold of the chain in the unbound form. Various interfacial properties of a large number of complexes of known 3-D structures thus classified are comparatively analyzed with an aim to identify structural descriptors that distinguish these two types of interfaces. We report that the interaction patterns across the interfaces of obligatory and non-obligatory components are different and contacts made by obligatory chains are predominantly non-polar. The obligatory chains have a higher number of contacts per interface ( 20 \pm 14 contacts per interface) than non- obligatory chains ( 13 \pm 6 contacts per interface). The involvement of main chain atoms is higher in the case of obligatory chains ( 16.9 %) compared to non- obligatory chains ( 11.2 %). The beta- sheet formation across the subunits is observed only among obligatory protein chains in the dataset. Apart from these, other features like residue preferences and interface area produce marginal differences and they may be considered collectively while distinguishing the two types of interfaces. Conclusion: These results can be useful in distinguishing the two types of interfaces observed in structures determined in large- scale in the structural genomics initiatives, especially for those multicomponent protein assemblies for which the biochemical characterization is incomplete