Functional Relevance of Protein Disorder: Why is Disorder Favourable?
For half a century, the central tenet of protein science has been grounded on the idea that the three-dimensional structure of a protein underlies its function. However, increasing evidence of natively unstructured but functional proteins is accumulating. Termed as intrinsically disordered proteins (IDPs), they populate a number of different conformations in isolation. Interestingly, as part of their function, some IDPs become fully or partly structured upon interaction with their binding partners. This process, known as coupled folding and binding raises the question what comes first – folding of the IDP or binding to its partner protein followed by folding. This thesis focuses on understanding the role of disorder in protein- protein interactions using biophysical characterization. Over-representation of IDPs in complex network and signalling pathways emphasizes the importance of disorder. Conformational flexibility in IDPs facilitates post-translational modifications, which provides a neat way to modulate the residual structure. This can alter affinity of IDPs to their partners and it is speculated that bound like structures of IDPs speed association. The impact of phosphorylation was explored in the KID/KIX system: phosphorylation modulates only the dissociation kinetics increasing the lifetime of the bound complex, which may be important in signalling processes. Further, phi-value analysis applied to investigate the mechanism of interaction reveals that non-native interactions play a key role in this reaction, before the IDP consolidates its final structure in the bound complex. Promiscuous interaction of IDPs with their partners often results in complexes with differing affinities. Members of BCL-2 family were explored, and the results indicate that IDPs bind to the same partner protein with marginal variation in the association rates, but significant differences in dissociation rates are observed. Thus, it seems that in such homologous but competing network of proteins, disorder facilitates complexes with differing affinities by modulating dissociation rate, again altering the lifetime of the bound complex. The work presented here demonstrates that disorder plays a role in altering complex lifetimes. Perhaps being disordered permits a level of plasticity to IDPs to adapt the rates at which they bind/unbind to many target proteins. This may be why disorder is conserved and abundant in proteins involved in intricate signalling networks.