Mutual Folding Induced by Binding 2.0 (MFIB 2.0) database is a repository for protein complexes that are formed from protein chains that are intrinsically disordered or unstable in isolation. As these proteins have no stable tertiary structure in their monomeric form, their folding is induced by the assembly of the complex. This phenomenon is often termed as cooperative folding and binding or mutual synergistic folding (MSF).

Intrinsically disordered proteins (IDPs) do not have a well-defined 3D structure, but along these terms, they still represent a wide spectrum of structural states. They can be extended, random coil-like chains, but also premolten globule-like and molten globule-like IDPs that can have a considerable amount of secondary structure.

In the case of many protein complexes, especially obligate homooligomers, there is no information on the structural states of the isolated monomers, because those do not exist alone and therefore cannot be studied. Because of these cases, MFIB 2.0 collects both, direct and indirect evidence on the disorder/instability of the monomers, where direct evidence means that there is explicit information on the structural state of the monomers in isolation, while indirect means that disorder/instability of the monomers is inferred from characteristic features of the complex.

IDPs in general perform functions vital to living organisms. Since the systematic study of IDPs has been undertaken, a wealth of information has been accumulated about their biological functions, mechanisms of action and their abundance in living organisms. This information is integrated into a large number of fundamental databases, such as UniProt and are also collected in IDP-specific databases, such as DisProt or MobiDB.

Most IDP functions depend on their interactions with protein partners, they are often hubs of interaction networks and often called interaction specialists. Upon interaction with ordered proteins, IDPs generally adopt a stable structure in a process termed coupled folding and binding. Such complexes are stored in the sister database of MFIB, called Disordered Binding Sites (DIBS). IDPs often recognize their partners via short linear motifs (SLiMs). Dedicated linear motif databases, such as ELM and LMPID, provide a nice collection of the known interaction modules of IDPs specialized to bind particular domain types. In addition, binding sites in IDPs that interact with ordered proteins can be predicted from the protein sequence alone using various bioinformatics tools, such as ANCHOR2.

MFIB 2.0 complements the research of IDP interactions by providing a curated and annotated set of complexes that are formed by IDPs alone. In these cases the emerging complex structure does not form on the template surface of an ordered protein. Instead, the folding of all participating protein partners happens at the same time, coupled to the interaction in a synergistic manner. MFIB 2.0 serves as a collection of such protein complexes together with their structural and functional annotations, as well as links and literature references that describe the experimental verification of mutual synergistic folding of the constituting protein chains or features of the complexes implying that the folding of the monomers is co-dependent.