*Article* **Physical Background of the Disordered Nature of "Mutual Synergetic Folding" Proteins**

**Csaba Magyar 1,†, Anikó Mentes 1,†, Erzsébet Fichó 1, Miklós Cserz ˝o 1,2 and István Simon 1,\***


Received: 28 September 2018; Accepted: 21 October 2018; Published: 26 October 2018

**Abstract:** Intrinsically disordered proteins (IDPs) lack a well-defined 3D structure. Their disordered nature enables them to interact with several other proteins and to fulfil their vital biological roles, in most cases after coupled folding and binding. In this paper, we analyze IDPs involved in a new mechanism, mutual synergistic folding (MSF). These proteins define a new subset of IDPs. Recently we collected information on these complexes and created the Mutual Folding Induced by Binding (MFIB) database. These protein complexes exhibit considerable structural variation, and almost half of them are homodimers, but there is a significant amount of heterodimers and various kinds of oligomers. In order to understand the basic background of the disordered character of the monomers found in MSF complexes, the simplest part of the MFIB database, the homodimers are analyzed here. We conclude that MFIB homodimeric proteins have a larger solvent-accessible main-chain surface area on the contact surface of the subunits, when compared to globular homodimeric proteins. The main driving force of the dimerization is the mutual shielding of the water-accessible backbones and the formation of extra intermolecular interactions.

**Keywords:** dehydron; homodimer; hydrogen bond; inter-subunit interaction; intrinsically disordered protein; ion pair; mutual synergistic folding; solvent-accessible surface area; stabilization center
