**Anikó Mentes** †**, Csaba Magyar** †**, Erzsébet Fichó and István Simon \***

Institute of Enzymology, Research Centre for Natural Sciences, Hungarian Academy of Sciences, Magyar Tudósok krt. 2., H-1117 Budapest, Hungary; mentes.aniko@ttk.mta.hu (A.M.);

magyar.csaba@ttk.mta.hu (C.M.); ficho.erzsebet@ttk.mta.hu (E.F.) **\*** Correspondence: simon.istvan@ttk.mta.hu; Tel.: +36-1-3826-710

† These authors contributed equally to the paper.

Received: 13 September 2019; Accepted: 15 October 2019; Published: 16 October 2019

**Abstract:** Several intrinsically disordered proteins (IDPs) are capable to adopt stable structures without interacting with a folded partner. When the folding of all interacting partners happens at the same time, coupled with the interaction in a synergistic manner, the process is called Mutual Synergistic Folding (MSF). These complexes represent a discrete subset of IDPs. Recently, we collected information on their complexes and created the MFIB (Mutual Folding Induced by Binding) database. In a previous study, we compared homodimeric MSF complexes with homodimeric and monomeric globular proteins with similar amino acid sequence lengths. We concluded that MSF homodimers, compared to globular homodimeric proteins, have a greater solvent accessible main-chain surface area on the contact surface of the subunits, which becomes buried during dimerization. The main driving force of the folding is the mutual shielding of the water-accessible backbones, but the formation of further intermolecular interactions can also be relevant. In this paper, we will report analyses of heterodimeric MSF complexes. Our results indicate that the amino acid composition of the heterodimeric MSF monomer subunits slightly diverges from globular monomer proteins, while after dimerization, the amino acid composition of the overall MSF complexes becomes more similar to overall amino acid compositions of globular complexes. We found that inter-subunit interactions are strengthened, and additionally to the shielding of the solvent accessible backbone, other factors might play an important role in the stabilization of the heterodimeric structures, likewise energy gain resulting from the interaction of the two subunits with different amino acid compositions. We suggest that the shielding of the β-sheet backbones and the formation of a buried structural core along with the general strengthening of inter-subunit interactions together could be the driving forces of MSF protein structural ordering upon dimerization.

**Keywords:** dehydrons; inter-subunit interactions; intrinsically disordered proteins; ion-pairs; mutual synergistic folding; solvent accessible surface area; stabilization centers
