1 │ INTRODUCTION
Usually, protein exert its biological function forming a pair with other protein as a result of interaction. Due to a great diversity of variously created protein complexes, the protein-protein interactions might be defined in several contexts. They are direct when proteins stay in close, physical, contact with other protein or indirect when interaction between proteins is maintained by intermediate in the complex. 1 In respect of a stability, protein-protein interaction might be stable and irreversible, i.e. permanent, or able to temporal association/dissociation, i.e. transient. In terms of binding affinity, the permanent interactions are usually obligate, i.e. including components that cannot exist independently, while transient are non-obligate, i.e. including constituents able to exist independently. 2 Irrespective of the constituents of interactions and a type of created complexes, the interactions between proteins are crucial for a right course of all cellular processes running in the various cellular structures. 3 One of them is chromatin, a multi-subunit complex contained over 1900 proteins participating in DNA templated processes, such as gene regulation, packaging and repair. 4 In chromatin, a highly abundant are histones. They represent five classes of proteins, i.e. H2A, H2B, H3, H4 and H1, responsible for organization of chromatin structure and modulation of its genetic and epigenetic functioning.5
Histone H1 is well-known as core component of chromatin implicated in maintenance of nucleosomal and linker DNA to promote chromatin compaction and formation of its higher order structure.6,7 Such canonical account of histone H1 functioning is nowadays supplemented by its activity realized through the interactions with proteins that differ in their cellular localization and belongs to distinct functional categories. They include spliceosome-associated proteins, core histone binding chaperone proteins, regulatory transcription proteins, DNA damage repair proteins and translation-associated proteins. 8 Thus, histone H1 extended paradigm point that it should be seen as multifunctional protein engaged in the course of diverse biological processes. However, a vast majority of known histone H1 partnering proteins refers to the one human histone H1 subtype, i.e. H1.0. To know a broader range of histone H1 activities realized via interaction with partner proteins, a successive analyzes concerning subsequent histone H1 subtypes are needed. This work is aimed at fill this gap through the characteristic of proteins that interact with five standard human histone H1 somatic subtypes (H1.1 – H1.5). To do this, the identification of histone H1 partnering proteins subcellular localization was performed together with prediction of their molecular function and biological processes. Besides, the histone H1 interaction networks were constructed to show a scope of their activity. Based on the secondary structure motifs and amino acid composition of proteins binding interfaces, a strength of protein-protein interaction and a type of formed complexes was appointed. Finally, an impact of intrinsic structural disorder on protein-protein interaction was determined to define a mechanism of protein complex formation. A scheme depicting the data processing stages is presented in the Fig.1.