3.6 │ Intrinsic disorder of histone H1 subtypes-interacting protein complexes
Because transient interactions were frequently presented as coupled to the protein disorder 48,49,50, it seems advisable to check in which extend such a phenomena influence on histone H1 subtypes interactions. In order to found the disordered regions in the interfaces of physically associated proteins, their sequences were analyzed with the DisEMBL predictor. For all of the contact regions in the histone H1 subtypes, a characteristic is an occurrence of disordered residues, in the amount from 50% to 80%. On the other hand, among the histone H1 interacting proteins are six having fully ordered interfaces and six ones possessing from 83% to 100% of the disordered residues (Figure 5). While histone H1 subtypes were already recognized as abundantly disordered proteins displaying the representative features of sequence and structure attributable to the intrinsic structural disorder51, a way of their interactions and a kind of formed complexes remained unknown. The current analysis allowed to show that histone H1 subtypes may create two types of complexes via disordered motifs. According to the categories of complexes introduced by Mészáros et al. 52, the first encompass interaction between disordered H1 and ordered partner through the coupled folding and binding and the second include interaction between disordered H1 and disordered partner through the mutual synergistic folding. If so, histone H1 have potential for creation a transient (weaker) interaction with ordered partner due to the decreasing binding strength evoked by loss of conformational entropy 53 as well as to form more stable complexes with disordered partner due to a difference in the compositions and variation of the backbone sequence.54 A participation of histone H1 in the weaker interactions results from its intrinsic disorder reaching above 90% of the sequence, while a share in the stronger ones provides a presence of MoRFs and APBRs motifs stabilizing the interactions.51 Thus, it seems that a specificity of interaction is determined by an individual sequence and/or structure properties of the binding partner. By generalizing, this finding confirm that intrinsic disorder is an inherent feature of histone H1 subtypes, essential for recognition and binding of an interacting partner. A disorder-dependent formation of various complexes seems to be crucial for multifaceted functioning of histone H1 subtypes.