3.1 │ Identification of histone H1 subtypes interacting proteins
The partner proteins of standard histone H1 somatic subtypes, i.e. H1.1 – H1.5, were retrieved from the APID analyzer by enter the UniProt accession number (AC) of a given histone H1 subtype. According to the decreasing amount of interacting proteins, the histone H1 subtypes takes the following order: H1.1 (190), H1.4 (133), H1.2 (123), H1.5 (71) and H1.3 (47) (Table S1 – S5). The vast majority of interactions, i.e. 93.6%, were indicated as indirect, meaning that proteins have only functional relationship without direct physical contact. Due to that 4.3% of interactions were not annotated, the binary interactions represent 2.1% only. They related to the pair of proteins that stay in the close contact and able to form a complex. From all of 564 proteins identified as interacting with histone H1 subtypes, 343 proteins are the partners for a given histone H1 subtype only. They represent more than half of all proteins interacting with histone H1.1 (68.9%), H1.2 (60.2%), H1.4 (62.4%) and H1.5 (50.7%) and almost half that interact with histone H1.3 (40.4%) (Figure 2A). The remained proteins are the partners characteristic of more than one H1 subtype. Usually for two subtypes, alike protein-lysine 6-oxidase interacting with the histone H1.1 and H1.5 and suppressor of SWI4 1 homolog identified as a partner of subtype H1.2 and H1.4. A less frequently represented are proteins which interact with three and four histone H1 subtypes. They include, among others, the DNA replication licensing factor MCM5 which is a partner of subtype H1.1, H1.2 and H1.4. Likewise, a 60S ribosomal protein L28 is involved in the interaction with subtype H1.1, H1.2 and H1.3 and a chromobox protein homolog 6 is a partner of subtype H1.1, H1.2, H1.4 and H1.5. A common partners for all histone H1 subtypes are nucleophosmin, interleukin-1 receptor-associated kinase 4 and protein kinase C alpha type. A pairwise grouping of histones H1 in relation to the common interacting proteins raises supposition that subtypes sharing more same partners possesses a similar activity. The subtypes H1.1 and H1.4 having about 30 same interacting proteins may display similar functional properties. In contrast, subtypes H1.3 and H1.4 possessing up to 5 same proteins can be considered as more functionally distinct (Figure 2B). The observed quantitative disproportions between common and individual partnering proteins indicate that histone H1 subtypes activity may be both specific and general, corresponding to the partial redundancy. 25