3.2 │ Localization of histone H1 subtypes interacting proteins
Localization of histone H1 subtypes partnering proteins was determined with highest localization score indicated in the ComPPI database. As it was shown in the Figure 3, the main subcellular compartment in which the proteins are located is nucleus and cytosol, characteristic of 46.0–65.0% and 22.0–40.0% proteins, respectively. Although no statistically significant difference in the localization of histone H1 subtypes partnering proteins was detected (F = 0.0052, p = 0.999), a disproportions of their emplacement shows on a diverse engagement of histone H1 subtypes in the processes running in the nucleus and cytosol. Whereas histone H1.1 have 65.0% of partner proteins located in the nucleus and 22.0% characteristic of a cytosol, it should be perceived as more active in the nucleus. In contrast, the histone H1.2, possessing 48.0% partnering proteins in the nucleus and 40.0% in the cytosol, might be similarly active in both regions of the cell. Likewise, almost twice more histone H1.4 interacting proteins originating from nucleus (59.0%) than from cytosol (30%) indicate that this histone H1 subtype may play more significant role in the nucleus. On the contrary, a participation of histone H1.3 in the processes running in the nucleus (46.0% partnering proteins) and in the cytosol (36.0% partnering proteins) can be considered as comparable. Other proteins interacting with histone H1 subtypes are the components of membrane, mitochondrion and secretory pathway and also remain in both nucleus and cytosol. Such a locations are generally characteristic of less than 5% of proteins, except for these which are the partners of histone H1.1 located in both nucleus and cytosol (6.0%) as well as the membrane proteins (10.0%) interacting with histone H1.3. Besides, a comparable proportions of the secretory pathway proteins designated as the partners of histone H1.1 – H1.4 (1.0 - 2.0%) differ from an amount of proteins predicted as interacting with histone H1.5 (9%). This suggest a stronger relationship of histone H1.5 with function of the endoplasmic reticulum and Golgi apparatus. Although a complete wide-genome distribution of H1 subtypes still needs to be definitively determined, a known reports regarding localization of H1 histones point to their association with a specific chromatin type and, thus, a characteristic role in chromatin-dependent cellular processes. Based on the findings of Izzo et al. 26, a disparate abundance of histone H1.1 and histone H1.2 – H1.5 are discernible at promoters, enhancers and CpG islands. Histone H1.1 is preferentially located in the PcG regions marked by H3K9me3 while histone H1.2 – H1.5 in the HP1-containing domains marked by H3K27me3. Thus, in contrast to the subtype H1.2 – H1.5, subtype H1.1 has more potential to form both active and repressed chromatin state. In another study27, a difference in emplacement in the genome was detected between histone H1.2 and other H1 subtypes. Whereas histone H1.0 and H1X was detected as associated with high CG content and gene-rich regions, histone H1.2 was indicated as located in the regions with low CG content and gene-poor. These reports pointing to peculiar distribution of histone H1 subtypes reflects simultaneously their differential influence on processes running in the chromatin. It seems that a similar relationship may concern the activity of histone H1 subtypes differently realized in various cell structures through their interaction with partner proteins.