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  • HMGN proteins are subject to extensive post

    2024-05-17

    HMGN proteins are subject to extensive post-translational modifications which influence both their mode of binding to Sodium phosphate monobasic and their functional activity. Like the other HMG protein families, HMGNs are substrates for many of the same enzymes that modify histone proteins (review in [175]). The histone acetyltransferase (HAT) enzyme p300, for example, acetylates both HMGN1 and HMGN2 whereas the HAT enzyme pCAF (p300/CBP-associated factor) acetylates only HMGN2. It has been demonstrated that acetylated HMGNs bind nucleosomes less tightly than unmodified proteins and therefore exhibit a decreased ability to unfold compact chromatin [24]. Likewise, phosphorylation decreases the interaction of HMGN proteins with nucleosomes [114]. Cell cycle-dependent phosphorylation of serine residues on HMGN1 (at S6, S20 and S24) specifically prevents it from binding to mitotic chromosome, increases its mobility inside the nucleus, inhibits its nuclear import and promotes its interaction with cytoplasmic 14.3.3 proteins [127,128]. Several protein kinases have been demonstrated to phosphorylate HMGNs, including PKC, PKA, CK2, GMP-dependent protein kinase and mitogen- and stress-activated kinases (MSKs) [175]. MSKs are the major kinases responsible for phosphorylating HMGN1 (at S6) and histone H3 (at S10) in what is called the “nucleosome response” which leads to the transcriptional activation of immediate early (IE) genes in mouse fibroblasts exposed to mitogenic or stress stimuli [152]. Phosphorylation of HMGN1 leads to a transient weakening of its binding to chromatin and allows kinases to subsequently access and phosphorylate the N-terminal tail of histone H3. Since phosphorylation of HMGN1 precedes that of H3, it has been suggested that HMGN1 plays an important role in modulating the “histone code” [88]. Numerous in vitro studies have demonstrated that HMGN1 proteins enhance transcription in the context of chromatin, suggesting that it acts as a genome-wide transcriptional coactivator [42,116,160]. Analysis of embryonic fibroblasts from mice, however, has shown that loss of HMGN1 leads to both up and down-regulation of gene expression [18,19], suggesting that specific subsets of genes may be differentially regulated by different HMGN proteins. Available evidence supports this idea but does not rule out the possibility that HMGN proteins can also have gene-specific activities. For example, it has been demonstrated that stable expression of HMGN3a and HMGN3b proteins in cells (Hepa-1) that contain no endogenous HMGN3 induces the up-regulation of a small subset of only 22 genes, one of which is the glycine transporter, Glyt1[166]. Likewise, experiments with embryonic fibroblasts from Hmgn1+/+ and mice examining the effect of HMGN1 on the heat-shock-induced transcriptional activation of the Hsp70 gene demonstrated that HMGN1 specifically enhances the rate of heat-shock-induced H3K14 acetylation in the Hsp70 promoter, thereby enhancing the rate of chromatin remodeling and the subsequent transcription during the early rounds of Hsp70 transcriptional activation [13]. Homozygous knock-out / mice are viable but subfertile, exhibit minor developmental abnormalities and are hypersensitive to various stress conditions, such as exposure to UV light or ionizing irradiation [18]. The incidence of spontaneous tumors in mice is also almost twice that of wild-type mice and cells derived from mice have an increased tumorigenic potential, as measured by colony formation in soft agar and generation of tumors in nude mice [18]. Following UV irradiation, the transcription profile of cells is altered but expression of the genes in the NER pathway is normal [19]. When cells are stably transfected with a plasmid that expresses wild-type HMGN1 protein (but not with a plasmid that expresses HMGN1 lacking the CHUD domain), the reconstituted cells are restored to a wild-type phenotype with respect to UV sensitivity and turmorigenic potential. Together these findings indicate that HMGN1 enhances the rate of repair of UV-induced lesions by decompacting chromatin and facilitating the access of NER proteins to the damaged sites [19,165].