Review
HMGN5/NSBP1: A new member of the HMGN protein family that affects chromatin structure and function

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Abstract

The dynamic nature of the chromatin fiber provides the structural and functional flexibility required for the accurate transcriptional responses to various stimuli. In living cells, structural proteins such as the linker histone H1 and the high mobility group (HMG) proteins continuously modulate the local and global architecture of the chromatin fiber and affect the binding of regulatory factors to their nucleosomal targets. HMGN proteins specifically bind to the nucleosome core particle through a highly conserved “nucleosomal binding domain” (NBD) and reduce chromatin compaction. HMGN5 (NSBP1), a new member of the HMGN protein family, is ubiquitously expressed in mouse and human tissues. Similar to other HMGNs, HMGN5 is a nuclear protein which binds to nucleosomes via NBD, unfolds chromatin, and affects transcription. This protein remains mainly uncharacterized and its biological function is unknown. In this review, we describe the structure of the HMGN5 gene and the known properties of the HMGN5 protein. We present recent findings related to the expression pattern of the protein during development, the mechanism of HMGN5 action on chromatin, and discuss the possible role of HMGN5 in pathological and physiological processes.

Introduction

Essential cellular processes such as transcription, DNA repair, replication, and recombination occur in the context of chromatin. Therefore, the architecture of the chromatin fiber plays a key role in the integration of endogenous and exogenous signals for proper adjustment of cellular responses to a constantly changing environment.

A hallmark of chromatin organization is the dynamic nature of the chromatin fiber [1], [2]. Histones in the nucleosome core particle, the basic unit of chromatin, are subjected to numerous posttranslational modifications, collectively known as the “histone code” that directly affects the structure of chromatin [3]. The pattern of histone modifications is established and maintained through the balancing action of chromatin-modifying enzymes that continuously add and remove modifications to histone tails. Histone modifications are recognized by chromatin-remodeling complexes thereby providing the molecular platform for chromatin remodeling [4]. The next level of complexity of chromatin fiber is the formation of higher-order chromatin structures, such as the 30-nm fiber, tertiary structures of chromatin loops, and entire chromosomes [5]. The dynamic properties of these structures are regulated by numerous chromatin-modifying factors including a complex network of nucleosome-binding chromatin architectural proteins, such as linker histone H1 [6] and high mobility group (HMG) proteins [7]. The activity of the linker histone H1 is required for the formation of higher-order chromatin structures and chromatin compaction, whereas HMGNs, such as HMGN1 and HMGN2, compete with histone H1 for the binding to nucleosomes and counteract this function [7], [8], [9], [14].

High mobility group N (HMGN) proteins are a family of ubiquitous nuclear proteins which are expressed in higher eukaryotes and specifically interact with nucleosomes without any known preference for the underlying DNA sequence [10]. This interaction is mediated by the nucleosome-binding domain (NBD), a highly conserved sequence motif that is the signature of the HMGN protein family. Additionally, all members of the HMGN family contain negatively charged and highly unstructured C-terminal regulatory domain which increases the affinity of HMGN binding to the nucleosome and is involved in transcriptional activation [11]. However, the molecular function of this domain is not fully understood. Based on these structural properties, six proteins have been identified as members of HMGN protein family: HMGN1 (HMG14), HMGN2 (HMG17), HMGN3a and 3b, HMGN4, and HMGN5, previously known as NBP-45, GARP45, and NSBP1.

Fluorescence recovery after photobleaching (FRAP) experiments demonstrate that HMGN proteins are highly dynamic and are constantly moving in the nucleus in a “stop and go” manner [12]. HMGNs destabilize linker histone H1 binding to nucleosomes by competing for chromatin binding sites [9], unfold higher-order chromatin structures [13], [14], and modulate transcription [15], [16]. In addition, HMGNs affect several histone modifications thus adjusting the chromatin architecture at the level of the nucleosome [17], [18], [19]. Also, it has been recently reported that recombinant HMGN proteins can counteract the ATP-dependent chromatin-remodeling activities in vitro in a reversible and dynamic manner [20].

Studies of HMGN1 knockout (KO) mice reveal that these mice are hypersensitive to UV and ionizing radiation and have a higher incidence of multiple malignant tumors and metastases [21], [22] The underlying molecular mechanism for the increased tumor burden of HMGN1 KO mice lies in the ability of HMGN1 to optimize the interaction of the DNA repair machinery with chromatin [23]. Aberrant expression of HMGN proteins during mouse development and cellular differentiation is detrimental for early embryonic development, myotube formation, and chondrocyte differentiation, indicating that proper regulation of HMGN protein expression is important for these processes [16], [24], [25].

HMGN5 protein was first identified in our laboratory based on its structural similarity to other HMGN proteins [26]. HMGN5 is a typical member of the HMGN family which is localized to the nucleus and contains a functional NBD and negatively charged C-terminus [26]. Unlike other HMGNs, the C-terminus of HMGN5 is unusually long (more than 300 amino acids long in the mouse protein and around 200 amino acids long in the human protein) and affects cellular localization and architectural properties of the protein [27].

HMGN5 remains mainly uncharacterized and its biological function is unknown. In this review, we summarize the available data on molecular and biochemical properties of HMGN5 gene and protein, expression pattern during development, and its role in the regulation of chromatin architecture and transcription. We discuss the potential association of HMGN5 with physiological and pathological processes, including cancer progression. We note that most of the recent data come from work on the mouse HMGN5 protein, and further research is required to elucidate the functional importance of this unusual chromatin architectural protein in other species.

Section snippets

The structure of the HMGN5 gene

Analysis of the databases of ortholog genes suggests that the Hmgn5 gene appeared late in evolution and is only present in rats, mice, cows, monkeys, and human. For other species such as dogs, pigs, chicken, and lower eukaryotes, no similar gene or protein is predicted. Genes coding for both mouse and human HMGN5 proteins are located at syntenic regions of the X chromosome (Xq13.3 for human gene and X D for mouse gene; Fig. 1A) and span approximately 8 kb regions. In contrast to the Hmgn1,

The properties of the HMGN5 protein

Human and mouse HMGN5 proteins show 59% amino acid identity (86% similarity) and are structurally similar (Fig. 1C). The N-terminal part of HMGN5 contains a nuclear localization signal and the NBD. RRSARLSA is the highly conserved functional domain of the NBD which defines the ability of HMGN proteins to specifically interact with nucleosomes [29]. Additional features of HMGN5 associated with HMGN origin include the asymmetric charge distribution along the molecule: the N-terminal region

Intracellular localization of HMGN5

As other HMGN proteins, HMGN5 is also localized to the nucleus of the cell [26]. More detailed analysis reveals that mouse HMGN5 protein is specifically localized to less-condensed euchromatic areas and is excluded from more condensed constitutive heterochromatin domains [27]. This localization is unusual because another member of the HMGN family, HMGN1, localizes to both eu- and heterochromatic areas of the nucleus [29]. The major structural distinction between these proteins which accounts

Expression pattern of HMGN5 during development

Despite the ubiquitous expression of HMGN5, expression levels of the protein vary significantly between tissues and cell lines [26], [28]. A query of the Gene Expression Omnibus (GEO) expression database suggests the highest expression of mouse HMGN5 protein in the pituitary gland, which is supported by the relatively high expression of the protein in the AtT20 mouse pituitary cell line [27].

As indicated by mRNA and protein analysis, both human and mouse HMGN5 proteins are ubiquitously

HMGN5 unfolds chromatin and modulates transcription

The concept of dynamic chromatin interactions is paramount in understanding how a relatively low amount of protein (sufficient to bind to less than 2% of the nucleosomes) has a global effect on chromatin architecture [40]. As evident from FRAP analysis, HMGN proteins are highly mobile. They are constantly moving through the nucleus, randomly interacting with nucleosomes and forming metastable protein complexes [8], [41]. The mobility properties of HMGNs are defined by the binding of the

HMGN5 association with physiological and pathological processes

Aberrant expression of HMGN proteins is associated with developmental defects, hypersensitivity to stress and increased tumorigenic potential in mice [50]. Although the molecular role of HMGN5 is not yet understood, there are indications in the literature that suggest the potential involvement of HMGN5 in disease and in normal cellular functions (Table 1 and references therein). For example, microarray analysis and overexpression experiments identified HMGN5 as a factor that induces

Conclusions and perspectives

Understanding chromatin architecture and its regulation is crucial for deciphering the dynamics of biological processes, such as development and differentiation. It is progressively becoming clear that the regulation of chromatin architecture requires coordinative efforts of multiple proteins and that HMGN protein family is an integral part of this network. The discovery of HMGN5 extends the functional potential of HMGNs in the regulation of the structure of the chromatin fiber and emphasizes

Acknowledgements

This project was supported by the Intramural Research Program of NIH, National Cancer Institute (NCI). We thank The Fellows Editorial Board for editorial assistance.

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