HMGN3a and HMGN3b, Two Protein Isoforms with a Tissue-specific Expression Pattern, Expand the Cellular Repertoire of Nucleosome-binding Proteins

Abstract

HMGN1 (HMG-14) and HMGN2 (HMG-17) are nuclear proteins that bind specifically to nucleosomes, reduce the compactness of the chromatin fiber, and enhance transcription from chromatin templates. Here we report that many vertebrates contain an additional type of HMGN protein named HMGN3 (Trip 7). The human HMGN3 gene is located on chromosome 6 and spans 32 kilobase pairs, which is nearly 10-fold longer than the closely related HMGN2 gene. However, the intron/exon boundaries of the HMGN3 gene are identical to those of HMGN1 and HMGN2. Unique within the HMGN family, the HMGN3 transcript undergoes alternative splicing and generates two different variants, HMGN3a and HMGN3b. The shorter variant, HMGN3b, arises from an additional splice site that truncates exon V and causes a frameshift. The resulting HMGN3b protein lacks the majority of the C-terminal chromatin-unfolding domain. Both splice variants are found in many vertebrates from frogs to man and are expressed in many tissues. The pattern of tissue-specific expression differs considerably from those of HMGN1 and HMGN2 at both the mRNA and the protein level. Our results expand the multiplicity of the HMGN protein family and raise the possibility that these nucleosome-binding proteins function as co-activators in tissue-specific gene expression.

Chromosomal Proteins HMGN3a and HMGN3b Regulate the Expression of Glycine Transporter 1

“…Hepa-1 cells were transfected with linearized pTK-Hyg and either pCI-INS2, pCI-INS2-N3a, or pCI-INS2-N3b and then selected in the presence of 400 g of hygromycin (BD Biosciences) per ml. Colonies were expanded and screened by Western blot analysis of 5% perchloric acid extracts using antibody 2752 against HMGN3 (36,53). The number of integrated plasmid copies in the clones selected for further study was estimated by Southern analysis to be between 1 and 10.…”

“…Unfixed eyes from 3-week-old C57BL/6J mice were embedded in tissue-freezing medium, and 16-m sections were generated as described previously (43). HMGN3 was detected with antibody 2752 raised in rabbit (53). GLYT1 protein was detected with an antibody raised in rabbit by using a synthetic peptide corresponding to the final 15 amino acids on the carboxyl terminus of GLYT1 (41).…”

“…Studies of HMGN2 expression during mouse development also revealed highest expression in tissues undergoing differentiation (27,28). However, the expression pattern of mouse HMGN3a/b is dis-tinct from those of HMGN1 and HMGN2, being highly expressed in the eye and brain (4,23,53). Taken together, the data raise the possibility that HMGNs function as coactivators in tissue-specific gene expression.…”

“…The structure of HMGN3 is very similar to those of HMGN1 and HMGN2 in that it contains domains homologous to the nucleosome binding domain, the bipartite nuclear localization signal, and the chromatin-unfolding domain. HMGN3 is expressed as two splice variants, HMGN3a and HMGN3b, and the latter lacks most of the C-terminal chromatin-unfolding domain (53). HMGN3b interacts with TR-RXR in a liganddependent manner and can promote thyroid hormone-dependent transcription from chromatin templates (2).…”

Immunohistochemical Localization of the Nucleosome-Binding Protein HMGN3 in Mouse Brain

“…A related protein named HMGN3 (Trip7) was recently detected in a HeLa cell library by a yeast two-hybrid assay through its ligand-dependent interaction with the thyroid hormone receptor TR ␤ 1 (Lee et al 1995). Both Northern and Western analysis demonstrated that HMGN3 is expressed in a tissue-specific manner in mice (West et al 2001) and that the highest level of expression was found in whole mouse brain extracts (West et al 2001). Interestingly, the closely related HMGN2 protein was expressed in most cells and its expression in the brain was not higher than in other tissues (West et al 2001).…”