Analysis of the Structure and Function of the Transcriptional Coregulator HOP^,

Abstract

Homeodomain-only protein (HOP) is an 8-kDa transcriptional corepressor that is essential for the normal development of the mammalian heart. Previous studies have shown that HOP, which consists entirely of a putative homeodomain, acts downstream of Nkx2.5 and associates with the serum response factor (SRF), repressing transcription from SRF-responsive genes. HOP is also able to recruit histone deacetylase (HDAC) activity, consistent with its ability to repress transcription. Unlike other classic homeodomain proteins, HOP does not appear to interact with DNA, although it has been unclear if this is because of an overall divergent structure or because of specific amino acid differences between HOP and other homeodomains. To work toward an understanding of HOP function, we have determined the 3D structure of full-length HOP and used a range of biochemical assays to define the parts of the protein that are functionally important for its repression activity. We show that HOP forms a classical homeodomain fold but that it cannot recognize double stranded DNA, a result that emphasizes the importance of caution in predicting protein function from sequence homology alone. We also demonstrate that two distinct regions on the surface of HOP are required for its ability to repress an SRF-driven reporter gene, and it is likely that these motifs direct interactions between HOP and partner proteins such as SRF- and HDAC-containing complexes. Our results demonstrate that the homeodomain fold has been co-opted during evolution for functions other than sequence-specific DNA binding and suggest that HOP functions as an adaptor protein to mediate transcriptional repression.

Potential utility of HOP homeobox gene promoter methylation as a marker of tumor aggressiveness in gastric cancer

“…In normal tissues, HOPX lacks several key conserved homeodomain residues for binding to DNA, but it inhibits serum response factor-dependent transcriptional activation by recruiting histone deacetylase activity (Chen et al, 2002). Thus, HOPX functions as an adaptor protein to regulate a delicate balance between proliferation and differentiation through negative-feedback regulation (Kook et al, 2006). However, stable transfection of HOPX into esophageal squamous cell carcinoma cell lines does not necessarily repress serum response factor activity, and this activity does not correlate with tumorigenic potential, thus indicating that mechanisms other than serum response factor inactivation have important roles in tumorigenesis .…”

“…Three spliced transcript variants, HOPX-a (NM 139212.2), HOPX-b (NM 139211.2) and HOPX-g (NM 032495.4), encode the same protein, which contains a putative homeodomain motif that acts as an adapter protein to mediate transcriptional repression (Kook et al, 2006). HOPX expression is ubiquitous in diverse types of normal tissue, but not in various malignant tissues, including lung cancer, esophageal squamous cell carcinoma and uterine endometrial cancer (Chen et al, 2002(Chen et al, , 2007Yamashita et al, 2008;Yamaguchi et al, 2009).…”

Integration of Bmp and Wnt signaling by Hopx specifies commitment of cardiomyoblasts

“…Although Hopx does not bind to DNA directly (22) it can interact with co-repressor complexes to inhibit gene expression. We performed ChIP-seq analysis of pooled chromatin derived from ~35 E9.5 microdissected murine embryonic hearts (table S2).…”

Association between non-coding polymorphisms of HOPX gene and syncope in hypertrophic cardiomyopathy

“…Binding to serum response element (SRE) which includes CArG box, SRF controls the transcription of genes including cellular immediate early genes, cytoskeletal and contractile proteins (11,12). Activity of the SRF itself is also under the control of some co-activators like myocardin (13) and corepressors like HOPX (14).…”

“…HOPX protein is known to act as an antagonist of SRF in pre-natal cardiomyocyte proliferation and post-natal cardiomyocyte hypertrophy (14). This antagonistic action of HOPX on SRF-mediated transcription is mediated by recruiting histone deacetylase (HDAC) (16).…”