Alternative titles; symbols
HGNC Approved Gene Symbol: RSF1
Cytogenetic location: 11q14.1 Genomic coordinates (GRCh38) : 11:77,660,009-77,872,232 (from NCBI)
HBXAP is involved in transcription repression, transcription coactivation when associated with hepatitis B virus X protein (HBX), and chromatin remodeling and spacing when associated with SNF2H (603375).
Using a variation of the yeast 2-hybrid screen with HBX as bait, Shamay et al. (2002) cloned HBXAP from a spleen cDNA library. The deduced 1,189-amino acid protein has a calculated molecular mass of 135 kD. It contains a PHD finger and 2 nuclear localization signals. RNA dot blot analysis of 50 different tissues detected ubiquitous expression of HBXAP, with highest expression in adrenal and pituitary glands, and lowest expression in skeletal muscle. SDS-PAGE indicated that in vitro translated HBXAP had an apparent molecular mass of about 240 kD. Western blot analysis detected endogenous HBXAP in 3 liver-derived cell lines. Endogenous HBXAP and epitope-tagged HBXAP expressed in COS-1 cells localized in nuclei, with no staining in the cytoplasm and nucleolar structures.
By yeast 2-hybrid analysis, followed by screening a spleen cDNA library, EST database analysis, and 5-prime RACE of a HeLa cell cDNA library, Shamay et al. (2002) cloned 3 splice variants of HBXAP. The deduced proteins, which the authors designated HBXAP-alpha, -beta, and -gamma, contain 1,431, 1,400, and 1,189 amino acids, respectively. All 3 proteins contain a central PHD finger, followed by a tandem repeat with the sequence RGKDISTI, a basic region, 2 nuclear localization signals, and long acidic stretches at the C terminus. In addition, both HBXAP-alpha and -beta, but not HBXAP-gamma, have an N-terminal domain conserved in several other proteins. Using Northern blot analysis, Shamay et al. (2002) detected HBXAP transcripts of about 5.0 and 10.0 kb in 3 breast cancer cell lines.
The remodeling and spacing factor (RSF) complex is composed of 2 subunits, SNF2H and the larger p325 subunit. Using peptide sequences from the p325 subunit to query an EST database, followed by screening HeLa cell and heart cDNA libraries, Loyola et al. (2003) cloned RSF1. The deduced 1,441-amino acid RSF1 protein has a calculated molecular mass of 164 kD. Loyola et al. (2003) stated that RSF1 is identical to the HBXAP-alpha variant, although they identified 10 additional N-terminal amino acids compared with HBXAP-alpha. In addition, they identified 3 nuclear localization signals near the C terminus of RSF1. Northern blot analysis detected 4 RSF1-related transcripts, 2 of about 11 kb, 1 of about 7.0 kb, and 1 of about 5.6 kb. Expression was high in heart, skeletal muscle, kidney, and placenta and low in brain and colon. SDS-PAGE detected native RSF1 at an apparent molecular mass of 200 to 300 kD. Treatment with alkaline phosphatase increased the migration rate of RSF1, indicating that it is phosphorylated.
Using several protein interaction assays, Shamay et al. (2002) confirmed direct interaction between the 1,189-amino acid HBXAP variant, HBXAP-gamma, and HBX. Examination of HBXAP-gamma deletion mutants indicated that the interaction required the PHD finger of HBXAP-gamma; however, a point mutation at a conserved his residue did not affect HBX binding, suggesting that the interaction is not mediated by the most conserved residues in the PHD finger. In transfected embryonic kidney cells, HBXAP-gamma increased hepatitis B virus transcription in an HBX-dependent manner, suggesting a role for this interaction in the virus life cycle. In the absence of HBX, HBXAP-gamma only marginally affected transcription from a nuclear factor kappa-B (NFKB; see 164011) reporter plasmid, but in the presence of HBX, it significantly increased NFKB transcription.
Shamay et al. (2002) found that transfection of HBXAP-gamma or the HBXAP PHD domain into a human hepatoma cell line could repress transcription from a cotransfected reporter plasmid.
Loyola et al. (2003) reconstituted the RSF complex by coinfection of viruses carrying the 2 RSF subunits, SNF2H and RSF1. The nucleosome-stimulated ATPase activity of the recombinant complex was comparable to that of native RSF. Characterization of the role of each subunit in the chromatin assembly reaction indicated that the nucleosome-dependent ATPase activity was a function of SNF2H, while RSF1 was the histone chaperone. RSF1 modulated the DNA-binding activity of SNF2H, and both components were required for chromatin assembly. The chromatin assembly reaction was unaffected by the phosphorylation state of RSF1. The HBXAP-gamma isoform was unable to interact with SNF2H, presumably due to the lack of 252 N-terminal amino acids compared with RSF1. Furthermore, SNF2H was unable to mediate chromatin assembly in the presence of HBXAP-gamma.
By FISH, Shamay et al. (2002) mapped the HBXAP gene to chromosome 11q13.4-q14.1. By genomic sequence analysis, Loyola et al. (2003) mapped the HBXAP gene to chromosome 11q13.
Shih et al. (2005) found amplification at 11q13.5 in 3 of 7 ovarian carcinomas, and RSF1 was the only gene consistently overexpressed in all tumors harboring the amplification. Patients with RSF1 amplification or overexpression had significantly shorter overall survival than those without. Overexpression of RSF1 in ovarian cancer cells stimulated cell proliferation and transformed nonneoplastic cells by conferring serum-independent and anchorage-independent growth, and RSF1 knockdown by short interfering RNA inhibited cell growth.
Loyola, A., Huang, J.-Y., LeRoy, G., Hu, S., Wang, Y.-H., Donnelly, R. J., Lane, W. S., Lee, S.-C., Reinberg, D. Functional analysis of the subunits of the chromatin assembly factor RSF. Molec. Cell. Biol. 23: 6759-6768, 2003. [PubMed: 12972596] [Full Text: https://doi.org/10.1128/MCB.23.19.6759-6768.2003]
Shamay, M., Barak, O., Doitsh, G., Ben-Dor, I., Shaul, Y. Hepatitis B virus pX interacts with HBXAP, a PHD finger protein to coactivate transcription. J. Biol. Chem. 277: 9982-9988, 2002. [PubMed: 11788598] [Full Text: https://doi.org/10.1074/jbc.M111354200]
Shamay, M., Barak, O., Shaul, Y. HBXAP, a novel PHD-finger protein, possesses transcription repression activity. Genomics 79: 523-529, 2002. [PubMed: 11944984] [Full Text: https://doi.org/10.1006/geno.2002.6717]
Shih, I.-M., Sheu, J. J.-C., Santillan, A., Nakayama, K., Yen, M. J., Bristow, R. E., Vang, R., Parmigiani, G., Kurman, R. J., Trope, C. G., Davidson, B., Wang, T.-L. Amplification of a chromatin remodeling gene, Rsf-1/HBXAP, in ovarian carcinoma. Proc. Nat. Acad. Sci. 102: 14004-14009, 2005. [PubMed: 16172393] [Full Text: https://doi.org/10.1073/pnas.0504195102]