Proline365 is a critical residue for the activity of XMI-ER1 in Xenopus embryonic development

The transcriptional cofactor MIER1-beta negatively regulates histone acetyltransferase activity of the CREB-binding protein

Background

Mier1 encodes a novel transcriptional regulator and was originally isolated as a fibroblast growth factor early response gene. Two major protein isoforms have been identified, MIER1α and β, which differ in their C-terminal sequence. Previously, we demonstrated that both isoforms recruit histone deacetylase 1 (HDAC1) to repress transcription. To further explore the role of MIER1 in chromatin remodeling, we investigated the functional interaction of MIER1 with the histone acetyltransferase (HAT), Creb-binding protein (CBP).

Findings

Using GST pull-down assays, we demonstrate that MIER1 interacts with CBP and that this interaction involves the N-terminal half (amino acids 1–283) of MIER1, which includes the acidic activation and ELM2 domains and the C-terminal half (amino acids 1094–2441) of CBP, which includes the bromo-, HAT, C/H3 and glutamine-rich domains. Functional analysis, using HEK293 cells, shows that the CBP bound to MIER1 in vivo has no detectable HAT activity. Histone 4 peptide binding assays demonstrate that this inhibition of HAT activity is not the result of interference with histone binding.

Conclusion

Our data indicate that an additional mechanism by which MIER1 could repress transcription involves the inhibition of histone acetyltransferase activity.

Protein expression of the transcriptional regulator MI-ER1 alpha in adult mouse tissues

MI-ER1 is a novel transcriptional regulator that plays a critical role in embryonic development and is differentially expressed in breast carcinoma. The MI-ER1 protein sequence is highly conserved among species, with 95% identity between mouse and humans and 72% between Xenopus and mouse. There are two major protein isoforms, MI-ER1alpha and MI-ER1beta, which differ in the sequence of their C-terminus. MI-ER1alpha is of particular interest because it contains a consensus LXXLL nuclear receptor interaction motif and the current study was undertaken to determine the expression pattern of MI-ER1alpha protein in adult mouse tissues. Immunohistochemical analysis of paraffin-embedded tissue using an MI-ER1alpha-specific antibody revealed that the majority of mouse adult tissues examined showed very weak or no immunoreactivity; these included tissues of the lung, liver, intestine, uterus, spleen, lymph node, bladder as well as skeletal muscle. Interestingly, a subset of endocrine tissues displayed intense staining for MI-ER1alpha. Specifically, the islets of Langerhans, the zona glomerulosa and medulla of the adrenal gland, the ovary and the hypothalamus were intensely stained. In addition, both anterior and posterior pituitary showed moderate immunoreactivity, as did the parafollicular cells of the thyroid gland and Leydig cells and spermatids in the testes. Negative endocrine tissues included follicular cells of the thyroid gland and the X zone of the adrenal cortex. A few non-endocrine tissues displayed moderate immunoreactivity; these included all tubules and collecting ducts in the kidney, myocardial and endocardial layers of the heart, the hippocampal formation, pyramidal neurons in the cortex and the ductal epithelium of the mammary gland. In all cases, MI-ER1alpha immunoreactivity was cytoplasmic. This study represents the first immunohistochemical analysis of MI-ER1alpha expression in mammals and our data suggest that this transcriptional regulator plays a role in specific endocrine pathways.

Developmentally regulated cytoplasmic retention of the transcription factor XMI-ER1 requires sequence in the acidic activation domain

Xmi-er1 is a fibroblast growth factor regulated immediate-early gene that is activated during mesoderm induction in Xenopus embryonic explants. This gene encodes a nuclear protein with potent transcriptional regulator activity and overexpression of XMI-ER1 in Xenopus embryos inhibits mesoderm induction and leads to truncations along the anteroposterior axis. We showed previously that XMI-ER1 is retained in the cytoplasm during cleavage stages and only begins to appear in the nucleus at mid-blastula. Such developmentally regulated nuclear translocation may represent an important mechanism for regulating XMI-ER1 activity in the early embryo. Here, we investigate different mechanisms that might control nuclear translocation of XMI-ER1. Using alpha-amanitin to inhibit transcription, we show that nuclear localization is not dependent on zygotic transcription. Nor is it the result of a developmentally regulated import pathway, as the XMI-ER1 nuclear localization signal (NLS) fused to beta-galactosidase (betagal) was able to direct nuclear translocation prior to mid-blastula. Fusion of an additional, heterologous NLS to the N-terminus of XMI-ER1 was not sufficient to overcome cytoplasmic retention, indicating that retention does not involve NLS masking, but rather binding to a cytoplasmic anchor. The anchoring molecule is not an RNA, as microinjection of RNase A did not affect the timing of nuclear translocation. Western blot analysis using antibodies that recognize phosphorylated residues revealed that, while XMI-ER1 is not itself phosphorylated, it is associated with two differentially phosphorylated proteins, suggesting that the anchoring mechanism may involve interaction with a cytoplasmic protein(s). A series of XMI-ER1 deletion mutants was utilized to map the putative retention domain. Our analysis revealed that amino acids 144-175, containing the fourth acidic stretch of the acidic activation domain, are required for retention. These results suggest that XMI-ER1 is retained in the cytoplasm of the early embryo by interaction of the region containing amino acids 144-175 with a cytoplasmic anchor.