Holocarboxylase Synthetase Represses Long Terminal Repeats through Epigenetic Synergies between Biotin and Methyl Donors, and Activates Immune Response by Catalyzing Biotinylation of Heat Shock Protein 72
Date of this Version
Xue, Jing. Holocarboxylase Synthetase Represses Long Terminal Repeats through Epigenetic Synergies between Biotin and Methyl Donors, and Activates Immune Response by Catalyzing Biotinylation of Heat Shock Protein 72. PhD Dissertation, University of Nebraska, Lincoln, 2013.
Holocarboxylase synthetase (HLCS) is the sole human protein biotin ligase in the human proteome. Other than its role as a coenzyme to the five carboxylases in intermediary metabolism, HLCS and biotin are involved in epigenetic gene regulation and immune response. Previously, it was shown that (1) HLCS physically interacts with histone methyl transferase EHMT-1 which creates histone H3 lysine 9 methylation (H3K9me) marks and biotinylates lysine (K) 161 in EHMT-1, thereby strenghtening the HLCS/EHMT-1 interaction; (2) biotin depletion and HLCS knockdown cause a loss of H3K9me marks leading to de-repression of long terminal repeats (LTRs) and chromosomal abnormalities; and (3) deletion of DNA methylation impairs HLCS-catalyzed biotinylation events in the epigenome. Here we demonstrate a novel paradigm that HLCS contributed to the repression of LTRs through mediating protein/protein interactions and synergies between methylation and biotinylation events. We show that (1) HLCS interacts physically with the maintenance DNA methyl transferase DNMT1 and the methyl CpG binding protein MeCP2; (2) HLCS overexpression causes a 200% increase of H3K9me marks in LTRs and a corresponding 84% decrease in mRNA coding for LTRs; and (3) erasure of DNA methylation abrogated HLCS-dependent enrichment of H3K9me and transcriptional repression of LTRs. It was thus concluded that HLCS mediated the integration of biotin and methyl dependent gene repression signals in the epigenome, which was further supported by cell culture studies where biotin and folate compensated for each other’s deficiency in the repression of LTR in human Jurkat and U937 cells, and interactively regulated the expression of proinflammatory cytokines through activation of the NF-κB pathway. We also demonstrate that HLCS is involved in cellular immune response by catalyzing biotinylation of the stress inducible heat shock protein 72 (HSP72). In screen for detecting novel biotinylated proteins in the proteome of the human embryonic kidney HEK293 cells, members of the heat shock superfamily of proteins, including HSP72, were overrepresented. At least five lysine residues in HSP72 are targeted for biotinylation, and HLCS-dependent biotinylation of HSP72 elicited the expression of the chemokine RANTES from HEK293 cells. In conclusion, HLCS and biotin have major impacts in epigenome and immune function.
Advisor: Janos Zempleni