Abstract
Mixed lineage leukemia 5 (MLL5) protein is a trithorax family histone 3 lysine 4 (H3K4) methyltransferase that regulates diverse biological processes, including cell cycle progression, hematopoiesis and cancer. The mechanisms by which MLL5 protein stability is regulated have remained unclear to date. Here, we showed that MLL5 protein stability is cooperatively regulated by O-GlcNAc transferase (OGT) and ubiquitin-specific protease 7 (USP7). Depletion of OGT in cells led to a decrease in the MLL5 protein level through ubiquitin/proteasome-dependent proteolytic degradation, whereas ectopic expression of OGT protein suppressed MLL5 ubiquitylation. We further identified deubiquitinase USP7 as a novel MLL5-associated protein using mass spectrometry. USP7 stabilized the MLL5 protein through direct binding and deubiquitylation. Loss of USP7 induced degradation of MLL5 protein. Conversely, overexpression of USP7, but not a catalytically inactive USP7 mutant, led to decreased ubiquitylation and increased MLL5 stability. Co-immunoprecipitation and co-immunostaining assays revealed that MLL5, OGT and USP7 interact with each other to form a stable ternary complex that is predominantly located in the nucleus. In addition, upregulation of MLL5 expression was correlated with increased expression of OGT and USP7 in human primary cervical adenocarcinomas. Our results collectively reveal a novel molecular mechanism underlying regulation of MLL5 protein stability and provide new insights into the functional interplay among O-GlcNAc transferase, deubiquitinase and histone methyltransferase.
Highlights
Mixed lineage leukemia 5 (MLL5) protein, a trithorax group protein and histone 3 lysine 4 (H3K4) methyltransferase, was originally identified in a segment of chromosome band 7q22 that is frequently deleted in human myeloid leukemia [1,2]
We showed that O-GlcNAc transferase (OGT) and ubiquitin-specific protease 7 (USP7) interact with MLL5 protein to form a stable protein complex in the cell nucleus
Studies over recent years have greatly expanded our knowledge of epigenetic regulations mediated by MLL5 protein in gene transcription, cell cycle progression, hematopoiesis and tumorigenesis
Summary
MLL5 protein, a trithorax group protein and histone 3 lysine 4 (H3K4) methyltransferase, was originally identified in a segment of chromosome band 7q22 that is frequently deleted in human myeloid leukemia [1,2]. Several studies using Mll5-deficient mice have strongly suggested a crucial role in both hematopoiesis and spermatogenesis [6,7,8,9]. Recent clinical research indicates that higher expression of MLL5 serves as a useful biomarker for predicting survival outcomes in patients with acute myeloid leukemia after chemotherapy treatment [12,13,14]. A recent whole-exome sequencing study linked MLL5 to autism spectrum disorder, a serious neurodevelopmental disorder [15]. Despite its extensive physiological and pathological functions, the mechanisms regulating MLL5 protein stability remain largely unknown
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