Expression data from 17 day old testes of wild type and UBR2-/- mice

Ubiquitylation of histones provides an important mechanism regulating chromatin remodeling and gene expression. Recent studies have revealed ubiquitin ligases involved in histone ubiquitylation, yet the responsible enzymes and the function of histone ubiquitination in spermatogenesis remain unclear. Here we show that the ubiquitin ligase UBR2, one of the recognition E3 components of the N-end rule proteolytic pathway, localizes to meiotic chromatin regions, including unsynapsed axial elements linked to chromatin inactivation, and mediates, in combination with the ubiquitin-conjugating enzyme HR6B, the ubiquitination of histone H2A. UBR2 interacts with HR6B and H2A and promotes the HR6B-H2A interaction and the HR6B-to-H2A transfer of ubiquitin. UBR2 and ubiquitinated H2A (uH2A) spatiotemporally mark meiotic chromatin regions subject to transcriptional silencing, and UBR2-deficient spermatocytes fail to induce the ubiquitination of H2A during meiosis. UBR2-deficient spermatocytes are profoundly impaired in transcriptional silencing of genes linked to unsynapsed axes of the X and Y chromosomes. We propose a model, in which UBR2 on axial elements of the X-Y pair enables HR6B on the linked chromatin domain to repeat histone ubiquitination cycles while scanning a string of nucleosomes. Our results suggest that histone ubiquitination in germ cells may be mediated by E3-E2 pairs distinct from those in somatic cells, providing a new insight into chromatin remodeling and gene expression regulation.

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An JY, Kim EA, Jiang Y, Zakrzewska A, Kim DE, Lee MJ, Mook-Jung I, Zhang Y, Kwon YT