Scmh1 has E3 ubiquitin ligase activity for Geminin and histone H2A and regulates Geminin stability directly or indirectly via transcriptional repression of Hoxa9 and Hoxb4.

Yasunaga S, Ohtsubo M, Ohno Y, Saeki K, Kurogi T, Tanaka-Okamoto M, Ishizaki H, Shirai M, Mihara K, Brock HW, Miyoshi J, Takihara Y

Polycomb-group (PcG) complex 1 acts as an E3 ubiquitin ligase both for histone H2A to silence transcription and for Geminin to regulate its stability. Scmh1 is a sub-stoichiometric component of PcG complex 1 that provides the complex with an interaction domain for Geminin. Scmh1 is unstable and regulated through the ubiquitin-proteasome system but its molecular roles are unknown, so we generated Scmh1-deficient mice to elucidate its function. Loss of Scmh1 caused derepression of Hoxb4 and Hoxa9, direct targets of PcG complex 1-mediated transcriptional silencing in hematopoietic cells. Double knockdown of Hoxb4 and Hoxa9 or transduction of a dominant-negative Hoxb4N>A mutant caused Geminin accumulation. Age-related transcriptional down-regulation of derepressed Hoxa9 also leads to Geminin accumulation. Transduction of Scmh1 lacking a Geminin-binding domain restored derepressed expression of Hoxb4 and Hoxa9 but did not down-regulate Geminin like full-length Scmh1. Each of Hoxb4 and Hoxa9 can form a complex with Roc1-Ddb1-Cul4a to act as an E3 ubiquitin ligase for Geminin. We suggest that Geminin dysregulation may be restored by derepressed Hoxb4 and Hoxa9 in Scmh1-deficient mice. These findings suggest that PcG and a subset of Hox genes compose a homeostatic regulatory system for determining expression level of Geminin.

Chromatin Shearing
LowCell ChIP kit

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December, 2012


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