first malignant brain tumor (MBT) repeat found in the sex comb on midleg family
The sex comb on midleg (Scm) family of Polycomb proteins includes Drosophila melanogaster Scm protein, as well as two vertebrate homologs of Scm, sex comb on midleg homolog 1 (SCMH1) and sex comb on midleg-like protein 2 (SCML2). Scm is a putative zinc finger protein that functions as a transcriptional regulator which represses multiple homeotic genes during embryonic stages. It coordinates Polycomb group (PcG) complexes, PRC1 (PcG-repressive complex 1) or PRC2 (PcG-repressive complex 2), and polymerizes to produce broad domains of PcG silencing. Both SCMH1 and SCML2 are components of PRC1, which acts as an E3 ubiquitin ligase both for histone H2A to silence transcription and for geminin to regulate its stability. SCMH1 provides the complex with an interaction domain for geminin. SCML2 is a germline-specific Polycomb protein that acts as a critical regulator of heterochromatin organization in spermatogenesis. Members of this family contain two MBT (malignant brain tumor) repeats. The MBT repeat is a protein module that recognizes methylated lysine residues on histones. MBT repeats contain a semi-aromatic cage to accommodate methyllysine and show specificity towards the lower methylation states of lysine. This model corresponds to the first MBT repeat that does not contain a semi-aromatic cage and may not bind methylated lysine residues.
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