Hybrid protein assembly-histone modification mechanism for PRC2-based epigenetic switching and memory

The histone modification H3K27me3 plays a central role in Polycomb-mediated epigenetic silencing. H3K27me3 recruits and allosterically activates Polycomb Repressive Complex 2 (PRC2), which adds this modification to nearby histones, providing a read/write mechanism for inheritance through DNA replication. However, for some PRC2 targets, a purely histone-based system for epigenetic inheritance may be insufficient. We address this issue at the Polycomb target FLOWERING LOCUS C (FLC) in Arabidopsis thaliana, as a narrow nucleation region of only ~three nucleosomes within FLC mediates epigenetic state switching and subsequent memory over many cell cycles. To explain the memory’s unexpected persistence, we introduce a mathematical model incorporating extra protein memory storage elements with positive feedback that persist at the locus through DNA replication, in addition to histone modifications. Our hybrid model explains many features of epigenetic switching/memory at FLC and encapsulates generic mechanisms that may be widely applicable.

Immunostimulatory Activity of Major Membrane Protein II fromMycobacterium tuberculosis

ABSTRACTPreviously, we observed that both major membrane protein II ofMycobacterium leprae(MMP-ML) and its fusion withM. bovisBCG (BCG)-derived heat shock protein 70 (HSP70) (Fusion-ML) are immunogenic and that recombinant BCG secreting either of these proteins effectively inhibits the multiplication ofM. lepraein mice. Here, we purifiedM. tuberculosis-derived major membrane protein II (MMP-MTB) and its fusion with HSP70 (Fusion-MTB) in a lipopolysaccharide-free condition and evaluated their immunostimulatory abilities. Both MMP-MTB and Fusion-MTB activated monocyte-derived dendritic cells (DC) in terms of phenotype and interleukin-12 (IL-12) production, but Fusion-MTB more efficiently activated them than MMP-MTB did. The IL-12 production was a consequence of the ligation of those recombinant proteins with Toll-like receptor 2. TheM. tuberculosis-derived andM. leprae-derived recombinant proteins activated naïve T cells of both CD4 and CD8 subsets, butM. tuberculosis-derived proteins were superior toM. leprae-derived proteins and fusion proteins were superior to MMP, regardless of the origin of the protein. Memory-type CD4+T cells obtained from BCG-vaccinated healthy individuals seem to be primed with MMP-MTB by the vaccination, and bothM. tuberculosis-derived recombinant proteins produced perforin-producing CD8+T cells from memory-type CD8+T cells. Further, infection of DC and macrophages withM. tuberculosisH37Ra and H37Rv induced the expression of MMP on their surface. These results indicate thatM. tuberculosis-derived MMP, as a sole protein or as part of a fusion protein, may be useful for developing new vaccinating agents against tuberculosis.