Department of Cell BiologyHarvard Medical School/HHMI
LHRRB, Room 517
240 Longwood Avenue
Boston, MA 02115
Lab Members: 10 postdoctoral fellows, 4 graduate students
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We are interested in epigenetic mechanisms that control cell identity and chromosome stability in eukaryotes. Specialized chromatin structures, called heterochromatin or silent chromatin, are associated with centromeres, telomeres, repetitive DNA elements, and with genes that regulate cell differentiation. A central property of these domains is that once assembled, they are epigenetically inherited during many cell divisions. The mechanism of establishment and epigenetic inheritance of heterochromatin involves cooperative interactions between silencing complexes that contain histone-modifying enzymes, and histones, DNA binding proteins, or noncoding RNA molecules. Our lab uses the budding and fission yeasts as model systems to study how heterochromatic domains are established and propagated.
We have shown that two types of multiprotein complexes, composed of previously identified as well as novel proteins, mediate silencing in budding yeast. Both complexes contain a universally conserved deacetylase enzyme called Sir2, which requires NAD as a co-factor. In a highly unusual reaction, Sir2 couples deacetylation to NAD breakdown and the synthesis of O-acetyl-ADP-ribose (AAR, also called OAADPR). In addition to the enzymatic activity of Sir2, Sir-Sir interactions are required for the spreading of silencing complexes along the chromatin fiber. The mechanism appears to involve the sequential deacetylation of histone tails and spreading of silencing complexes by self-associations coupled to binding to deacetylated histone tails. We have proposed that this type of mechanism is a conserved feature of heterochromatin assembly in distantly related organisms such as yeast, fruit fly, and human. More recently, we have demonstrated a role for AAR in promoting the assembly of the SIR silencing complex and have uncovered a role for perinuclear chromosome tethering in maintenance of genome stability in yeast.
Our work in fission yeast is focused on understanding the role of the RNA interference (RNAi) pathway in mediating heterochromatin formation. We have identified several multiprotein complexes that are targeted to specific DNA repeats by small interfering RNAs (siRNAs). Our studies suggest that noncoding centromeric RNAs form scaffolds for the cooperative assembly of RNAi and histone-modifying complexes, leading to heterochromatin formation. The conserved RNAi protein, Argonaute, plays a key role in these pathways. The fission yeast Argonaute (Ago1) physically links noncoding RNA with histone-modifying activities and also acts as a surveillance factor that samples the entire transcriptome through its RNA binding activities. Ago1 surveillance leads to the generation of a new class of small RNAs, called primal RNAs or priRNAs, which act on antisense targets to initiate silencing.
Last Update: 8/22/2013