Date of Award
yeast apoptosis, chromatin remodeling, chormatin compaction, chromatin structure
Chromatin compaction is a hallmark property of apoptosis, a highly coordinated suicide mechanism generally believed to be confined to vertebrates. However, invertebrates such as the budding yeast, Saccharomyces cerevisiae, display an apoptotic-like phenotypes including chromatin condensation, although its functions and mechanism are unclear. One mechanism that alters chromatin structure is the covalent modification of histones, which associates with DNA to form the nucleosome, the fundamental unit of chromatin. Phosphorylation of histone H2B at serine 14 (H2BS14ph), catalyzed by Mst1 kinase, has been linked to chromatin compaction during mammalian apoptosis. I extended these results to yeast by demonstrating that Ste20 kinase, a yeast orthologue of Mst1, directly phosphorylates H2B at serine 10 (H2BS10ph) in a hydrogen peroxide-induced cell death pathway. Unlike Mst1, Ste20 translocates into the nucleus in a caspase-independent fashion to mediate phosphorylation of H2B. H2BS10ph is dependent on the removal of acetylation mark on adjacent lysine residue, (H2BK11ac), which exists in growing yeast. During yeast apoptosis, the HDAC Hos3 deacetylates K11, which in turn, mediates H2BS10ph by Ste20 kinase. My studies underscore a concerted series of enzyme reactions governing histone modifications that promote a switch from cell proliferation to cell death. Moreover, the conservation of targeted H2B phosphorylation and the enzyme system point to an ancient, late-stage chromatin remodeling event that likely governs cellular homeostasis in a wide range of organisms. H2B phosphorylation may mediate apoptotic chromatin compaction by 1) directly affecting internucleosomal contacts and histone DNA interaction (â€œcis mechanismâ€), or 2) recruiting binding partners that then induce and direct downstream functions (â€œtransâ€ mechanisms). Peptides corresponding to the phosphorylated form of yeast H2B and human H2B have the intrinsic ability to form â€œaggregatesâ€ in SDS polyacrylamide gel electrophoresis. In addition, nucleosome array containing yeast S10E or human S14E H2B fold into compacted conformation as measured by analytical ultracentrifugation. Moreover, an interaction between the forkhead homology-associated domain 1 (FHA1) of Rad53 and H2BS10ph was uncovered. This interaction inactivates the DNA damage checkpoint pathway and promotes apoptotic chromatin condensation. Thus, both mechanisms may contribute to chromatin remodeling event that govern apoptotic chromatin compaction in a pathway conserved from yeast to humans.
Ahn-Upton, Sung Hee, "Regulation of Histone Covalent Modifications During Yeast Apoptosis" (2007). Student Theses and Dissertations. 8.