| Description |
Protein phosphatase 2A (PP2A) is a major serine/threonine-specific phosphatase and regulates a significant array of cellular events. This dissertation primarily describes the novel role of PP2A in Wnt signaling and apoptosis. First, PP2A and its B56 regulatory subunit inhibit Wnt signaling in Xenopus . PP2A is required for beta-catenin degradation in vitro . A PP2A heterotrimer containing A, C, and B56 subunits was co-immunoprecipitated with axin. A, C, and B56 subunits each have ventralizing ability in Xenopus embryos. B56 was epistatically positioned downstream of GSK3beta and axin but upstream of beta-catenin. Second, B56-targeted PP2A is required for survival and protects from apoptosis in Drosophila . Loss of A, C, or B56 subunits by RNA interference (RNAi) induced apoptosis in S2 cells, which requires the presence of specific caspases. Epistasis analysis placed B56-targeted PP2A functionally upstream of Apaf-1, Reaper and Hid, and p53. Loss of B56-targeted PP2A in Drosophila embryos by RNAi resulted in abortion of embryo development and this phenotype was rescued by co-RNAi of Drice. Third, two conserved domains in B subunits mediate binding to the A subunit of PP2A. B subunits have no detectable sequence homology among different families. In vitro expression of a series of B56alpha fragments identified two distinct domains that independently bound to the A subunit. Sequence alignment of these A subunit-binding domains recognized conserved residues in B/PR55 and B '' /PR72 family members that serve a similar function. Fourth, to examine whether the B56beta gene within 11q12 is a tumor suppressor mutated in neuroblastoma, the DNA and RNA samples from neuroblastoma patients and cell lines were analyzed and no mutations were identified in the coding regions of the B56beta gene. Finally, to identify novel regulatory subunits of PP2A in S. cerevisiae , biochemical approaches for purifying PP2A-associated novel regulators were undertaken. Although the A and C subunit complex in the cdc55delta rts1delta strain was purified, no obvious novel B subunits were identified. Together, this dissertation extended our understanding of PP2A's function and regulation, especially, the novel role of B56-targeted PP2A in the regulation of apoptosis. |
