The cell cycle is negatively regulated by members of the pocket protein family, which consists of the tumor suppressor pRB and two closely related paralogs, p107 and p130. In their hypophosphorylated state, they are associated with E2F transcription factors which result in the repression of transcription of E2F-dependent genes that are required for cell cycle progression. The phosphorylation state of pocket proteins during the cell cycle is determined at least in part by an equilibrium between inducible CDKs and the serine/threonine protein phosphatase PP2A. Protein Phosphatase 2A (PP2A), is a serine/threonine phosphatase that functions as as a collection of trimeric holoenzymes. The trimeric PP2A holoenzyme is composed of the "A" scaffolding subunit, the "C" catalytic subunit, and a "B" regulatory subunit. The B subunit is the major determinant in substrate specificity and subcellular localization. Two holoenzymes consisting of the core PP2A dimer and either the B55α or PR70 regulatory subunits have been implicated in the activation of p107/p130 and pRB, respectively. While the phosphorylation state of p107 is very sensitive to forced changes of B55α levels in human cell lines, regulation of p107 in response to physiological modulation of PP2A/B55α has not been previously elucidated. In this thesis, I show that FGF1, which induces maturation and cell cycle exit in chondrocytes, triggers rapid accumulation of p107/PP2A/B55α complexes coinciding with p107 dephosphorylation without an increase in B55α protein expression in RCS cells. Reciprocal solution-based mass-spectrometry analysis identified the PP2A/B55α complex as a major component of a subset of p107 complexes, which also contain E2F/DPs, DREAM subunits and cyclin/CDK complexes. p107 is one of the major partners of B55α, which also associates with pRB in RCS cells. FGF1 induces dephosphorylation of p107, its translocation to the nucleus, remodeling of p107 complexes, and enhances its interaction with E2F4 and other p107 partners. Consistent with an essential role of B55α in the rapid activation of p107 in chondrocytes, limited ectopic expression of B55α results in marked dephosphorylation of p107, while B55α knockdown results in hyperphosphorylation. More importantly, limited knockdown of B55α dramatically delays FGF1 induced dephosphorylation of p107. Moreover, dephosphorylation of p107 in response to FGF1 treatment results in selective recruitment of p107 to regulated genes including CMYC. Our results suggest a model where FGF1 mediates rapid dephosphorylation and activation of p107 independently of the CDK activities that maintain p130 and pRB hyperphosphorylated for several hours post p107 dephosphorylation in maturing chondrocytes. Additionally, we provide preliminary evidence that PPP2R2A may act as a haploinsufficient tumor suppressor in prostate cancer cell lines. PPP2R2A is hemizygously deleted in various prostate cancer cell lines and tumor samples. We identified three cell lines that express less B55α the gene product of PPP2R2A, than cell lines that are reported to have both alleles intact. Furthermore, ectopic expression of B55α in PC3 cells results in a phenotype reminiscent of senescence, ultimately leading to cell death. These cells are unable to form colonies in soft agar and have increased DNA content and euploidy. Combined with their larger cell and nuclear size, this suggests that ectopic expression of B55α in PC3 cells results in endoreplication. Altogether these suggest that reduced B55α expression in these cells confers a growth advantage in PCa cell lines, which is extinguished when B55α is reintroduced, supporting the notion that hemizygous deletion of PPP2R2A in prostate tumors may help promote tumorigenesis. / Molecular Biology and Genetics