During the G 0 /G 1 -S phase transition, the timely synthesis and degradation of key regulatory proteins is required for normal cell cycle progression. Two of these proteins, c-Myc and cyclin E, are recognized by the Cdc4 E3 ligase of the Skp1/Cul1/Rbx1 (SCF) complex. SCF Cdc4 binds to a similar phosphodegron sequence in c-Myc and cyclin E proteins resulting in ubiquitylation and degradation of both proteins via the 26 S proteosome. Since the prolyl isomerase Pin1 binds the c-Myc phosphodegron and participates in regulation of c-Myc turnover, we hypothesized that Pin1 would bind to and regulate cyclin E turnover in a similar manner. Here we show that Pin1 regulates the turnover of cyclin E in mouse embryo fibroblasts. Pin1 binds to the cyclin E-Cdk2 complex in a manner that depends on Ser 384 of cyclin E, which is phosphorylated by Cdk2. The absence of Pin1 results in an increased steady-state level of cyclin E and stalling of the cells in the G 1 /S phase of the cell cycle. The cellular changes that result from the loss of Pin1 predispose Pin1 null mouse embryo fibroblasts to undergo more rapid genomic instability when immortalized by conditional inactivation of p53 and sensitizes these cells to more aggressive Ras-dependent transformation and tumorigenesis.Mitogenic stimuli initiate a sequence of events that result in the entry of quiescent cells into S phase (1). Critical for this important transition is the ordered synthesis and degradation of transcription factors such as c-Jun and c-Myc (Myc) and cyclins such as cyclin D and cyclin E (1). Deregulation of the turnover of these proteins, such that they remain active at inappropriate times during cell cycle progression, is frequently found in human cancer. Thus, an understanding of the molecular mechanisms that regulate protein turnover is crucial to provide insight into the oncogenic process. Three proteins important for the progression of cells into S phase, c-Jun, c-Myc, and cyclin E, are ubiquitylated by a common member of the Skp1/ Cul1/Rbx1 (SCF) 3 group of ubiquitin enzymes in which the F-box component, which serves as the ubiquitin E3 ligase, is Cdc4 (SEL-10, Fbw7, Ago) (2). SCF Cdc4 binds to a component of each protein that has been termed the "phosphodegron" (3) to promote the ubiquitylation and degradation of these proteins via the 26 S proteosome. c-Jun and Myc have an additional binding protein in common, the peptidyl prolyl cis/trans-isomerase Pin1, which binds and isomerizes prolyl bonds in the context of phospho-Ser/ Thr-Pro motifs (4, 5).Previously, we described the mechanism by which Pin1 promotes Myc degradation (5). The Cdc4 phosphodegron of Myc is present in a domain termed Myc box 1 (MB1), containing the sequence LPpTPPLpSP (where pT represents phosphothreonine and pS represents phosphoserine), in which the two phosphorylation events occur sequentially and are catalyzed by ERK (Ser 62 ) and GSK3␤ (Thr 58 ), respectively (6, 7). Pin1 binds to the doubly phosphorylated motif in a manner requiring phospho-Thr 58 and promotes a conformationa...

Signals that regulate T cell homeostasis are not fully understood. G protein-coupled receptors (GPCR), such as the chemokine receptors, may affect homeostasis by direct signaling or by guiding T cell migration to distinct location-restricted signals. Here, we show that blockade of Gai-associated GPCR signaling by treatment with pertussis toxin led to T cell atrophy and shortened life-span in T cell-replete hosts and prevented T cell homeostatic growth and proliferation in T cell-deficient hosts. In vitro, however, neither GPCR inhibition nor chemokine stimulation affected T cell atrophy, survival, or proliferation. These findings suggest that GPCR signals are not trophic stimuli, but instead may be required for migration to distinct trophic signals, such as IL-7 or selfpeptide/MHC. Surprisingly, while chemokines did not affect atrophy, atrophic T cells displayed increased chemokine-induced chemotaxis that was prevented by IL-7 and submitogenic anti-CD3 antibody treatment. This increase in migration was associated with increased levels of GTP-bound Rac and the ability to remodel actin. These data suggest a novel mechanism of T cell homeostasis wherein GPCR may promote T cell migration to distinct location-restricted homeostatic trophic cues for T cell survival and growth. Homeostatic trophic signals, in turn, may suppress chemokine sensitivity and cytoskeletal remodeling, to inhibit further migration. IntroductionMaintenance of lymphocyte homeostasis is crucial to allow rapid protective responses to foreign pathogens and simultaneously prevent diseases of the immune system, such as autoimmunity or immunodeficiency. One mechanism to maintain immune homeostasis is the requirement of cells to receive signals from their microenvironment to prevent activation of a default cell-intrinsic death pathway [1]. When deprived of these signals, cells undergo cellular atrophy prior to an ultimate commitment to apoptosis. Atrophy is characterized by decreased cell size and metabolism and inhibition of the cell cycle [2][3][4]. A second mechanism to maintain immune homeostasis is the spontaneous proliferation of lymphocytes in lymphopenic animals [5]. This cytokine and self-antigen-driven cell expansion may play an important role to increase lymphocyte numbers when they are limiting. A third homeostatic process is regulation of T cell migration to allow even T cell distribution and immunosurveillance [6,7]. Lymphocyte chemotaxis is guided primarily by Gaiassociated G protein-coupled receptor (GPCR) signaling mediated by chemokine receptors [7]. The c c cytokine IL-7 and interactions between the TCR and self-peptide/MHC maintain T cell viability, allow homeostatic proliferation in vivo and can maintain basal T cell size and metabolism in vitro [3,5,8,9]. The role of GPCR in these T cell homeostatic processes, however, is not known. These signals may directly affect T cell homeostasis through activation of similar signaling pathways as activated by c c cytokines and the TCR. In particular, the survival-promoting phosphatidylinosit...