Estrogen receptor-alpha (ER[alpha]) is a member of nuclear receptor superfamily of transcription factors. It is known to regulate carcinogenic gene expression programs that are involved in the development and progression of breast cancer. The transcriptional function of ER[alpha] is mediated by a C-terminal AF2 and an N-terminal AF1 activation domains. Ligand-dependent AF2 activity is well-characterized and serves as a basis for hormonal therapy for breast cancer. In contrast, structural and functional mechanisms governing AF1 functions remain poorly understood. AF1 activity of ER[alpha] is regulated by phosphorylation stemming from hormone, peptide growth factors, and second messenger pathways. Paradoxically, phosphorylation results in contrasting responses (differentiation and growth, protein stability and degradation, agonist and antagonist activities). How phosphorylation translates into diverse outcome is not clearly understood. The work presented in this thesis has uncovered a post-translation modification beyond phosphorylation that regulates the function and fate of ER[alpha]. I found that phosphorylation-dependent prolyl cis/trans isomerase, Pin1, causes structural changes at the AF1 region of ER[alpha]. These local changes allosterically regulate DNA binding and dimerization activities, enhancing overall ER[alpha] transcriptional function. Pin1 also stabilizes ER[alpha] protein by blocking its ubiquitination and degradation by the proteasome. Further studies in understanding the role of Pin1 in breast cancer led us to uncover the importance of Pin1 in proliferation of ER[alpha]-positive breast cancer cells and mammary tumors in rodent models. Pin1 overexpression was sufficient to overcome the antagonistic effects of tamoxifen and also contributed to tamoxifen resistance in breast cancer cells. Finally, the clinical relevance of Pin1 activity was confirmed by our findings in human breast tumors, where Pin1 levels were correlated with ER[alpha] protein levels, and ER[alpha]-positive tumor patients with high Pin1 levels had poor overall survival. Overall, the findings in this thesis have identified a new regulatory mechanism governing ER[alpha] AF1 function in breast cancer and discovered Pin1 as an important component modulating ER[alpha] protein levels and transactivation functions.

The enzyme Pin1 is a peptidyl-prolyl cis-trans isomerase consisting structurally of two domains, an N-terminal WW protein interaction domain and a C-terminal PPIase catalytic domain. Both domains bind a phosphorylated serine/threonine-proline motif, however, a precise mechanism regarding how binding to interactors is coordinated by both domains has not yet been determined. Although multiple models exist to explain this process, it appears that the interactions may be substrate-specific. With regards to a well-studied Pin1 interactor, CDC25C, we hypothesize that binding occurs via the simultaneous model. This model suggests that two binding sites, each having low affinity, may bind in concert producing a higher affinity interaction. To investigate this we chose to employ a peptide-based approach, using human CDC25C-derived peptides which contained the two identified Pin1 binding sites in phosphorylated and non-phosphorylated combinations. These peptides were utilized in two independent assays, surface plasmon resonance and fluorescence polarization, to elucidate the domain- and phosphorylation-requirements of the Pin1-CDC 25C interaction. We showed that the interaction is phosphorylation-dependent, and is optimal when full- length, wild-type Pin1 binds to a doubly-phosphorylated peptide. Collectively, our results support our hypothesis that the Pin1-CDC25C interaction occurs via the simultaneous model, and requires both domains.

Pin1 is a human protein classified as a peptidyl-prolyl cis/trans isomerase. The protein regulates the conformation of phosphorylated protein substrates by rotating the peptide bond between phosphorylated serine/threonine residues that precede proline residues. Structurally, Pin1 consists of an N-terminal WW domain and a C-terminal PPIase domain. The PPIase domain catalyzes cis/trans isomerization of peptide bonds in substrate proteins that contain the aforementioned consensus motif. We hypothesize that Pin1 binding is positively impacted when two phospho-acceptor sites on peptides derived from mitotic phosphatase CDC25C, a known Pin1-interacting protein, are phosphorylated. Using nuclear magnetic resonance and fluorescence polarization, binding affinities of CDC25C peptides to Pin1 were calculated. The results indicate that doubly-phosphorylated peptides bound to Pin1 have lower dissociation constants and consequently greater binding affinities, than complexes containing non- or singly-phosphorylated peptides, at the equivalent residues. This suggests that Pin1 has two independent phospho-binding sites that when bound, increase substrate binding affinity.

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Organized on a combined basis of chronology and of structural and functional hierarchy, This comprehensive text describes all aspects of proteins--biosynthesis, evolution, dynamics, ligand binding, catalysis, and energy transduction--not just their structures. This edition (first was 1984) is thoroughly updated--especially in the area of protein biosynthesis--and features end-of-chapter exercises and problems, many of which require the student to consult the cited literature in order to obtain the answer. Annotation copyright by Book News, Inc., Portland, OR