Structural and functional characterization of Pin1BP1: A novel Pin1-interacting protein regulating apoptosis

Pin1 recognizes and isomerizes pSer/pThr-Pro bonds, a mechanism critical for mitotic progression in human and yeast cells. This post-phosphorylation regulation precisely modulates phosphoprotein function. To explore Pin1-regulated pathways in the cell cycle and apoptosis, we identified Pin1BP1 through yeast two-hybrid screening. RT-PCR confirmed that Pin1BP1 is ubiquitously expressed. Sequence analysis revealed that Pin1BP1 contains an N-terminal nuclear localization signal, a proline-rich region, and a C-terminal Zn²⁺ binding domain. Consistently, immunocytochemistry showed predominant nuclear localization of Pin1BP1. Co-immunoprecipitation and GST pull-down assays demonstrated that Pin1 interacts directly with Pin1BP1 in a phosphorylation-independent manner. Peptide array and AlphaFold3-based structural analysis identified key Pin1BP1 residues interacting with Pin1's WW and PPIase domains, stabilizing Pin1BP1. Functional studies demonstrated Pin1BP1 overexpression induces apoptosis in HeLa cells, enhancing stability and activity through Pin1 interaction. Kaplan-Meier analysis linked low Pin1BP1 expression to poor breast cancer prognosis, suggesting Pin1BP1 as a biomarker and therapeutic target. These findings highlight Pin1BP1's apoptotic role and relevance to cancer biology.