The physiological functions of the beta-amyloid precursor protein (APP) may include nuclear signaling. To characterize the role of the APP adaptor proteins Fe65, Jip1b, X11α (MINT1) and the chromatin-associated protein Tip60, we analyzed their interactions by confocal microscopy and co-immunoprecipitations. AICD corresponding to S3-cleaved APP bound to Fe65 that transported it to nuclei and docked it to Tip60. These proteins formed AICD-Fe65-Tip60 (AFT) complexes that were concentrated in spherical nuclear spots. γ-Secretase inhibitors prevented AFT-complex formation with AICD derived from full-length APP. The APP adaptor protein Jip1b also transported AICD to nuclei and docked it to Tip60, but AICD-Jip1b-Tip60 (AJT) complexes had different, speckle-like morphology. By contrast, X11α trapped AICD in the cytosol. Induced AICD expression identified the APP-effector genes APP, BACE, Tip60, GSK3β and KAI1, but not the Notch-effector gene Hes1 as transcriptional targets. These data establish a role for APP in nuclear signaling, and they suggest that therapeutic strategies designed to modulate the cleavage of APP affect AICD-dependent signaling.

p31comet opposes the activities of the Mad2 spindle assembly checkpoint protein, localizes to unattached kinetochores, and like many checkpoint proteins, turns over rapidly at that site. Depletion of p31comet prevents timely passage into anaphase, showing that mitotic progression requires an active mechanism for silencing the spindle checkpoint.

The mitotic spindle drives chromosome movement during mitosis and attaches to chromosomes at dedicated genomic loci named centromeres. Centromeres are epigenetically specified by their histone composition, namely the presence of the histone H3 variant CENP-A, which is regulated during the cell cycle by its dynamic expression and localization. Here, we combined biochemical methods and quantitative imaging approaches to investigate a new function of CUL4-RING E3 ubiquitin ligases (CRL4) in regulating CENP-A dynamics. We found that the core components CUL4 and DDB1 are required for centromeric loading of CENP-A, but do not influence CENP-A maintenance or pre-nucleosomal CENP-A levels. Interestingly, we identified RBBP7 as a substrate-specific CRL4 adaptor required for this process, in addition to its role in binding and stabilizing soluble CENP-A. Our data thus suggest that the CRL4 complex containing RBBP7 (CRL4 RBBP7 ) might regulate mitosis by promoting ubiquitin-dependent loading of newly synthesized CENP-A during the G1 phase of the cell cycle.

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