The Apc5 Subunit of the Anaphase-Promoting Complex/Cyclosome Interacts with Poly(A) Binding Protein and Represses Internal Ribosome Entry Site-Mediated Translation

Abstract

The anaphase-promoting complex/cyclosome (APC/C) is a multisubunit ubiquitin ligase that mediates the proteolysis of cell cycle proteins in mitosis and G₁. We used a yeast three-hybrid screen to identify proteins that interact with the internal ribosome entry site (IRES) of platelet-derived growth factor 2 mRNA. Surprisingly, this screen identified Apc5, although it does not harbor a classical RNA binding domain. We found that Apc5 binds the poly(A) binding protein (PABP), which directly binds the IRES element. PABP was found to enhance IRES-mediated translation, whereas Apc5 overexpression counteracted this effect. In addition to its association with the APC/C complex, Apc5 binds much heavier complexes and cosediments with the ribosomal fraction. In contrast to Apc3, which is associated only with the APC/C and remains intact during differentiation, Apc5 is degraded upon megakaryocytic differentiation in correlation with IRES activation. Expression of Apc5 in differentiated cells abolished IRES activation. This is the first report implying an additional role for an APC/C subunit, apart from its being part of the APC/C complex.

We thank N. Sonenberg, M. Wickens, N. Standard, G. Dreyfuss, J. Hershey, J. Gannon, C. Hoog, and D. Kornitzer for timely gifts of plasmids and antibodies used in this study; Yuri Svitkin for Krebs-2 cells and the PABP-depletion protocol; and A. Yahalom, E. Oron, and D. Chamovitz for assistance with gel filtration chromatography.

This work was supported by grants from the Chief Scientist's Office of Ministry of Health, Israel; Israel Cancer Association; the U.S.-Israel Binational Science Foundation; and the Israel Science Foundation Administration by the Academy of Sciences and Humanities (the Charles H. Revson Foundation) to O.E.-S. N. Koloteva-Levine's fellowship was supported by the Israel Cancer Research Fund.