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J. Biol. Chem., Vol. 279, Issue 47, 48598-48606, November 19, 2004

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The Scaffolding A/Tpd3 Subunit and High Phosphatase Activity Are Dispensable for Cdc55 Function in the Saccharomyces cerevisiae Spindle Checkpoint and in Cytokinesis^*

Roni Koren, Liat Rainis, and Tamar Kleinberger

From the Gonda Center of Molecular Microbiology and The Rappaport Family Institute for Research in the Medical Sciences, Faculty of Medicine, Technion-Israel Institute of Technology, Bat Galim, Haifa, 31096, Israel

Protein serine/threonine phosphatase 2A (PP2A) is a multifunctional enzyme whose trimeric form consists of a scaffolding A subunit, a catalytic C subunit, and one of several regulatory B subunits (B, B', and B''). The adenovirus E4orf4 protein associates with PP2A by directly binding the B or B' subunits. An interaction with an active PP2A containing the B subunit, or its homologue in yeast, Cdc55, is required for E4orf4-induced apoptosis in mammalian cells and for induction of growth arrest in Saccharomyces cerevisiae. In this work, Cdc55 was randomly mutagenized by low-fidelity PCR amplification, and Cdc55 mutants that lost the ability to transduce the E4orf4 toxic signal in yeast were selected. The mutations obtained by this protocol inhibited the association of Cdc55 with E4orf4, or with the PP2A-AC subunits, or both. Functional analysis revealed that a mutant that does not bind Tpd3, the yeast A subunit, as well as wild type Cdc55 in a tpd3 background, can form a heterodimer with the catalytic subunit. This association requires C subunit carboxyl methylation. The residual phosphatase activity associated with Cdc55 in the absence of Tpd3 is sufficient to maintain a partially active spindle checkpoint and to prevent cytokinesis defects.

Received for publication, August 16, 2004

^* This work was supported by grants from the Israel Science Foundation, founded by the Israel Academy of Sciences and Humanities Grant 374/00, the Israeli Ministry of Sciences, and the Fund for the Promotion of Research at the Technion (to T. K.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Both authors contributed equally to this work.

To whom correspondence should be addressed. Tel.: 972-4-829-5257; Fax: 972-4-829-5225; E-mail: tamark{at}tx.technion.ac.il.

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