Differential expression, localization and activity of two alternatively spliced isoforms of human APC regulator CDH1

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Abstract
The timely destruction of key regulators through ubiquitin-mediated proteolysis ensures the orderly progression of the cell cycle. The APC (anaphase-promoting complex) is a major component of this degradation machinery and its activation is required for the execution of critical events. Recent studies have just begun to reveal the complex control of the APC through a regulatory network involving WD40 repeat proteins CDC20 and CDH1. In the present paper, we report on the identification and characterization of human CDH1β, a novel alternatively spliced isoform of CDH1. Both CDH1α and CDH1β can bind to the APC and stimulate the degradation of cyclin B1, but they are differentially expressed in human tissues and cells. CDH1α contains a nuclear localization signal which is absent in CDH1β. Intracellularly, CDH1α appears in the nucleus whereas CDH1β is a predominantly cytoplasmic protein. The forced overexpression of CDH1α in cultured cells correlates with the reduction of nuclear cyclin A, but the steady-state amount of cyclin A does not change noticeably in CDH1β-overexpressed cells. In Xenopus embryos, ectopic overexpression of human CDH1α, but not of CDH1β, induces cell-cycle arrest during the first G1 phase at the mid-blastula transition. Taken together, our findings document the differential expression, subcellular localization and cell-cycle-regulatory activity of human CDH1 isoforms.