Alterations in ubiquitin-proteasome system (UPS) have been implicated in the etiology of human cardiovascular diseases. Skp1/Cul1/F-box (SCF) ubiquitin E3 ligase complex plays a pivotal role in ubiquitination of cardiac proteins. However, a specific ubiquitin E3 ligase responsible for the destruction of cardiac transcription factors such as Nkx2-5, IsI1, Mef2C, and Tbx5 remains elusive to date. Here, we show that a novel F-box containing Fbxo25 is cardiac-specific and acts as an ubiquitin E3 ligase for cardiac transcription factors. Fbxo25 expression was nuclei-specific in vitro and cardiomyocytes. Expression level of Fbxo25 was higher in a fetal heart than an adult. Moreover, Fbxo25 expression was increased along with those of cardiac-specific genes during cardiomyocyte development from ESCs. Fbxo25 expression facilitated protein degradation of Nloc2-5, ISI1, Hand1, and Mef2C. Especially, Fbxo25 ubiquitinated Nkx2-5, IsI1, and Hand1. Altogether, Fbxo25 acts as an ubiquitin E3 ligase to target cardiac transcription factors including Nkx2-5, IsI1, and Hand1, indicating that cardiac protein homeostasis through Fbxo25 has a pivotal impact on cardiac development. (C) 2011 Published by Elsevier Inc.