Phosphorylation-Dependent Ubiquitination of Cyclin D1 by the SCF^{FBX4-αB Crystallin} Complex

Growth factor-dependent accumulation of the cyclin D1 proto-oncogene is balanced by its rapid phosphorylation-dependent proteolysis. Degradation is triggered by threonine 286 phosphorylation, which promotes its ubiquitination by an unknown E3 ligase. We demonstrate that Thr286-phosphorylated cyclin D1 is recognized by a Skp1-Cul1-F box (SCF) ubiquitin ligase where FBX4 and αB crystallin govern substrate specificity. Overexpression of FBX4 and αB crystallin triggered cyclin D1 ubiquitination and increased cyclin D1 turnover. Impairment of SCF^{FBX4-αB crystallin} function attenuated cyclin D1 ubiquitination, promoting cyclin D1 overexpression and accelerated cell-cycle progression. Purified SCF^{FBX4-αB crystallin} catalyzed polyubiquitination of cyclin D1 in vitro. Consistent with a putative role for a cyclin D1 E3 ligase in tumorigenesis, FBX4 and αB crystallin expression was reduced in tumor-derived cell lines and a subset of primary human cancers that overexpress cyclin D1. We conclude that SCF^{FBX4-αB crystallin} is an E3 ubiquitin ligase that promotes ubiquitin-dependent degradation of Thr286-phosphorylated cyclin D1.