βTrCP2 is a protein that in humans is encoded by the FBXW11gene. This gene encodes a member of the F-box protein family which is characterized by an approximately 40 residue structural motif, the F-box. The F-box proteins constitute one of the four subunits of ubiquitin protein ligase complex called SCFs, which often, but not always, recognize substrates in a phosphorylation-dependent manner. F-box proteins are divided into 3 classes:
The protein encoded by FBXW11 belongs to the Fbxw class as, in addition to an F-box, this protein contains multiple WD40 repeats. This protein is homologous to Xenopus βTrCP, yeastMet30, NeurosporaScon2 and DrosophilaSlimb. In mammals, in addition to βTrCP2, a paralog protein also exists, but, so far, their functions appear redundant and indistinguishable.
Discovery
Human βTrCP was originally identified as a cellular ubiquitin ligase that is bound by the HIV-1Vpuviral protein to eliminate cellular CD4 by connecting it to the proteolytic machinery. Subsequently, βTrCP was shown to regulate multiple cellular processes by mediating the degradation of various targets. Cell cycle regulators constitute a major group of βTrCP substrates. During S phase, βTrCP keeps CDK1 in check by promoting the degradation of the phosphatase CDC25A, whereas in G2, βTrCP contributes to CDK1 activation by targeting the kinase WEE1 for degradation. In early mitosis, βTrCP mediates the degradation of EMI1, an inhibitor of the APC/C ubiquitin ligase complex, which is responsible for the anaphase-metaphase transition and mitotic exit. Furthermore, βTrCP controls APC/C by targeting REST, thereby removing its transcriptional repression on MAD2, an essential component of the spindle assembly checkpoint that keeps APC/C inactive until all chromatids are attached to the spindle microtubles.
Functions
βTrCP plays important roles in regulating cell cycle checkpoints. In response to genotoxic stress, it contributes to turn off CDK1 activity by mediating the degradation of CDC25A in collaboration with Chk1, thereby preventing cell cycle progression before the completion of DNA repair. During recovery from DNA replication and DNA damage, βTrCP instead targets Claspin in a Plk1-dependent manner. βTrCP has also emerged as an important player in protein translation, cell growth and survival. In response to mitogens, PDCD4, an inhibitor of the translation initiation factor eIF4A, is rapidly degraded in a βTrCP- and S6K1-dependent manner, allowing efficient protein translation and cell growth. βTrCP also cooperates with mTOR and CK1α to induce the degradation of DEPTOR, thereby generating an auto-amplification loop to promote the full activation of mTOR. At the same time, βTrCP mediates the degradation of the pro-apoptotic protein BimEL to promote cell survival. βTrCP also associates with phosphorylated IkappaBalpha and beta-catenin destruction motifs, probably functioning in multiple transcriptional programs by regulating the NF-kappaB and the WNT pathways.
βTrCP behaves as an oncoprotein in some tissues. Elevated levels of βTrCP expression have been found in colorectal, pancreatic, hapatoblastoma, and breast cancers.
