Polyubiquitination of the B-Cell Translocation Gene 1 and 2 Proteins Is Promoted by the SCF Ubiquitin Ligase Complex Containing βTrCP

Abstract

B-cell translocation gene 1 and 2 (BTG1 and BTG2) are members of the BTG/Tob antiproliferative protein family, which is able to regulate the cell cycle and cell proliferation. We previously reported that BTG1, BTG2, Tob, and Tob2 are degraded via the ubiquitin-proteasome pathway. In this study, we investigated the mechanism of polyubiquitination of BTG1 and BTG2. Since the Skp1-Cdc53/Cullin 1-F-box protein (SCF) complex functions as one of the major ubiquitin ligases for cell cycle regulation, we first examined interactions between BTG proteins and components of the SCF complex, and found that BTG1 and BTG2 were capable of interacting with the SCF complex containing Cullin-1 (a scaffold protein) and Skp1 (a linker protein). As the SCF complex can ubiquitinate various target proteins by substituting different F-box proteins as subunits that recognize different target proteins, we next examined which F-box proteins could bind the two BTG proteins, and found that Skp2, β-transducin repeat-containing protein 1 (βTrCP1), and βTrCP2 were able to associate with both BTG1 and BTG2. Furthermore, we obtained evidence showing that βTrCP1 enhanced the polyubiquitination of both BTG1 and BTG2 more efficiently than Skp2 did, and that an F-box truncated mutant of βTrCP1 had a dominant negative effect on this polyubiquitination. Thus, we propose that BTG1 and BTG2 are subjected to polyubiquitination, more efficiently when it is mediated by SCF βTrCP than by SCF Skp2 .Key words antiproliferative protein; B-cell translocation gene; ubiquitin ligase; Skp1-Cdc53/Cullin 1-F-box protein; F-box protein; ubiquitin B-cell translocation gene 1 and 2 (BTG1 and BTG2) proteins belong to the BTG/transducer of ErbB2 (Tob) antiproliferative protein family, which shares a highly conserved N-terminal domain that bears no similarity to any other known domain.1,2) The exogenous expression of BTG/Tob family members results in the inhibition of cell proliferation in a variety of cell types, [3][4][5][6] implying that the BTG/Tob family is a novel class of protein family involved in cell cycle progression, cell proliferation, and tumor suppression. In fact, BTG/ Tob expression is reduced in several cancer tissues. 7,8) Intriguingly, the conserved BTG/Tob domain is implicated as a region that allows for interaction with a variety of transcription factors, suggesting that BTG/Tob contributes to the regulation of DNA-binding of sequence-specific transcription factors. [9][10][11] BTG/Tob family members also interact with subunit 7 of the chemokine (C-C motif) receptor 4 (CCR4)-negative regulator of transcription (NOT) complex (CNOT7) or CNOT8 6,[12][13][14] ; the CCR4-NOT complex regulates transcription and mRNA turnover.15,16) Furthermore, it was reported that BTG2 and Tob participate in mRNA turnover by promoting mRNA deadenylating activity of the CCR4-NOT complex.

PABPN1L mediates cytoplasmic mRNA decay as a placeholder during the maternal‐to‐zygotic transition

“…BTG family proteins are short-lived in somatic cells and are promptly degraded by the ubiquitin-proteasome system [45,46]. However, the mechanisms regulating the stability of BTG4 during the MZT have not yet been clarified.…”

“…BTG1 and BTG2 are polyubiquitinated by the SKP1-Cullin 1-Fbox protein (SCF) ubiquitin E3 ligase and then degraded. b-transducin repeat-containing protein 1 (bTrCP1) is the substrate adaptor of SCF in mediating BTG2 polyubiquitination [45,46]. In Co-IP experiments, bTrCP1 interacted with BTG4 ( Fig 7C) but not with PABPN1L ( Fig EV5F).…”

Ubiquitination-mediated degradation of cell cycle-related proteins by F-box proteins

“…Additionally, both β-TRCP1 and β-TRCP2 can recognize and degrade BTG (B-cell translocation gene), thereby regulating the cell cycle and cell proliferation (Sasajima et al, 2012). The centriolar protein Plk4 (Polo-like kinase 4) plays a role in cell cycle via regulation of centriole biogenesis and maintaining constant centriole number.…”

Recent advances in SCF ubiquitin ligase complex: Clinical implications

“…Since many substrates of β-TRCP have been identified to play a critical role in cell cycle, apoptosis, and migration, dysregulated β-TRCP is involved in tumorigenesis. For example, some cell cycle regulators including Emi1[12], Cdc25A [13, 14], Wee1A [15], cyclin D1 [16], and BTG [17] are the substrates of β-TRCP. REST is degraded by means of β-TRCP during the G2 phase of the cell cycle to allow transcriptional derepression of Mad2, which is an essential component of the spindle assembly checkpoint [18].…”