Cyclin F-Mediated Degradation of SLBP Limits H2A.X Accumulation and Apoptosis upon Genotoxic Stress in G2

Abstract

Summary SLBP (stem-loop binding protein) is a highly conserved factor necessary for the processing, translation, and degradation of H2AFX and canonical histone mRNAs. We identified the F-box protein cyclin F, a substrate recognition subunit of an SCF (Skp1-Cul1-F-box protein) complex, as the G2 ubiquitin ligase for SLBP. SLBP interacts with cyclin F via an atypical CY motif, and mutation of this motif prevents SLBP degradation in G2. Expression of an SLBP stable mutant results in increased loading of H2AFX mRNA onto polyribosomes, resulting in increased expression of H2A.X (encoded by H2AFX). Upon genotoxic stress in G2, high levels of H2A.X lead to persistent γH2A.X signaling, high levels of H2A.X phosphorylated on Tyr142, high levels of p53, and induction of apoptosis. We propose that cyclin F co-evolved with the appearance of stem-loops in vertebrate H2AFX mRNA to mediate SLBP degradation, thereby limiting H2A.X synthesis and cell death upon genotoxic stress.

Guard the guardian: A CRL4 ligase stands watch over histone production

“…Notably, proteasomal degradation of SLBP in G2 is restricted to vertebrates and evolutionary correlates with the occurrence of SCF cyclin F complexes. 10 As mentioned earlier, SLBP degradation in G2 was also attributed to the activity of CRL4 WDR23 .…”

“FEM1”nism controls SLBP stability during cell cycle

“…Hence, these elegant studies from the Pagano group together revealed a synergy between CRL2 FEM1A/B/C and SCF Cyclin F to orchestrate the timely degradation of SLBP during the G1 and G2 phases, 6,7 respectively, thereby synchronizing histone and nucleosome metabolism with cell cycle progression (Fig. 1).…”

“…At the end of S phase, SLBP is phosphorylated at Thr-61 and Thr-62 by CK2 and cyclin A2-CDK1, 4,5 respectively, facilitating its degradation by SCF Cyclin F in the following G2 phase, which is largely dependent on an atypical RxL motif present in SLBP. 6 Interestingly, there is evidence that SLBP may also be degraded during the G1 phase when Cyclin F is not abundantly expressed. However, the E3 ligase responsible for its decay in G1 phase remained elusive until this report.…”

“…7 In a recent paper published in Cell Cycle, Dankert and colleagues report that SLBP is degraded by CRL2 FEM1A/B/C during the G1 phase, 7 in addition to its controlled degradation by SCF Cyclin F in G2 phase. 6 Through a series of rigorous mutagenesis analysis, the authors identified that FEM1A, FEM1B, and FEM1C bound to distinct motifs within the N-terminus of SLBP. As a result, depletion of individual FEM1A, FEM1B, or FEM1C led a modest effect, while depleting all 3 FEM1 isoforms led to a more dramatic accumulation, of SLBP protein abundance during the G1 phase with minimal effects to SLBP abundance during the G2 phase, where SCF Cyclin F is largely controls its stability.…”

“…Indeed, a stable SLBP mutant that is insensitive to Cyclin F-mediated degradation led to constant translation of H2A.X throughout the cell cycle, subsequently conferring increased cellular sensitivity to apoptosis upon DNA damage. 6 However, it warrants further in-depth studies including generation of conditional knockout (KO) and knockin (KI) mice to explore to what extent the aberrancy in histone processing due to elevated SLBP protein abundance contributes to human pathological conditions such as sex determining or polycystic ovary defects associated with FEM1 deficiency.…”