SUMMARY Cereblon (CRBN), a substrate receptor for the cullin–RING ubiquitin ligase 4 (CRL4) complex, is a direct protein target for thalidomide teratogenicity and antitumor activity of immunomodulatory drugs (IMiDs). Here we report that glutamine synthetase (GS) is an endogenous substrate of CRL4CRBN. Upon exposing cells to high glutamine concentration, GS is acetylated at lysines 11 and 14, yielding a degron that is necessary and sufficient for binding and ubiquitylation by CRL4CRBN and degradation by the proteasome. Binding of acetylated degron peptides to CRBN depends on an intact thalidomide-binding pocket but is not competitive with IMiDs. These findings reveal a feedback loop involving CRL4CRBN that adjusts GS protein levels in response to glutamine and uncover a new function for lysine acetylation.

SUMMARY Small ubiquitin-like modifier (SUMO) modification has emerged as an important regulatory mechanism during embryonic development. However, it is not known whether SUMOylation plays a role in the development of the immune system. Here, we show that SUMO-specific protease 1 (SENP1) is essential for the development of early T and B cells. STAT5, a key regulator of lymphoid development, is modified by SUMO-2 and is specifically regulated by SENP1. In the absence of SENP1, SUMO-2 modified STAT5 accumulates in early lymphoid precursors, resulting in a block in its acetylation and subsequent signaling. These results demonstrate a crucial role of SENP1 in the regulation of STAT5 activation during early lymphoid development.

Edited by Ashok Venkitaraman and Wilhelm JustKeywords: SUMO SENP RPA Repair Replication a b s t r a c t To maintain genomic integrity, a cell must utilize multiple mechanisms to protect its DNA from the damage generated by environmental agents or DNA metabolism. SUMO (small ubiquitin-like modifier) can regulate protein stability, protein cellular location, and protein-protein interactions. In this review, we summarize the current understanding of the roles of SUMOylation and de-SUMOylation in DNA damage response (DDR) and DNA repair with a specific focus on the role of RPA SUMOylation in homologous recombination (HR).

Glutamine synthetase (GS) plays an essential role in metabolism by catalyzing the synthesis of glutamine from glutamate and ammonia. Our recent study showed that CRBN, a direct protein target for the teratogenic and antitumor activities of immunomodulatory drugs such as thalidomide, lenalidomide, and pomalidomide, recognizes an acetyl degron of GS, resulting in ubiquitylation and degradation of GS in response to glutamine. Here, we report that valosin-containing protein (VCP)/p97 promotes the degradation of ubiquitylated GS, resulting in its accumulation in cells with compromised p97 function. Notably, p97 is also required for the degradation of all four known CRBN neo-substrates [Ikaros family zinc finger proteins 1 (IKZF1) and 3 (IKZF3), casein kinase 1α (CK1α), and the translation termination factor GSPT1] whose ubiquitylation is induced by immunomodulatory drugs. Together, these data point to an unexpectedly intimate relationship between the E3 ubiquitin ligase CRL4 CRBN and p97 pathways.lutamine plays important roles in many cellular processes, including oxidative metabolism and ATP generation, biosynthesis of proteins, lipids, and nucleic acids, and cell growth and proliferation through the regulation of the mTOR signaling pathway, translation, and autophagy (1, 2). In mammals, it is the most abundant amino acid in plasma with a concentration of 0.5-0.9 mM (3), accounting for ∼20% of its free amino acid pool. Glutamine synthetase (GS) is the only enzyme that is capable of de novo synthesis of glutamine and also functions to detoxify glutamate and ammonia, depending on tissue localization. Skeletal muscles and lungs are major sites of glutamine synthesis, whereas cells of the gut and the immune system, such as lymphocytes and macrophages, consume large amounts of glutamine in plasma (4). GS protects neurons against excitotoxicity by converting glutamate into glutamine in brain, detoxifies ammonia in liver, and maintains physiologic pH in kidney (5). In an attempt to investigate the role of GS in development, He et al. generated GS-knockout mice and reported that GS is essential in early embryogenesis, because deletion of the murine GLUL gene causes lethality at the blastocyst stage (embryonic day 3.5) (6). Interestingly, mouse ES cells maintain pluripotency and proliferate when grown in the absence of exogenous glutamine (7). However, inhibition of GS with the small molecule methionine sulfoximine (MSO) is sufficient to block the proliferation of ES cells in glutamine-free medium (7). In humans, congenital systemic glutamine deficiency caused by homozygous GS mutations results in multiorgan failure and neonatal death (8).Recent studies highlight the importance of glutamine metabolism in metabolic reprogramming, because many tumor cells display "glutamine addiction" (9). Activation of oncogenes such as MYC, KRAS, and HIF1α and/or loss of tumor suppressor genes including p53 can directly mediate the reprogramming of glutamine metabolism by selectively activating their downstream signaling or metabolic pathway...