Editors' ChoicePosttranslational Modifications

Controlling glutamine metabolism by acetylation

Sci. Signal.  29 Mar 2016:
Vol. 9, Issue 421, pp. ec73
DOI: 10.1126/scisignal.aaf7581

Glutamine is not only an amino acid found in proteins but also a critical regulatory metabolite. Glutamine comes from the diet and from cellular biosynthesis through the action of the enzyme glutamine synthetase (GS), which is inhibited when glutamine is abundant. Van Nguyen et al. identified a molecular mechanism for this inhibition—acetylation-mediated degradation. A screen for proteins that showed altered stability in cells exposed to thalidomide, which targets and inhibits the E3 ubiquitin ligase adaptor protein cereblon (CRBN), identified GS as stabilized by thalidomide. CRISPR/Cas9-mediated knockout of CRBN in cultured cells confirmed that GS abundance and its glutamine-induced reduction involved CRBN, and in vitro ubiquitylation assays confirmed that CRBN mediated GS ubiquitylation in the presence of the appropriate E3 ligase components. In Crbn-knockout mice, the glutamine-induced reduction in GS abundance was compromised in kidney, lung, and skeletal muscle—but not in brain or liver—indicating that there are multiple mechanisms controlling GS activity. Mutational analysis and mass spectrometry analysis identified lysines 11 and 14 in a KxxK motif in the N-terminal region of GS as acetylated, and showed that acetylation was necessary for CRBN-mediated ubiquitylation and degradation of GS. Inhibition of histone acetylase activity blocked GS acetylation, whereas inhibition of histone deacetylase activity enhanced glutamine-induced GS degradation. Peptides acetylated at both lysines bound to recombinant tagged CRBN and showed that both lysines had to be acetylated (or charge neutralized by mutation to alanine) for binding. Thus, this study identifies a role for acetylation in the formation of a degron and suggests that acetyldegrons may participate in substrate recognition for other proteins targeted by CRBN (see Koirala and Potts).

This study also may explain the mechanism of action of immunomodulatory drugs related to thalidomide that are used for multiple myeloma. Klysz et al. showed that the abundance of glutamine was critical to the fate decision made by T cells. When glutamine was limited, naïve CD4+ T cells differentiated into Treg cells, which suppress immune responses, even in the presence of cytokines that promote effector T cell differentiation. Multiple myeloma occurs when B cells proliferate uncontrollably. Thus, drugs that disrupt glutamine metabolism, such as those that inhibit CRBN, may alter glutamine metabolism that limits the division of these cells or that alters the production of a regulatory immune cell that limits these cells. How glutamine metabolism regulates B cells and their precursors is an area for investigation.

T. Van Nguyen, J. E. Lee, M. J. Sweredoski, S.-J. Yang, S.-J. Jeon, J. S. Harrison, J.-H. Yim, S. G. Lee, H. Handa, B. Kuhlman, J.-S. Jeong, J. M. Reitsma, C.-S. Park, S. Hess, R. J. Deshaies, Glutamine triggers acetylation-dependent degradation of glutamine synthetase via the thalidomide receptor cereblon. Mol. Cell 61, 809­–820 (2016). [PubMed]

S. Koirala, P. R. Potts, An acetyldegron triggers CRBN to take down the “Q.” Mol. Cell 61, 795–796 (2016). [PubMed]

D. Klysz, X. Tai, P. A. Robert, M. Craveiro, G. Cretenet, L. Oburoglu, C. Mongellaz, S. Floess, V. Fritz, M. I. Matias, C. Yong, N. Surh, J. C. Marie, J. Huehn, V. Zimmermann, S. Kinet, V. Dardalhon, N. Taylor, Glutamine-dependent α-ketoglutarate production regulates the balance between T helper 1 cell and regulatory T cell generation. Sci. Signal. 8, ra97 (2015). [Abstract]