Highlights of Selected Talks Related to Posttranslational Modifications in Cell Signaling
6 February 2012
Nancy R. Gough
Tony Hunter (Salk Institute for Biological Studies) provided an overview of protein phosphorylation and other posttranslational modifications that are important for mediating signal transduction and cellular regulation. He discussed the effect of cancer- associated mutations in kinases and how to decide whether these are good candidates for development of inhibitors. It turns out that many are tumor suppressors and, therefore, not good candidates for the development of inhibitors for oncogenic therapies.
Another theme of his talk was the chemistry of phosphorylation and why this posttranslational modification is particularly suitable for biological systems. He noted that it is important to recognize that phosphomimetic mutations in proteins may not always truly mimic phosphorylated residues due to major differences in the chemistry. He also reminded the audience that kinases can work in reverse and that it is the high ATP/ADP ratio in biological systems that drives the reaction in the direction of protein phoshorylation and not ATP generation through dephosphorylation of the protein.
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A. Moritz, Y. Li, A. Guo, J. Villén, Y. Wang, J. MacNeill, J. Kornhauser, K. Sprott, J. Zhou, A. Possemato, J. M. Ren, P. Hornbeck, L. C. Cantley, S. P. Gygi, J. Rush, M. J. Comb, Akt-RSK-S6 Kinase Signaling Networks Activated by Oncogenic Receptor Tyrosine Kinases. Sci. Signal. 3, ra64 (2010). [Abstract] [Full Text]
Gerald Hart (Johns Hopkins Medical School) described the posttranslational modification of O-GlcAcylation and the crosstalk between this modification and phosphorylation. O- GlcNAcylation is a reversible modification mediated by O- GlcNAc transferase (OGT), which adds this modification to target proteins, and glycanase, which removes it. This area of biology is relatively understudied because of the technical challenges associated with detecting this modification; thus, relatively little is known about the regulation of the enzymes that control this modification, and few large-scale studies of the occurrence of this modification and its biological consequences have been done. It is likely that the dynamic turnover of this modification is more important than the absolute stoichometry of the modification.
O-GlcNAcylation is regulated by nutrient status, and the O-GlcAcylation of proteins is often reciprocal with phosphorylation. For example, in RNA polymerase II, there are initially many O-GlcNAc groups added to the C-terminal domain, and these have to be removed and then replaced with phosphate groups for the polymerase to mediate transcription. Histones are also O-GlcNAcylated, and at least one modification is located where the histone contacts DNA, begging the question of whether this could be another part of the histone code. A dynamic interplay between O-GlcNAcylation and phosphorylation occurs during the cell cycle. O-GlcNAcylation of kinases can change their specificity or kinetics as compared to the phosphorylated form of the kinase.
K. Sakabe, Z. Wang, G. W. Hart, β-N-acetylglucosamine (O-GlcNAc) is part of the histone code. Proc. Natl. Acad. Sci. U.S.A. 107, 19915-19920 (2010). [PubMed]
N. E. Zachara, K. Vosseller, G. W. Hart, Detection and analysis of proteins modified by O-linked N- acetylglucosamine. Current Protocols Protein Sci. UNIT 12.8.1-12.8.33 (2011). [PubMed]
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Z. Wang, N. D. Udeshi, C. Slawson, P. D. Compton, K. Sakabe, W. D. Cheung, J. Shabanowitz, D. F. Hunt, G. W. Hart, Extensive Crosstalk Between O-GlcNAcylation and Phosphorylation Regulates Cytokinesis. Sci. Signal. 3, ra2 (2010). [Abstract] [Full Text]
Federico Mayor, Jr. (Universidad Autónoma de Madrid) presented results from his lab's investigation into the functions of G protein-coupled receptor (GPCR) kinase 2 (GRK2), especially on those functions that are not related to desensitization of GPCRs. He described how GRK2 may contribute to epithelial cell migration by promoting histone deacetylase 6 (HDAC6)-mediated deactylation of tubulin; phosphorylation of HDAC6 by GRK2 promotes its deacetylase activity. The ability of GRK2 to target HDAC6 is regulated by extracellular signal regulated kinase (ERK)1/2 in response to epidermal growth factor (EGF). It appears that EGF signaling redirects GRK2 away from GPCRs and toward HDAC6, which enhances cell motility. However, the role of GRK2 in chemoattraction may be more complex, because GRK2 activity toward chemokine receptors initially decreases migration. Thus, open questions remain regarding whether there is a temporal aspect to GRK2 signaling or whether GRK2 is serving as an integrator of multiple signals.
GRK2 has also been implicated in vasculogenesis and angiogenesis. When GRK2 is reduced in mice (GRK2+/-), TGF-β signaling is shifted to favor that mediated by the TGF-β receptor ALK5. Thus, GRK2 appears to serve as a switch in the TGF-β cascade. GRK+/- and GRK2-/- mice show impaired angiogenesis during development, which may explain the embryonic lethality of the knockout animals and also the impaired angiogenesis of xenografted tumors.
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