Metabolism Without Modification: Obesity-associated metabolic disease has rapidly become a public health priority in the developed world and is being addressed through prevention strategies aimed at lifestyle changes and through pharmacological approaches. Barnett et al. (p. 1689 , published online 18 November) designed a drug that inhibits the action of ghrelin, a circulating peptide hormone that increases fat mass and food intake. The drug, a bisubstrate analog called GO-CoA-Tat, is a selective antagonist of ghrelin O-acyltransferase (GOAT), an enzyme that catalyzes a posttranslational modification that is essential for ghrelin activity. Injection of GO-CoA-Tat into wild-type mice on a high-fat diet improved glucose tolerance and reduced weight gain, probably through changes in metabolic activity. Because GO-CoA-Tat is a peptide-based drug that requires repeated injection, it is unsuitable for clinical use, but GOAT does represent a potentially valuable target for future drug development efforts in metabolic disease.

The explosion of scientific interest in protein kinase-mediated signaling networks has led to the infusion of new chemical methods and their applications related to the analysis of phosphorylation pathways. We highlight some of these chemical biology approaches across three areas. First, we discuss the development of chemical tools to modulate the activity of protein kinases to explore kinase mechanisms and their contributions to phosphorylation events and cellular processes. Second, we describe chemical techniques developed in the past few years to dissect the structural and functional effects of phosphate modifications at specific sites in proteins. Third, we cover newly developed molecular imaging approaches to elucidate the spatiotemporal aspects of phosphorylation cascades in live cells. Exciting advances in our understanding of protein phosphorylation have been obtained with these chemical biology approaches, but continuing opportunities for technological innovation remain.

Semisynthetic constructs of protein kinase CK2 incorporating nonhydrolyzable post-translational modifications now demonstrate the interplay between phosphorylation and glycosylation in regulating CK2 stability, controlling substrate specificity and modifying interactions with regulatory proteins. Protein serine-threonine kinase casein kinase II (CK2) is involved in a myriad of cellular processes including cell growth and proliferation through its phosphorylation of hundreds of substrates, yet how CK2 function is regulated is poorly understood. Here we report that the CK2 catalytic subunit CK2α is modified by O-linked β-N-acetyl-glucosamine (O-GlcNAc) on Ser347, proximal to a cyclin-dependent kinase phosphorylation site (Thr344). We use protein semisynthesis to show that phosphorylation of Thr344 increases the cellular stability of CK2α by strengthening its interaction with Pin1, whereas glycosylation of Ser347 seems to be antagonistic to Thr344 phosphorylation and permissive to proteasomal degradation. By performing kinase assays with site-specifically phospho- and glyco-modified CK2α in combination with CK2β and Pin1 binding partners on human protein microarrays, we show that the kinase substrate selectivity of CK2 is modulated by these specific post-translational modifications. This study suggests how a promiscuous protein kinase can be regulated at multiple levels to achieve particular biological outputs.