Protein kinases phosphorylate substrates in the context of specific phosphorylation site sequence motifs. The knowledge of the specific sequences that are recognized by kinases is useful for mapping sites of phosphorylation in protein substrates and facilitates the generation of model substrates to monitor kinase activity. Here, we have adapted a positional scanning peptide library method to a microarray format that is suitable for the rapid determination of phosphorylation site motifs for tyrosine kinases. Peptide mixtures were immobilized on glass slides through a layer of a tyrosine-free Y33F mutant avidin to facilitate the analysis of phosphorylation by radiolabel assay. A microarray analysis provided qualitatively similar results in comparison with the solution phase peptide library “macroarray” method. However, much smaller quantities of kinases were required to phosphorylate peptides on the microarrays, which thus enabled a proteome scale analysis of kinase specificity. We illustrated this capability by microarray profiling more than 80% of the human nonreceptor tyrosine kinases (NRTKs). Microarray results were used to generate a universal NRTK substrate set of 11 consensus peptides for in vitro kinase assays. Several substrates were highly specific for their cognate kinases, which should facilitate their incorporation into kinase-selective biosensors.

Mixed Lineage Leukemia 1 (MLL1) protein is a member of the SET1 (or MLL) family of histone methyltransferases. In humans, this family consists of six members: MLL1-4, SETd1A, and SETd1B (1-8). The SET1 family catalyzes methylation of histone 3 lysine 4 (H3K4), 3 an epigenetic mark that is associated with active transcription (9 -12). The human SET1 family is composed of large proteins with several well characterized functional domains involved in chromatin binding and protein-protein interactions (13, 14) (Fig. 1A). Although some of these domains differ among family members, all share a C-terminal SET (suppressor of variegation, enhancer of Zeste, trithorax) domain that confers H3K4 methyltransferase activity (15). Like many chromatin-modifying enzymes, the SET1 family works as part of multiprotein complexes that contain binding partners involved in enzymatic regulation and gene targeting. Although the majority of isolated SET1 family SET domains catalyze weak H3K4 monomethylation (H3K4me1), enhanced methylation is observed in the context of a "core complex" (16). The minimal core complex required for enhanced methylation is composed of the SET1/MLL SET domain and a subcomplex called WRAD (WD-40 repeat protein 5 (WDR5), retinoblastoma-binding protein 5 (RbBP5), absent small homeotic 2-like (ASH2L), and dumpy-30 (DPY-30)) (17)(18)(19)(20). Interestingly, SET1 family core complexes preferentially catalyze different levels of H3K4 methylation in a manner that correlates with their evolutionary lineage (16). Whereas SETd1A/B core complexes catalyze mono-, di-, and trimethylation of H3K4 (H3K4me1, H3K4me2, and H3K4me3, respectively), the MLL1 and MLL4 (also known as MLL2) core complexes predominantly catalyze mono-and dimethylation (16). In contrast, MLL2 and MLL3 core complexes catalyze predominantly H3K4 monomethylation (16). In cells, different levels of H3K4 methylation are localized to distinct genomic regions and are associated with distinct functional outcomes (21-23). Assembly of the MLL1 core complex requires a direct interaction between MLL1 and WDR5, whereby WDR5 acts to stabilize the interaction between the MLL1 SET domain and the RbBP5/ASH2L heterodimer (18,24). The MLL1-WDR5 interaction occurs via the conserved Win (WDR5 interaction) * This work was supported, in whole or in part, by the National Institutes of Health Grant R01CA140522 (to M. S. C.). The authors declare that they have no conflicts of interest with the contents of this article. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

The Janus kinases (or Jak kinases) mediate cytokine and growth factor signal transduction. Acquired or inherited Jak mutations can result in dysregulation of Jak-mediated signal transduction and can be critical to disease acquisition in neoplasias including acute myeloid, acute lymphoblastic and acute megakaryoblastic leukemias, and in rare X-linked severe combined immunodeficiency. The discovery of an acquired Jak2 point mutation, V617F, in significant numbers of patients with classical myeloproliferative disorders has increased the interest in development of Jak2-specific tyrosine kinase inhibitors and consequently there are now over 20 publically available structures of Jak kinase domains that describe all four family members, Jak1, Jak2, Jak3, and Tyk2. Here we review the recent advances in understanding the druggable structure and function of the Jak family, with a focus on the structural biology of the Jak kinase domain. We will discuss how these advances impact the development of Jak-targeted therapeutics.

Polybromo‐1 ( PBRM 1) is an important tumor suppressor in kidney cancer. It contains six tandem bromodomains ( BD s), which are specialized structures that recognize acetyl‐lysine residues. While BD 2 has been found to bind acetylated histone H3 lysine 14 (H3K14ac), it is not known whether other BD s collaborate with BD 2 to generate strong binding to H3K14ac, and the importance of H3K14ac recognition for the molecular and tumor suppressor function of PBRM 1 is also unknown. We discovered that full‐length PBRM 1, but not its individual BD s, strongly binds H3K14ac. BD s 2, 4, and 5 were found to collaborate to facilitate strong binding to H3K14ac. Quantitative measurement of the interactions between purified BD proteins and H3K14ac or nonacetylated peptides confirmed the tight and specific association of the former. Interestingly, while the structural integrity of BD 4 was found to be required for H3K14ac recognition, the conserved acetyl‐lysine binding site of BD 4 was not. Furthermore, simultaneous point mutations in BD s 2, 4, and 5 prevented recognition of H3K14ac, altered promoter binding and gene expression, and caused PBRM 1 to relocalize to the cytoplasm. In contrast, tumor‐derived point mutations in BD 2 alone lowered PBRM 1's affinity to H3K14ac and also disrupted promoter binding and gene expression without altering cellular localization. Finally, overexpression of PBRM 1 variants containing point mutations in BD s 2, 4, and 5 or BD 2 alone failed to suppress tumor growth in a xenograft model. Taken together, our study demonstrates that BD s 2, 4, and 5 of PBRM 1 collaborate to generate high affinity to H3K14ac and tether PBRM 1 to chromatin. Mutations in BD 2 alone weaken these interactions, and this is sufficient to abolish its molecular and tumor suppressor functions.