Characterization of an Engineered Src Kinase to Study Src Signaling and Biology

Protein arginine N-methyltransferase activity determination with filter binding and phosphor screening (FBAPS) assay

We developed a cost-effective assay to measure protein arginine N-methyltransferase (PRMT) activity in a medium-throughput manner by combining P81 filter binding and phosphor screening (FBAPS). Recombinantly-expressed PRMT1 and coactivator-associated arginine methyltransferase 1 (CARM1) were used to develop the FBAPS assay using GST fusions of glycine- and arginine-rich (GAR) protein and polyA binding protein 1 (PABP1(437-488)) as substrates, respectively, and radiolabelled S-adenosyl-L-[methyl-¹⁴C]-methionine as cofactor. Methylation reactions were spotted onto P81 filter paper in a dot blot apparatus and radioactive signals were measured both by phosphor imaging and liquid scintillation counting. Kinetic parameters (K_M, k_cat) for enzymes and substrates were determined, and IC₅₀ values were obtained for well-characterized inhibitors. FBAPS yielded kinetic parameters with no statistically significant difference to what was obtained using liquid scintillation counting. The IC₅₀ values obtained by the FBAPS assay for PRMT1 and CARM1 were comparable to values reported in literature. The FBAPS assay is a modification to the P81 filter binding assay with a dot blot apparatus that allows for processing of samples in a multi-well format, moderately increasing throughput. Signal detection by phosphor imaging offers an affordable and quantitative method that can be used to screen several inhibitors simultaneously against PRMT enzymes with high accuracy.

Engineered Allosteric Regulation of Protein Function

Allosteric regulation of proteins has been utilized to study various aspects of cell signaling, from unicellular events to organism-wide phenotypes. However, traditional methods of allosteric regulation, such as constitutively active mutants and inhibitors, lack tight spatiotemporal control. This often leads to unintended signaling consequences that interfere with data interpretation. To overcome these obstacles, researchers employed protein engineering approaches that enable tight control of protein function through allosteric mechanisms. These methods provide high specificity as well as spatial and temporal precision in regulation of protein activity in vitro and in vivo. In this review, we focus on the recent advancements in engineered allosteric regulation and discuss the various bioengineered allosteric techniques available now, from chimeric GPCRs to chemogenetic and optogenetic switches. We highlight the benefits and pitfalls of each of these techniques as well as areas in which future improvements can be made. Additionally, we provide a brief discussion on implementation of engineered allosteric regulation approaches, demonstrating that these tools can shed light on elusive biological events and have the potential to be utilized in precision medicine.

Control of SRC molecular dynamics encodes distinct cytoskeletal responses by specifying signaling pathway usage

Upon activation by different transmembrane receptors, the same signaling protein can induce distinct cellular responses. A way to decipher the mechanisms of such pleiotropic signaling activity is to directly manipulate the decision-making activity that supports the selection between distinct cellular responses. We developed an optogenetic probe (optoSRC) to control SRC signaling, an example of a pleiotropic signaling node, and we demonstrated its ability to generate different acto-adhesive structures (lamellipodia or invadosomes) upon distinct spatio-temporal control of SRC kinase activity. The occurrence of each acto-adhesive structure was simply dictated by the dynamics of optoSRC nanoclusters in adhesive sites, which were dependent on the SH3 and Unique domains of the protein. The different decision-making events regulated by optoSRC dynamics induced distinct downstream signaling pathways, which we characterized using time-resolved proteomic and network analyses. Collectively, by manipulating the molecular mobility of SRC kinase activity, these experiments reveal the pleiotropy-encoding mechanism of SRC signaling.

Src family kinase expression and subcellular localization in macrophages: implications for their role in CSF-1-induced macrophage migration

A major role of colony-stimulating factor-1 is to stimulate the differentiation of mononuclear phagocytic lineage cells into adherent, motile, mature macrophages. The colony-stimulating factor-1 receptor transduces colony-stimulating factor-1 signaling, and we have shown previously that phosphatidylinositol 3-kinase p110δ is a critical mediator of colony-stimulating factor-1-stimulated motility through the colony-stimulating factor-1 receptor pY721 motif. Src family kinases are also implicated in the regulation of macrophage motility and in colony-stimulating factor-1 receptor signaling, although functional redundancy of the multiple SFKs expressed in macrophages makes it challenging to delineate their specific functions. We report a comprehensive analysis of individual Src family kinase expression in macrophage cell lines and primary macrophages and demonstrate colony-stimulating factor-1-induced changes in Src family kinase subcellular localization, which provides clues to their distinct and redundant functions in macrophages. Moreover, expression of individual Src family kinases is both species specific and dependent on colony-stimulating factor-1-induced macrophage differentiation. Hck associated with the activated colony-stimulating factor-1 receptor, whereas Lyn associated with the receptor in a constitutive manner. Consistent with this, inhibitor studies revealed that Src family kinases were important for both colony-stimulating factor-1 receptor activation and colony-stimulating factor-1-induced macrophage spreading, motility, and invasion. Distinct colony-stimulating factor-1-induced changes in the subcellular localization of individual SFKs suggest specific roles for these Src family kinases in the macrophage response to colony-stimulating factor-1.