Lck is a lymphocyte-specific tyrosine kinase involved in T cell activation, which is essential for the human immune response. Upon antigen engagement with the T Cell Receptor (TCR), Lck phosphorylates the CD3ζ chain of the TCR, transducing intercellular signaling that activates T cells. Recent studies have demonstrated that Lck’s phosphotransferase activity is not only important for T cell activation, but that Lck also plays a critical role in scaffolding the interaction between the phosphorylated CD3ζ chain of the TCR and the kinase ZAP70 using its regulatory domains. Lck’s phosphotransferase activity has been shown to be toxic to yeast, with increased activity correlating with decreased yeast-growth rates. Using a yeast growth-based deep mutational scan (DMS), we calculated the activity scores of ~5,000 single amino acid variants of Lck’s kinase domain. Through this DMS, we identified all positions on the kinase domain that are amenable to substitution without perturbing kinase activity. In particular, we focused on positions where we could install cysteine residues on the kinase domain without perturbing kinase activity. Currently, we are expressing these cysteine variants in primary T cells, and applying parallel chemoselective profiling methods to quantify changes in the electrophilic reactivity of the cysteine side chains upon TCR stimulation. The expected changes in alkylation of the cysteine side chains upon TCR stimulation will provide insight into changes in the conformational flexibility of Lck, accessibility of the substituted residue sites, and intramolecular protein-protein interactions (PPIs) of Lck upon TCR stimulation. Ultimately, this insight into the conformational dynamics of Lck can be applied to deepen our understanding of basic immunology and the T cell activation signaling cascade.

As tool compounds aimed at profiling drug leads, covalent small molecule probes targeting amino acid residues present great value to the field of chemical proteomics. Recent progress has expanded the search for these irreversible inhibitors to those targeting lysine residues on select kinases. These small molecule probes are typically composed of an aminophilic group that binds lysine, a scaffold that directs the inhibitor to the residue of interest, and a reporter group that facilitates visualization of binding on a gel. Here we present the organic synthesis of numerous novel small molecule inhibitors, as well as their live cell labeling behavior. The inhibitors of choice are designed with a variety of electrophilic groups that serve as aminophiles targeting lysine. Each inhibitor is built around either a Foretinib or Xo44 scaffold, and uses a transcyclooctene (TCO) click handle as the reporter group. The TCO handle rapidly binds tetrazine, allowing for the linking of a fluorophore to the inhibitor, which reports labeling of lysines through SDS-PAGE. An initial live cell labeling assay reveals clear binding activity in a foretinib-based inhibitor with a squarate electrophile, as well as minor bands in select sulfonyl-fluoride based inhibitors. Through mass spec (MS) proteomics, the protein targets of these promising inhibitors will be identified. Assuming MS reveals selective, high affinity binding, these small molecule probes aid the profiling of drug leads, and expand the range of targeted covalent inhibitors available for chemical proteomics.