Label Transfer Reagents for the Investigation of Protein Kinase Complexes

Abstract

Protein kinases are essential enzymes for cellular signaling, and are often regulated by participation in protein complexes. The mitogen-activated protein kinase (MAPK) p38 is involved in multiple pathways, and its regulation depends on its interactions with other signaling proteins. However, the weak and transient nature of these interactions makes the identification of p38 interacting proteins challenging. For this reason, we have developed label transfer reagents (LTRs) which allow labeling of p38 signaling complexes. These LTRs leverage the potency and selectivity of known p38 inhibitors to place a photo-crosslinker and tag in the vicinity of p38 and its binding partners. Upon UV irradiation, proteins that are in close proximity to p38 are covalently crosslinked, and labeled proteins are detected and/or purified through an orthogonal chemical handle. Here we demonstrate that p38-selective LTRs efficiently label a diversity of p38 binding partners, including substrates and activators. Furthermore, these LTRs can be used in immunoprecipitations for study of proteins not exogenously expressible. Several limitations of LTRs are also explored. Finally, copper-catalyzed click chemistry is optimized in a quantitative fashion for the labeling and purification of alkynylated proteins.