Mapping Cellular Protein Lysine Methylation Using Targeted-Mass Spectrometry

Lysine methylation is a dynamic posttranslation modification (PTM) involved in the regulation of numerous biological processes. Depending on the context, site- and state-specific lysine methylation has been shown to regulate key features of proteins, such as stability, activity, and interactions. Assessing site- and state-specific lysine methylation within the proteome is difficult without access to antibodies that are able to detect the modification site. As the commercially available options for reliable antibodies that are specific to a methylation site and state are limited, mass spectrometry (MS)-based methods are more appealing as they enable detection of site- and state-specific methylation events with broad applicability. Here, we describe the use of immunoprecipitation and targeted-MS to detect and perform relative quantification of site-specific lysine methylation events occurring on endogenous proteins of interest. As an example, detection and relative quantification of hypoxia-responsive PGC1α-K224me2 and RNF20-K610me3/K614me3 lysine methylation, occurring in hypoxia-treated human colorectal cancer cells, are described.