Spatial organization and temporal dynamics are essential properties of cellular signaling. Post-translational modifications (PTMs) to proteins relay biological signals by exerting precise spatiotemporal control over protein function, localization, and lifetime. A systems-level experimental map of post-translationally modified proteins across subcellular space and time would transform our understanding of how PTMs orchestrate complex cellular functions in response to biological stimuli. However, such a map is inaccessible with current technologies.