Protein tyrosine phosphatases regulate a myriad of essential subcellular signaling events, yet they remain difficult to study in their native biophysical context. Here we develop a minimally disruptive optical approach to toggle the activity of protein tyrosine phosphatase 1B (PTP1B)-an important regulator of receptor tyrosine kinases and a therapeutic target for the treatment of diabetes, obesity, and cancer-and we use that approach to probe both the structure and intracellular function of this enzyme. Our conservative architecture for photocontrol, which consists of a protein-based light switch fused to an allosteric regulatory element, preserves the native structure, activity, and subcellular localization of PTP1B, affords changes in activity that match those elicited by post-translational modifications inside the cell, and permits experimental analyses of the molecular basis of optical modulation. This work provides a framework for using optogenetic systems to examine both the biophysical basis and spatial context of cell signaling. 3 The enzymatic phosphorylation of tyrosine residues is centrally important to cellular function. It controls the location and timing of cellular differentiation, movement, proliferation, and death 1-4 ; its misregulation can cause cancer, diabetes, and neurodegenerative diseases, among other disorders 5-7 . Methods to toggle the activity of phosphorylation-regulating enzymes without interfering with their native structure or cellular organization could, thus, enable detailed analyses of the mechanisms by which cells process essential chemical signals 8,9 .Optogenetic actuators-genetically encoded proteins that undergo light-induced changes in conformation-provide a powerful means of controlling enzyme activity over time and space.As protein fusion partners, they have enabled optical manipulation of biomolecular transport, binding, and catalysis with millisecond and submicron resolution 10,11 . Common strategies to integrate optogenetic actuators into enzymes include (i) attachment near an active site, where they control substrate access 12,13 , (ii) insertion within a catalytic domain, where they afford activity-modulating structural distortions 14 , and (iii) fusion to N-or C-termini, where they direct subcellular localization 15 or guide domain assembly 16 . These approaches have generated powerful tools for stimulating phosphorylation-mediated signaling networks; their reliance on disruptive structural modifications, however, has precluded their use in both (i) biophysical analyses of native intra-domain control systems (allosteric networks) and (ii) biochemical studies of native regulatory effects-that is, changes in activity that match, rather than artificially exceed, those caused by post-translational modifications of an enzyme under study.Protein tyrosine phosphatase 1B (PTP1B) is an important regulatory enzyme for which minimally disruptive architectures for photocontrol could prove particularly informative. This enzyme catalyzes the hydrolytic dephosphorylation of tyros...