Controlling the interaction between CaMKII and Calmodulin with a photocrosslinking unnatural amino acid.

Using unnatural amino acid mutagenesis, we made a mutant of CaMKII that forms a covalent linkage to Calmodulin upon illumination by UV light. Like wild-type CaMKII, the L308BzF mutant stoichiometrically binds to Calmodulin, in a calcium-dependent manner. Using this construct, we demonstrate that Calmodulin binding to CaMKII, even under these stochiometric conditions, does not perturb the CaMKII oligomeric state. Furthermore, we were able to achieve activation of CaMKII L308BzF by UV-induced binding of Calmodulin, which, once established, is further insensitive to calcium depletion. In addition to the canonical auto-inhibitory role of the regulatory segment, inter-subunit crosslinking in the absence of CaM indicates that kinase domains and regulatory segments are substantially mobile in basal conditions. Characterization of CaMKIIL308BzF in vitro, and its expression in mammalian cells, suggests it could be a promising candidate for control of CaMKII activity in mammalian cells with light.

CaMKII autophosphorylation can occur between holoenzymes without subunit exchange.

The dodecameric protein kinase CaMKII is expressed throughout the body. The alpha isoform is responsible for synaptic plasticity and participates in memory through its phosphorylation of synaptic proteins. Its elaborate subunit organization and propensity for autophosphorylation allow it to preserve neuronal plasticity across space and time. The prevailing hypothesis for the spread of CaMKII activity, involving shuffling of subunits between activated and naive holoenzymes, is broadly termed subunit exchange. In contrast to the expectations of previous work, we found little evidence for subunit exchange upon activation, and no effect of restraining subunits to their parent holoenzymes. Rather, mass photometry, crosslinking mass spectrometry, single molecule TIRF microscopy and biochemical assays identify inter-holoenzyme phosphorylation (IHP) as the mechanism for spreading phosphorylation. The transient, activity-dependent formation of groups of holoenzymes is well suited to the speed of neuronal activity. Our results place fundamental limits on the activation mechanism of this kinase.

A Membrane-Permeable and Immobilized Metal Affinity Chromatography (IMAC) Enrichable Cross-Linking Reagent to Advance In†Vivo Cross-Linking Mass Spectrometry.

Cross-linking mass spectrometry (XL-MS) is an attractive method for the proteome-wide characterization of protein structures and interactions. Currently, the depth of in vivo XL-MS studies is lagging behind the established applications to cell lysates, because cross-linking reagents that can penetrate intact cells and strategies to enrich cross-linked peptides lack efficiency. To tackle these limitations, we have developed a phosphonate-containing cross-linker, tBu-PhoX, that efficiently permeates various biological membranes and can be robustly enriched using routine immobilized metal ion affinity chromatography. We have established a tBu-PhoX-based in vivo XL-MS approach that enables cross-links in intact human cells to be identified in high numbers with substantially reduced analysis time. Collectively, the developed cross-linker and XL-MS approach pave the way for the comprehensive XL-MS characterization of living systems.