We investigated the impact of bound calmodulin (CaM)-target compound structure on the affinity of calcium (Ca) by integrating coarse-grained models and all-atomistic simulations with nonequilibrium physics. We focused on binding between CaM and two specific targets, Ca/CaM-dependent protein kinase II (CaMKII) and neurogranin (Ng), as they both regulate CaM-dependent Ca signaling pathways in neurons. It was shown experimentally that Ca/CaM (holoCaM) binds to the CaMKII peptide with overwhelmingly higher affinity than Ca-free CaM (apoCaM); the binding of CaMKII peptide to CaM in return increases the Ca affinity for CaM. However, this reciprocal relation was not observed in the Ng peptide (Ng), which binds to apoCaM or holoCaM with binding affinities of the same order of magnitude. Unlike the holoCaM-CaMKII peptide, whose structure can be determined by crystallography, the structural description of the apoCaM-Ng is unknown due to low binding affinity, therefore we computationally generated an ensemble of apoCaM-Ng structures by matching the changes in the chemical shifts of CaM upon Ng binding from nuclear magnetic resonance experiments. Next, we computed the changes in Ca affinity for CaM with and without binding targets in atomistic models using Jarzynski's equality. We discovered the molecular underpinnings of lowered affinity of Ca for CaM in the presence of Ng by showing that the N-terminal acidic region of Ng peptide pries open the β-sheet structure between the Ca binding loops particularly at C-domain of CaM, enabling Ca release. In contrast, CaMKII peptide increases Ca affinity for the C-domain of CaM by stabilizing the two Ca binding loops. We speculate that the distinctive structural difference in the bound complexes of apoCaM-Ng and holoCaM-CaMKII delineates the importance of CaM's progressive mechanism of target binding on its Ca binding affinities.

Reactively canceling movements is a vital feature of the motor system to ensure safety. This behavior can be studied in the laboratory using the stop signal task. There remains ambiguity about whether a point-of-no-return exists, after which a response cannot be aborted. A separate question concerns whether motor system inhibition associated with attempted stopping persists when stopping is unsuccessful. We address these two questions using electromyography (EMG) in two stop signal task experiments. Experiment 1 (n = 24) involved simple right and left index finger responses in separate task blocks. Experiment 2 (n = 28) involved a response choice between the right index and pinky fingers. To evaluate the approximate point-of-no-return, we measured EMG in responding fingers during the 100 ms preceding the stop signal and observed significantly greater EMG amplitudes during failed than successful stop trials in both experiments. Thus, EMG differentiated failed from successful stopping prior to the stop signal, regardless of whether there was a response choice. To address whether motor inhibition persists after failed stopping, we assessed EMG peak-to-offset durations and slopes (i.e., the rate of EMG decline) for go, failed stop, and successful stop (partial response EMG) trials. EMG peak-to-offset was shorter and steeper in failed stop trials compared to go and successful stop partial response EMG trials, suggesting motor inhibition persists even when failing to stop. These findings indicate EMG is sensitive to a point at which participants can no longer successfully stop an ongoing movement and suggest the peak-to-offset time of response-related EMG activity during failed stopping reflects stopping-related inhibition.

No abstract

No abstract

No abstract