Dataset Information

Opposing Intermolecular Tuning of Ca2+ Affinity for Calmodulin by Neurogranin and CaMKII Peptides.

ABSTRACT: We investigated the impact of bound calmodulin (CaM)-target compound structure on the affinity of calcium (Ca2+) by integrating coarse-grained models and all-atomistic simulations with nonequilibrium physics. We focused on binding between CaM and two specific targets, Ca2+/CaM-dependent protein kinase II (CaMKII) and neurogranin (Ng), as they both regulate CaM-dependent Ca2+ signaling pathways in neurons. It was shown experimentally that Ca2+/CaM (holoCaM) binds to the CaMKII peptide with overwhelmingly higher affinity than Ca2+-free CaM (apoCaM); the binding of CaMKII peptide to CaM in return increases the Ca2+ affinity for CaM. However, this reciprocal relation was not observed in the Ng peptide (Ng13-49), 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-Ng13-49 is unknown due to low binding affinity, therefore we computationally generated an ensemble of apoCaM-Ng13-49 structures by matching the changes in the chemical shifts of CaM upon Ng13-49 binding from nuclear magnetic resonance experiments. Next, we computed the changes in Ca2+ affinity for CaM with and without binding targets in atomistic models using Jarzynski's equality. We discovered the molecular underpinnings of lowered affinity of Ca2+ for CaM in the presence of Ng13-49 by showing that the N-terminal acidic region of Ng peptide pries open the ?-sheet structure between the Ca2+ binding loops particularly at C-domain of CaM, enabling Ca2+ release. In contrast, CaMKII peptide increases Ca2+ affinity for the C-domain of CaM by stabilizing the two Ca2+ binding loops. We speculate that the distinctive structural difference in the bound complexes of apoCaM-Ng13-49 and holoCaM-CaMKII delineates the importance of CaM's progressive mechanism of target binding on its Ca2+ binding affinities.

SUBMITTER: Zhang P

PROVIDER: S-EPMC5374985 | biostudies-literature | 2017 Mar

REPOSITORIES: biostudies-literature

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Opposing Intermolecular Tuning of Ca2+ Affinity for Calmodulin by Neurogranin and CaMKII Peptides.

Zhang Pengzhi P Tripathi Swarnendu S Trinh Hoa H Cheung Margaret S MS

Biophysical journal 20170301 6

We investigated the impact of bound calmodulin (CaM)-target compound structure on the affinity of calcium (Ca2+) by integrating coarse-grained models and all-atomistic simulations with nonequilibrium physics. We focused on binding between CaM and two specific targets, Ca2+/CaM-dependent protein kinase II (CaMKII) and neurogranin (Ng), as they both regulate CaM-dependent Ca2+ signaling pathways in neurons. It was shown experimentally that Ca2+/CaM (holo ...[more]

PMID: 28355539

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Project description:Calmodulin-dependent kinase II (CaMKII) has long been known to play an important role in learning and memory as well as long term potentiation (LTP). More recently it has been suggested that it might be involved in the time averaging of synaptic signals, which can then lead to the high precision of information stored at a single synapse. However, the role of the scaffolding molecule, neurogranin (Ng), in governing the dynamics of CaMKII is not yet fully understood. In this work, we adopt a rule-based modeling approach through the Monte Carlo method to study the effect of Ca2+ signals on the dynamics of CaMKII phosphorylation in the postsynaptic density (PSD). Calcium surges are observed in synaptic spines during an EPSP and back-propagating action potential due to the opening of NMDA receptors and voltage dependent calcium channels. Using agent-based models, we computationally investigate the dynamics of phosphorylation of CaMKII monomers and dodecameric holoenzymes. The scaffolding molecule, Ng, when present in significant concentration, limits the availability of free calmodulin (CaM), the protein which activates CaMKII in the presence of calcium. We show that Ng plays an important modulatory role in CaMKII phosphorylation following a surge of high calcium concentration. We find a non-intuitive dependence of this effect on CaM concentration that results from the different affinities of CaM for CaMKII depending on the number of calcium ions bound to the former. It has been shown previously that in the absence of phosphatase, CaMKII monomers integrate over Ca2+ signals of certain frequencies through autophosphorylation (Pepke et al, Plos Comp. Bio., 2010). We also study the effect of multiple calcium spikes on CaMKII holoenzyme autophosphorylation, and show that in the presence of phosphatase, CaMKII behaves as a leaky integrator of calcium signals, a result that has been recently observed in vivo. Our models predict that the parameters of this leaky integrator are finely tuned through the interactions of Ng, CaM, CaMKII, and PP1, providing a mechanism to precisely control the sensitivity of synapses to calcium signals. Author Summary not valid for PLOS ONE submissions.

Project description:Ca(2+) channels and calmodulin (CaM) are two prominent signalling hubs that synergistically affect functions as diverse as cardiac excitability, synaptic plasticity and gene transcription. It is therefore fitting that these hubs are in some sense coordinated, as the opening of Ca(V)1-2 Ca(2+) channels are regulated by a single CaM constitutively complexed with channels. The Ca(2+)-free form of CaM (apoCaM) is already pre-associated with the isoleucine-glutamine (IQ) domain on the channel carboxy terminus, and subsequent Ca(2+) binding to this 'resident' CaM drives conformational changes that then trigger regulation of channel opening. Another potential avenue for channel-CaM coordination could arise from the absence of Ca(2+) regulation in channels lacking a pre-associated CaM. Natural fluctuations in CaM concentrations might then influence the fraction of regulable channels and, thereby, the overall strength of Ca(2+) feedback. However, the prevailing view has been that the ultrastrong affinity of channels for apoCaM ensures their saturation with CaM, yielding a significant form of concentration independence between Ca(2+) channels and CaM. Here we show that significant exceptions to this autonomy exist, by combining electrophysiology (to characterize channel regulation) with optical fluorescence resonance energy transfer (FRET) sensor determination of free-apoCaM concentration in live cells. This approach translates quantitative CaM biochemistry from the traditional test-tube context into the realm of functioning holochannels within intact cells. From this perspective, we find that long splice forms of Ca(V)1.3 and Ca(V)1.4 channels include a distal carboxy tail that resembles an enzyme competitive inhibitor that retunes channel affinity for apoCaM such that natural CaM variations affect the strength of Ca(2+) feedback modulation. Given the ubiquity of these channels, the connection between ambient CaM levels and Ca(2+) entry through channels is broadly significant for Ca(2+) homeostasis. Strategies such as ours promise key advances for the in situ analysis of signalling molecules resistant to in vitro reconstitution, such as Ca(2+) channels.

Dataset Information

Opposing Intermolecular Tuning of Ca2+ Affinity for Calmodulin by Neurogranin and CaMKII Peptides.

Publications

Opposing Intermolecular Tuning of Ca<sup>2+</sup> Affinity for Calmodulin by Neurogranin and CaMKII Peptides.

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