Chemical shift assignments of calmodulin bound to the β-subunit of a retinal cyclic nucleotide-gated channel (CNGB1)

Chemical shift assignments of calmodulin bound to a cytosolic domain of GluN2A (residues 1004-1024) from the NMDA receptor

N-methyl-D-aspartate receptors (NMDARs) consist of glycine-binding GluN1 and glutamate-binding GluN2 subunits that form tetrameric ion channels. NMDARs in the neuronal post-synaptic membrane are important for controlling neuroplasticity and synaptic transmission in the brain. Calmodulin (CaM) binds to the cytosolic C0 domains of both GluN1 (residues 841-865) and GluN2 (residues 1004-1024) that may play a role in the Ca2+-dependent desensitization of NMDAR channels. Mutations that disrupt Ca2+-dependent desensitization of NMDARs are linked to Alzheimer's disease, depression, stroke, epilepsy, and schizophrenia. NMR chemical shift assignments are reported here for Ca2+-saturated CaM bound to the GluN2A C0 domain of NMDAR (BMRB no. 51821).

Chemical shift assignments of calmodulin bound to the GluN1 C0 domain (residues 841-865) of the NMDA receptor

Neuroplasticity and synaptic transmission in the brain are regulated by N-methyl-D-aspartate receptors (NMDARs) that consist of hetero-tetrameric combinations of the glycine-binding GluN1 and glutamate-binding GluN2 subunits. Calmodulin (CaM) binds to the cytosolic C0 domain of GluN1 (residues 841-865) that may play a role in the Ca2+-dependent inactivation (CDI) of NMDAR channel activity. Dysregulation of NMDARs are linked to various neurological disorders, including Alzheimer's disease, depression, stroke, epilepsy, and schizophrenia. Here, we report complete NMR chemical shift assignments of Ca2+-saturated CaM bound to the GluN1 C0 domain of the human NMDAR (BMRB no. 51715).

Chemical shift assignments of calmodulin under standard conditions at neutral pH

The Ca²⁺ sensor protein, calmodulin (CaM) is ubiquitously expressed in all cells where it binds to hundreds of different target proteins, including dozens of enzymes, receptors, ion channels and numerous Ca²⁺ transporters. The only published NMR chemical shift assignments for Ca²⁺-bound CaM (in the absence of a target) have been determined under acidic conditions: at pH 6.5/310 K (BMRB 6541) and pH 6.3/320 K (BMRB 547). However, some CaM/target complexes are not soluble under these conditions. Also, amide chemical shifts are very sensitive to pH and temperature, which can cause large baseline errors when using the existing chemical shift assignments of free CaM to calculate chemical shift perturbations caused by target binding at neutral pH and physiological temperature. We report complete NMR chemical shift assignments of Ca²⁺-saturated CaM under a set of standard conditions at neutral pH and 308 K that will enable more accurate chemical shift comparison between free CaM and CaM/target complexes (BMRB 51289).

Chemical shift assignments of calmodulin bound to a C-terminal site (residues 1120-1147) in the β-subunit of a retinal cyclic nucleotide-gated channel (CNGB1)

Retinal cyclic nucleotide-gated (CNG) channels consist of two protein subunits (CNGA1 and CNGB1). Calmodulin (CaM) binds to two separate sites within the cytosolic region of CNGB1: CaM binding to an N-terminal site (human CNGB1 residues 565-587, called CaM1) decreases the open probability of CNG channels at elevated Ca²⁺ levels in dark-adapted photoreceptors, whereas CaM binding to a separate C-terminal site (CNGB1 residues 1120-1147, called CaM2) may increase channel open probability in light activated photoreceptors. We recently reported NMR chemical shift assignments of Ca²⁺-saturated CaM bound to the CaM1 site of CNGB1 (BMRB no. 51222). Here, we report complete NMR chemical shift assignments of Ca²⁺-saturated CaM bound to the C-terminal CaM2 site of CNGB1 (BMRB no. 51447).

NMR Structures of Calmodulin Bound to Two Separate Regulatory Sites in the Retinal Cyclic Nucleotide-Gated Channel

Retinal cyclic nucleotide-gated (CNG) channels (composed of three CNGA1 and one CNGB1 subunits) exhibit a Ca²⁺-induced reduction in channel open probability mediated by calmodulin (CaM). Defects in the Ca²⁺-dependent regulation of CNG channels may be linked to autosomal recessive retinitis pigmentosa and other inherited forms of blindness. Here, we report the NMR structure and binding analysis of CaM bound to two separate sites within CNGB1 (CaM1: residues 565-589 and CaM2: residues 1120-1147). Our binding studies reveal that CaM1 binds to the Ca²⁺-bound CaM N-lobe with at least fivefold higher affinity than it binds to the CaM C-lobe. By contrast, the CaM2 site binds to the Ca²⁺-bound CaM C-lobe with higher affinity than it binds to the N-lobe. CaM1 and CaM2 both exhibited very weak binding to Ca²⁺-free CaM. We present separate NMR structures of Ca²⁺-saturated CaM bound to CaM1 and CaM2 that define key intermolecular contacts: CaM1 residue F575 interacts with the CaM N-lobe while CaM2 residues L1129, L1132, and L1136 each make close contact with the CaM C-lobe. The CNGB1 mutation F575E abolishes CaM1 binding to the CaM N-lobe while L1132E and L1136E each abolish CaM2 binding to the CaM C-lobe. Thus, a single CaM can bind to both sites in CNGB1 in which the CaM N-lobe binds to CaM1 and the CaM C-lobe binds to CaM2. We propose a Ca²⁺-dependent conformational switch in the CNG channel caused by CaM binding, which may serve to attenuate cGMP binding to CNG channels at high cytosolic Ca²⁺ levels in dark-adapted photoreceptors.