Binding of calmodulin changes the calcineurin regulatory region to a less dynamic conformation

Case report: A novel PPP3CA truncating mutation within the regulatory domain causes severe developmental and epileptic encephalopathy in a Chinese patient

Introduction

Developmental and epileptic encephalopathy 91 (DEE91; OMIM#617711) is a severe neurodevelopmental disorder caused by heterozygous PPP3CA variants. To the best of our knowledge, only a few DEE91 cases have been reported.

Results

This study reports a boy who experienced recurrent afebrile convulsions and spasms at the age of 2 months. After being given multiple antiepileptic treatments with levetiracetam, adrenocorticotropic hormone (ACTH), prednisone, topiramate, and clonazepam, his seizures were not completely relieved. At the age of 4 months, the patient exhibited delayed neuromotor development and difficulty in feeding; at the age of 6 months, he was diagnosed with developmental regression with recurrent spasms and myoclonic seizures that could respond to vigabatrin. At the age of 1 year and 4 months, the patient showed profound global developmental delay (GDD) with intermittent absence seizures. Whole-exome sequencing (WES) identified a novel loss-of-function variant c.1258_1259insAGTG (p. Val420Glufs^*32) in PPP3CA.

Conclusion

This finding expands the genetic spectrum of the PPP3CA gene and reinforces the theory that DEE91-associated truncating variants cluster within a 26-amino acid region in the regulatory domain (RD) of PPP3CA.

Calcineurin

The serine/threonine phosphatase calcineurin acts as a crucial connection between calcium signaling the phosphorylation states of numerous important substrates. These substrates include, but are not limited to, transcription factors, receptors and channels, proteins associated with mitochondria, and proteins associated with microtubules. Calcineurin is activated by increases in intracellular calcium concentrations, a process that requires the calcium sensing protein calmodulin binding to an intrinsically disordered regulatory domain in the phosphatase. Despite having been studied for around four decades, the activation of calcineurin is not fully understood. This review largely focuses on what is known about the activation process and highlights aspects that are currently not understood. Video abstract.

Solid-film sampling method for the determination of protein secondary structure by Fourier transform infrared spectroscopy

Fourier transform infrared (FTIR) spectroscopy is one of the widely used vibrational spectroscopic methods in protein structural analysis. The protein solution sample loaded in demountable CaF₂ liquid cell presents a challenge and is limited to high concentrations. Some researchers attempted the simpler solid-film sampling method for the collection of protein FTIR spectra. In this study, the solid-film sampling FTIR method was studied in detail. The secondary structure components of some globular proteins were determined by this sampling method, and the results were consistent with those data determined by the traditional solution sampling FTIR method and X-ray crystallography, indicating that this sampling method is feasible and efficient for the structural characterization of proteins. Furthermore, much lower protein concentrations (~0.5 mg/mL) were needed to obtain high-quality FTIR spectra, which expands the application of FTIR spectroscopy to almost the same concentration range used for circular dichroism and fluorescence spectroscopy, making comparisons among three commonly used techniques possible in protein studies. Graphical Abstract ᅟ.

Cloning and Stress-Induced Expression Analysis of Calmodulin in the Antarctic Alga Chlamydomonas sp. ICE-L

Calmodulin (CaM) is a Ca²⁺-binding protein that plays a role in several Ca²⁺ signaling pathways, which dynamically regulates the activities of hundreds of proteins. The ice alga Chlamydomonas sp. ICE-L, which has the ability to adapt to extreme polar conditions, is a crucial primary producer in Antarctic ecosystem. This study hypothesized that Cam helps the ICE-L to adapt to the fluctuating conditions in the polar environment. It first verified the overall length of Cam, through RT-PCR and RACE-PCR, based on partial Cam transcriptome library of ICE-L. Then, the nucleotide and predicted amino acid sequences were, respectively, analyzed by various bioinformatics approaches to gain more insights into the computed physicochemical properties of the CaM. Potential involvements of Cam in responding to certain stimuli (i.e., UVB radiation, high salinity, and temperature) were investigated by differential expression, measuring its transcription levels by means of quantitative RT-PCR. Results showed that CaM was indeed inducible and regulated by high UVB radiation, high salinity, and nonoptimal temperature conditions. Different conditions had different expression tendencies, which provided an important basis for investigating the adaptation mechanism of Cam in ICE-L.

Calmodulin in complex with the first IQ motif of myosin-5a functions as an intact calcium sensor

The motor function of vertebrate myosin-5a is inhibited by its tail in a Ca²⁺-dependent manner. We previously demonstrated that the calmodulin (CaM) bound to the first isoleucine-glutamine (IQ) motif (IQ1) of myosin-5a is responsible for the Ca²⁺-dependent regulation of myosin-5a. We have solved the crystal structure of a truncated myosin-5a containing the motor domain and IQ1 (MD-IQ1) complexed with Ca²⁺-bound CaM (Ca²⁺-CaM) at 2.5-Å resolution. Compared with the structure of the MD-IQ1 complexed with essential light chain (an equivalent of apo-CaM), MD-IQ1/Ca²⁺-CaM displays large conformational differences in IQ1/CaM and little difference in the motor domain. In the MD-IQ1/Ca²⁺-CaM structure, the N-lobe and the C-lobe of Ca²⁺-CaM adopt an open conformation and grip the C-terminal and the N-terminal portions of the IQ1, respectively. Remarkably, the interlobe linker of CaM in IQ1/Ca²⁺-CaM is in a position opposite that in IQ1/apo-CaM, suggesting that CaM flip-flops relative to the IQ1 during the Ca²⁺ transition. We demonstrated that CaM continuously associates with the IQ1 during the Ca²⁺ transition and that the binding of CaM to IQ1 increases Ca²⁺ affinity and substantially changes the kinetics of the Ca²⁺ transition, suggesting that the IQ1/CaM complex functions as an intact Ca²⁺ sensor responding to distinct calcium signals.