PubMed · 8165280
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Y Ohya. 1994. [Multifunctional protein, calmodulin].. https://pubmed.ncbi.nlm.nih.gov/8165280/
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Bridge-like lipid transfer proteins (BLTPs) play fundamental roles in cellular lipid redistribution between organellar membranes. They comprise bridge domains spanning organelles at contact sites that allow lipids to transit through the cytosol between adjacent membranes. The assembly of BLTPs into complexes with adaptor proteins enables lipid transfer. To address the mechanisms underlying the assembly and regulation of BLTP complexes, we used cryo-EM to resolve the structure of one such BLTP, the Parkinson's disease protein VPS13C, at near-atomic resolution. The structure identifies a lipid-transfer-nonpermissive conformation, in which the built-in C-terminal VAB adaptor module blocks the end of the lipid transfer bridge, interfering with lipid delivery. We also identify calmodulin (CaM), central to calcium signaling, as a constitutive VPS13C interactor. Calcium induces conformational changes in the VPS13C-CaM complex, suggesting calcium regulation of VPS13 function. Altogether, this structure of intact VPS13C serves as a starting point for understanding its regulation and that of other VPS13 proteins.
Calmodulin (CaM) plays a central role in cardiac excitation-contraction coupling by regulating ion channels, including the L-type calcium (Ca2+) channel Cav1.2 and the voltage-gated potassium (K+) channel Kv7.1. Mutations in CaM are linked to severe arrhythmogenic disorders such as Long QT syndrome (LQTS), yet the molecular mechanisms remain incompletely understood. Here, we investigate the structural and functional consequences of the arrhythmia-associated CaM variant D133H. Biophysical analysis revealed that D133H destabilises Ca2+ binding at the C-terminal lobe of CaM, altering its Ca2+-dependent conformational changes. Electrophysiological recordings demonstrated that CaM D133H impairs Ca2+-dependent inactivation (CDI) of Cav1.2, prolonging Ca2+ influx, while also reducing activation of Kv7.1, thereby limiting repolarising K+ currents. Together, these dual defects converge to prolong action potential duration, providing a mechanistic basis for arrhythmogenesis in LQTS. Our findings establish that CaM D133H perturbs both Ca2+ and K+ channel regulation, highlighting a shared pathway by which calmodulinopathy mutations disrupt cardiac excitability.
Glutamate decarboxylase (GAD) catalyzes the decarboxylation of glutamate to CO2 and gamma-aminobutyrate (GABA). GAD is ubiquitous in prokaryotes and eukaryotes, but only plant GAD has been shown to bind calmodulin (CaM). Here, we assess the role of the GAD CaM-binding domain in vivo. Transgenic tobacco plants expressing a mutant petunia GAD lacking the CaM-binding domain (GADdeltaC plants) exhibit severe morphological abnormalities, such as short stems, in which cortex parenchyma cells fail to elongate, associated with extremely high GABA and low glutamate levels. The morphology of transgenic plants expressing the full-length GAD (GAD plants) is indistinguishable from that of wild-type (WT) plants. In WT and GAD plant extracts, GAD activity is inhibited by EGTA and by the CaM antagonist trifluoperazine, and is associated with a CaM-containing protein complex of approximately 500 kDa. In contrast, GADdeltaC plants lack normal GAD complexes, and GAD activity in their extracts is not affected by EGTA and trifluoperazine. We conclude that CaM binding to GAD is essential for the regulation of GABA and glutamate metabolism, and that regulation of GAD activity is necessary for normal plant development. This study is the first to demonstrate an in vivo function for CaM binding to a target protein in plants.