ABSTRACT
Store-operated calcium entry (SOCE), an important mechanism of Ca2+ signaling in a wide range of cell types, is mediated by stromal interaction molecule (STIM), which senses the depletion of endoplasmic reticulum Ca2+ stores and binds and activates Orai channels in the plasma membrane. This inside-out mechanism of Ca2+ signaling raises an interesting question about the evolution of SOCE: How did these two proteins existing in different cellular compartments evolve to interact with each other? We investigated the gating mechanism of Caenorhabditis elegans Orai channels. Our analysis revealed a mechanism of Orai gating by STIM binding to the intracellular 2-3 loop of Orai in C. elegans that is radically different from Orai gating by STIM binding to the N and C termini of Orai in mammals. In addition, we found that the conserved hydrophobic amino acids in the 2-3 loop of Orai1 are important for the oligomerization and gating of channels and are regulated via an intramolecular interaction mechanism mediated by the N and C termini of Orai1. This study identifies a previously unknown SOCE mechanism in C. elegans and suggests that, while the STIM-Orai interaction is conserved between invertebrates and mammals, the gating mechanism for Orai channels differs considerably.
Subject(s)
Caenorhabditis elegans/metabolism , Calcium Channels/metabolism , Calcium/metabolism , Ion Channel Gating , ORAI1 Protein/metabolism , Stromal Interaction Molecule 1/metabolism , Amino Acid Sequence , Animals , Caenorhabditis elegans/genetics , Calcium Channels/chemistry , Calcium Channels/genetics , Calcium Signaling , Cell Membrane/metabolism , Endoplasmic Reticulum/metabolism , Evolution, Molecular , HEK293 Cells , Humans , ORAI1 Protein/chemistry , ORAI1 Protein/genetics , Sequence Homology , Stromal Interaction Molecule 1/chemistry , Stromal Interaction Molecule 1/geneticsABSTRACT
Plants produce numerous calmodulin isoforms that exhibit differential gene expression patterns and sense different Ca2+ signals. This diversity results in different physiological responses to particular stimuli. Gm-CaM-4 and -5 are two divergent calmodulin isoforms from the soybean (Glycine max) that have been reported to be involved in plant disease resistance. However, little is known about the pathway by which these specific isoforms transduce the defense signal and up-regulate pathogenesis-related (PR) genes. Here we report that overexpression of GmCaM-4/-5 induces constitutive PR gene expression and enhances disease resistance in wild-type Arabidopsis, but not in the nim1 mutant of Arabidopsis. GmCaM-4/-5 also appear to activate trans-acting elements that bind to cis-acting elements in the Arabidopsis PR-1 promoter. Thus up-regulation of PR genes by these GmCaM isoforms is dependent on NIM1 (Non immunity 1) and unknown transcription factors.