Plants can sense and respond to mechanical stimuli, like animals. An early mechanism of mechanosensing and response is speculated to be governed by as-yet-unidentified sensory complexes containing a Ca 2؉ -permeable, stretch-activated (SA) channel. However, the components or regulators of such complexes are poorly understood at the molecular level in plants. Here, we report the molecular identification of a plasma membrane protein (designated Mca1) that correlates Ca 2؉ influx with mechanosensing in Arabidopsis thaliana. MCA1 cDNA was cloned by the functional complementation of lethality of a yeast mid1 mutant lacking a putative Ca 2؉ -permeable SA channel component. Mca1 was localized to the yeast plasma membrane as an integral membrane protein and mediated Ca 2؉ influx. Mca1 also increased [Ca 2؉ ]cyt upon plasma membrane distortion in Arabidopsis. The growth of MCA1-overexpressing plants was impaired in a high-calcium but not a low-calcium medium. The primary roots of mca1-null plants failed to penetrate a harder agar medium from a softer one. These observations demonstrate that Mca1 plays a crucial role in a Ca 2؉ -permeable SA channel system that leads to mechanosensing in Arabidopsis. We anticipate our findings to be a starting point for a deeper understanding of the molecular mechanisms of mechanotransduction in plants.calcium ͉ calcium channel ͉ calcium uptake ͉ mechanosensing

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Calcium plays a critical part in the regulation of cell growth, and growth factors stimulate calcium entry into cells through calcium-permeable channels. However, the molecular nature and regulation of calcium-permeable channels are still unclear at present. Here we report the molecular characterization of a calcium-permeable cation channel that is regulated by insulin-like growth factor-I (IGF-I). This channel, which we name growth-factor-regulated channel (GRC), belongs to the TRP-channel family and localizes mainly to intracellular pools under basal conditions. Upon stimulation of cells by IGF-I, GRC translocates to the plasma membrane. Thus, IGF-I augments calcium entry through GRC by regulating trafficking of the channel.

BackgroundSweet taste receptor is expressed in the taste buds and enteroendocrine cells acting as a sugar sensor. We investigated the expression and function of the sweet taste receptor in MIN6 cells and mouse islets.Methodology/Principal FindingsThe expression of the sweet taste receptor was determined by RT–PCR and immunohistochemistry. Changes in cytoplasmic Ca2+ ([Ca2+]c) and cAMP ([cAMP]c) were monitored in MIN6 cells using fura-2 and Epac1-camps. Activation of protein kinase C was monitored by measuring translocation of MARCKS-GFP. Insulin was measured by radioimmunoassay. mRNA for T1R2, T1R3, and gustducin was expressed in MIN6 cells. In these cells, artificial sweeteners such as sucralose, succharin, and acesulfame-K increased insulin secretion and augmented secretion induced by glucose. Sucralose increased biphasic increase in [Ca2+]c. The second sustained phase was blocked by removal of extracellular calcium and addition of nifedipine. An inhibitor of inositol(1, 4, 5)-trisphophate receptor, 2-aminoethoxydiphenyl borate, blocked both phases of [Ca2+]c response. The effect of sucralose on [Ca2+]c was inhibited by gurmarin, an inhibitor of the sweet taste receptor, but not affected by a Gq inhibitor. Sucralose also induced sustained elevation of [cAMP]c, which was only partially inhibited by removal of extracellular calcium and nifedipine. Finally, mouse islets expressed T1R2 and T1R3, and artificial sweeteners stimulated insulin secretion.ConclusionsSweet taste receptor is expressed in β-cells, and activation of this receptor induces insulin secretion by Ca2+ and cAMP-dependent mechanisms.

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