Background: The left and right AWC olfactory neurons in Caenorhabditis elegans differ in their functions and in their expression of chemosensory receptor genes; in each animal, one AWC randomly takes on one identity, designated AWC^OFF, and the contralateral AWC becomes AWC^ON. Signaling between AWC neurons induces left-right asymmetry through a gap junction network and a claudin-related protein, which inhibit a calcium-regulated MAP kinase pathway in the neuron that becomes AWC^ON. Results: We show here that the asymmetry gene olrn-1 acts downstream of the gap junction and claudin genes to inhibit the calcium-MAP kinase pathway in AWC^ON. OLRN-1, a protein with potential membrane-association domains, is related to the Drosophila Raw protein, a negative regulator of JNK mitogen-activated protein (MAP) kinase signaling. olrn-1 opposes the action of two voltage-activated calcium channel homologs, unc-2 (CaV2) and egl-19 (CaV1), which act together to stimulate the calcium/calmodulin-dependent kinase CaMKII and the MAP kinase pathway. Calcium channel activity is essential in AWC^OFF, and the two AWC neurons coordinate left-right asymmetry using signals from the calcium channels and signals from olrn-1. Conclusion: olrn-1 and voltage-activated calcium channels are mediators and targets of AWC signaling that act at the transition between a multicellular signaling network and cell-autonomous execution of the decision. We suggest that the asymmetry decision in AWC results from the intercellular coupling of voltage-regulated channels, whose cross-regulation generates distinct calcium signals in the left and right AWC neurons. The interpretation of these signals by the kinase cascade initiates the sustained difference between the two cells.

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