In a subset of the olfactory sensory neurons ONE-GC($) membrane guanylate cyclase is a central component of two odorant-dependent cyclic GMP signaling pathways. These odorants are uroguanylin and CO2. The present study was designed to decipher the biochemical and molecular differences between these two odorant signaling mechanisms. The study shows (1) in contrast to Uroguanylin, CO2 transduction mechanism is Ca2+-independent. (2) CO2 transduction site, like that of uroguanylin-neurocalcin 8, resides in the core catalytic domain, aa 880-1028, of ONE-GC. (3) The site. however, does not overlap the signature neurocalcin 8 signal transduction domain, (LSEPIE913)-L-908. Finally, (4) this study negates the prevailing concept that CO2 uniquely signals ONE-GC activity (Sun et al. [19]; Guo et al. [21]). It demonstrates that it also signals the activation of photoreceptor membrane guanylate cyclase ROS-GC1. These results show an additional new transduction mechanism of the membrane guanylate cyclases and broaden our understanding of the molecular mechanisms by which different odorants using a single guanylate cyclase can regulate diverse cyclic GMP signaling pathways. (C) 2009 Elsevier Inc. All rights reserved.