Identification and characterization of a novel β subunit of soluble guanylyl cyclase that is active in the absence of a second subunit and is relatively insensitive to nitric oxide

Previously characterized soluble guanylyl cyclases form alpha-beta heterodimers that can be activated by the gaseous messenger, nitric oxide. In mammals, four subunits have been cloned, named alpha 1, alpha 2, beta 1, and beta 2. We have identified a novel soluble guanylyl cyclase isoform from the nervous system of the insect Manduca sexta that we have named M. sexta guanylyl cyclase beta 3 (MsGC-beta 3). It is most closely related to the mammalian beta subunits but has several features that distinguish it from previously identified soluble cyclases. Most importantly, MsGC-beta 3 does not need to form heterodimers to form an active enzyme because guanylyl cyclase activity can be measured when it is expressed alone in COS-7 cells. Moreover, this activity is only weakly enhanced in the presence of the nitric oxide donor, sodium nitroprusside. Several of the amino acids in rat beta 1 subunits, previously identified as being important in heme binding or necessary for nitric oxide activation, are substituted with nonsimilar amino acids in MsGC-beta 3. There are also an additional 315 amino acids C-terminal to the catalytic domain of MsGC-beta 3 that have no sequence similarity to any known protein. Northern blot analysis shows that MsGC-beta 3 is primarily expressed in the nervous system of Manduca.