Identification and characterization of a novel beta 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 heterodi mers that can be activated by the gaseous messenger, nitric oxide. In mamma ls, four subunits have been cloned, named alpha 1, alpha 2, beta 1, and bet a 2. We have identified a novel soluble guanylyl cyclase isoform from the n ervous system of the insect Manduca sexta that we have named M. sexta guany lyl cyclase beta 3 (MsGC-beta 3). It is most closely related to the mammali an beta subunits but has several features that distinguish it from previous ly identified soluble cyclases. Most importantly, MsGC-beta 3 does not need to form heterodimers to form an active enzyme because guanylyl cyclase act ivity 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 d onor, sodium nitroprusside. Several of the amino acids in rat beta 1 subuni ts, previously identified as being important in heme binding or necessary f or 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 kn own protein. Northern blot analysis shows that MsGC-beta 3 is primarily exp ressed in the nervous system of Manduca.