Soluble guanylyl cyclases in invertebrates: Targets for NO and O2

Abstract

Soluble guanylyl cyclase (sGC) is probably the most prevalent target for NO in almost all species and tissues (Lucas et al., 2000). The native enzyme is usually a heterodimer (see below for exceptions to this) containing a single heme group (Lucas et al., 2000). In mammals, four subunits have been identified: α1, α2, β1 and β2. The major functional enzyme is the α1/β1 heterodimer (Lucas et al., 2000). The α2 is very similar in sequence to the α1 subunit and the α2/β1 heterodimer has similar properties to the α1/β1 enzyme (Russworm et al., 1998, Gibb et al., 2003). One significant difference is that the α2 subunit interacts with the scaffold protein PSD95 and hence likely has a different subcellular distribution (Russworm et al., 2001). All four mammalian subunits are homologous proteins with a similar arrangement of functional domains (Figure 1). The subunits can be divided into a C-terminal catalytic domain that catalyses the conversion of GTP to cGMP and an N-terminal regulatory domain that functions as a heme binding region and is required for NO activation of the enzyme (Figure 1). A variety of studies have identified several residues in each of these domains that are required for function and are indicated in Figure 1. The α1/β1 heterodimer binds a single heme group per heterodimer and the β1 subunit is primarily responsible for this interaction (Lucas et al., 2000). The Fe 2+ ion in the center of the heme group interacts with His105 (identified as H105 in Figure 1) of the β1 subunit (Zhao et al., 1998). The heme group interacts with three residues in the β1 subunit via hydrogen bonds that form the YXS/TXR motif (Schmidt et al., 2004). In addition, two cysteines (at positions C78 and C214 in the rat β1) subunit that are necessary for NO activation (Freibe et al., 1997). The relative positions of these residues are shown in Figure 1. Modeling of the catalytic domain of sGC, based on the crystal structure of the catalytic domain of mammalian adenylyl cyclase, predicts that there are 17 residues that contact the GTP substrate (Liu et al., 1997). The active site is at the interface between the two subunits, so each subunit provides a subset of specific residues (10 for the β subunit and 7 for the α subunit, which are marked on the catalytic domain in Figure 1). This model predicts that a single GTP molecule will bind per heterodimer (Liu et al., 1997). The β2 subunit has quite different functional characteristics and appears to be a member of a different subfamily of sGC subunits, which we have termed the atypical sGCs (Morton, 2004a). Although the β2 subunit will form active heterodimers with both the α1 and α2, it has the unusual property that it is also active as a homodimer (Koglin et al., 2001; Gibb et al., 2003). Each of these enzymes are sensitive to NO, but they are all less potently activated by NO compared to the α1/β1 and α2/β1 heterodimers (Gibb et al., 2003). The regulatory domain of the β2 subunit contains His104, Cys78, Cys214 and the YXS/TXR motif (Figure 1), which suggests that the α1/β2 and α2/...

Behavioral Responses to Hypoxia in Drosophila Larvae Are Mediated by Atypical Soluble Guanylyl Cyclases

“…Transient expression of different combinations of the Drosophila atypical sGCs in COS-7 cells showed that Gyc-89Da and Gyc-89Db form obligate heterodimers with Gyc-88E, and are potently activated in the absence of O 2 , therefore suggesting a function as O 2 sensors (Morton 2004b;Vermehren et al 2006;Morton and Vermehren 2007). Gyc-88E is also active as a homodimer (Morton 2004b;Huang et al 2007), directly binds O 2 and is also activated by reduced O 2 (Huang et al 2007).…”

Atypical soluble guanylyl cyclases control brain size inDrosophila

“…Previous studies have shown that Gyc88E is active in the absence of additional subunits, while Gyc89Da and Gyc89Db enhanced the activity of Gyc88E when co-expressed, suggesting that these enzymes acted likely as heterodimers (Morton and Vermehren 2007;Morton 2011). It is important to mention that Gyc89Db is expressed in the brain during larval development (Langlais et al 2004)and produces cyclic GMP in response to low oxygen in vitro (Morton 2004;Vermehren et al 2006).…”

Behavioral responses to hypoxia and hyperoxia in Drosophila larvae

“…7 The expression patterns of the Drosophila atypical sGCs showed that in larvae they are almost exclusively expressed in sensory and central neurons 14,15,18 and this distribution together with their biochemical properties suggested that they were ideal candidates for the O 2 sensors mediating behavioral responses to hypoxia. 19,20 Drosophila larvae respond rapidly to hypoxia by leaving their normal feeding positions (partially buried in their food) and initiating exploratory behaviors. 21 By inactivating the neurons that express Gyc-89Da with tetanus toxin (TTxN) we showed that these neurons were required for this behavioral response 19 but our initial efforts at establishing a role for the atypical sGCs in responses to hypoxia were hampered by the apparent redundancy between Gyc89Da and Gyc-89Db.…”