Cytosolic adenylyl cyclase defines a unique signaling molecule in mammals

Abstract

Mammals have nine differentially regulated isoforms of G protein-responsive transmembrane-spanning adenylyl cyclases. We now describe the existence of a distinct class of mammalian adenylyl cyclase that is soluble and insensitive to G protein or Forskolin regulation. Northern analysis indicates the gene encoding soluble adenylyl cyclase (sAC) is preferentially expressed in testis. As purified from rat testis cytosol, the active form of sAC appears to be a fragment derived from the full-length protein, suggesting a proteolytic mechanism for sAC activation. The two presumptive catalytic domains of sAC are closely related to cyanobacterial adenylyl cyclases, providing an evolutionary link between bacterial and mammalian signaling molecules.Adenylyl cyclase (AC) is the effector molecule of one of the most widely used signal transduction pathways. Its product, cAMP, mediates cellular responses to nutritional conditions and extracellular signals in organisms from prokaryotes to higher eukaryotes. In metazoans, a seemingly ubiquitous membrane-associated AC activity is encoded by a family of transmembrane adenylyl cyclases (tmACs) that mediate cellular responses to external stimuli. In mammals, nine distinct tmAC genes differing in their patterns of expression and regulatory properties have thus far been identified. Their catalytic activities are differentially regulated by G proteins and other signaling molecules in response to stimuli such as hormones and neurotransmitters (1, 2).In addition, another type of AC activity had been described in mammals. A soluble enzymatic activity was detected in cytosolic extracts from mammalian testis (3). Soluble AC activity appeared to be biochemically and chromatographically different from the tmACs and soluble guanylyl cyclases previously described in testis (4-6). Unlike the known tmACs, its biochemical activity depended on the divalent cation Mn 2ϩ (3), was insensitive to G protein regulation (6), and displayed approximately 10-fold lower affinity for substrate, ATP (K m Ϸ1 mM) (4, 7, 8) than the tmACs (K m Ϸ100 M) (9). Based on these studies, this soluble form of AC was predicted to be molecularly distinct from tmACs (8, 10).A soluble form of AC would define a novel means for generating cAMP and would imply that the second messenger could be generated at a distance from the membrane, closer to its required site of action. However, the molecular evidence confirming that soluble AC represents a distinct form of AC had been lacking. We now describe purification, molecular cloning, and functional expression of the cDNA encoding the soluble form of AC (sAC). The full-length cDNA predicts a protein of 187 kDa, whereas the catalytically active purified form of the enzyme is only 48 kDa, suggesting a proteolytic mechanism of activation for sAC. Two distinct regions within the catalytically active portion of sAC are very similar to catalytic portions of ACs from cyanobacteria and myxobacteria, providing a link between bacterial and mammalian signaling systems. MATERIALS AND METH...

Mice deficient for soluble adenylyl cyclase are infertile because of a severe sperm-motility defect

“…In testes, at least two categories have been described: first, the membrane-associated ACs that are regulated by G proteinassociated receptors (8)(9)(10)(11) and second, the structurally distinct soluble Mn 2ϩ/Ϫ -dependent form that is modulated by bicarbonate and is insensitive to G protein modifiers (12,13). The amino acid sequence of the two catalytic domains (C1 and C2) indicates that sAC is related more closely to the ancestral cyanobacterial ACs than to the mammalian membrane-associated ACs (5,14). Moreover, the ability of the cyanobacterial ACs to respond to bicarbonate stimuli shows that this metabolic pathway is highly conserved.…”

Bicarbonate activation of adenylyl cyclase via promotion of catalytic active site closure and metal recruitment

“…However, the catalytic domains of mammalian sAC are more closely related to bacterial class III ACs than to any other known mammalian cyclases 15 . We therefore studied the regulation of the catalytic domain of the sAC homolog CyaC from the cyanobacterium S. platensis (26% sequence identity), which carries all sequence properties specific for sAC enzymes 14 .…”

A soluble adenylyl cyclase from sea urchin spermatozoa

“…After its cloning [36], mammalian sAC has been found in different cell compartments such as the nucleus [37][38][39], mitochondria and in membrane microdomains, such as caveolae, and lipid rafts, which are also known as the low density-detergent insoluble membrane fraction (LD-DIM) (reviewed in [40]). It is now known that in both nuclei and mitochondria the main source of cAMP synthesis is from sAC.…”