## 7 cAMP in Apicomplexan Parasites Activates the Guanine Exchange Factor (GEF) EPAC and Rap1  
Adenylylcyclases [87] and guanylylcyclases [88] have been identified in _Plasmodium_ together with phosphodiesterases [89] and their cAMP/cGMP dependent-kinases [90]. Since their structural peculiarities and their biochemistry have been extensively reviewed [90], the focus here will be on the small GTPase Rap1 (rhoptry associated-protein-1) which is activated by cAMP as the second messenger. This effect is mediated by the guanine nucleotide exchange factor (GEF) EPAC, which is directly activated by cAMP and independent from protein kinase A. Thus Rap-1 has an important function in invasion of the RBC. Two adenylyl cyclases and two guanyl cyclases occur in _Plasmodium_. Adenylyl cyclase alpha (\(\mathit{PfAC}_{\alpha}\)) seems to be involved in sporozoite exocytosis while \(\mathrm{PfAC}_{\beta}\) is essential for the blood stages as shown in a rodent model by deletion experiments [90]. Moreover, the authors hypothesized that \(\mathit{PfAC}_{\beta}\) acts as a pH sensor. Both proteins differ in their structural architecture. \(\mathit{PfAC}_{\alpha}\) contains six predicted transmembrane domains with significant homology to voltage-gated K\({}^{+}\) channels and a single carboxy-terminal catalytic domain homologous to soluble adenylylcyclases (sAC). In contrast, \(\mathit{PfAC}_{\beta}\) has no predicted transmembrane regions and possesses two sAC-like AC catalytic domains. \(\mathit{PfAC}_{\beta}\) is an orthologue of the bicarbonate-sensitive adenylyl cyclases. To date, two pharmacological inhibitors i.e., KH7 (\(E\))-2-(\(1H\)-Benzo[d]imidazol-2-ylthio)-\(N^{\prime}\)-(5-bromo-2-hydroxybenzylidene) propanehydrazide) and 2-catechol estrogens (CE) [15] showed inhibition of \(\mathrm{PfAC}_{\beta}\) in the micromolar range. The inhibitory effect might be explained by the fact that either the imidazole- or the catechol- structure can complex the metal ion (magnesium) in the catalytic site. During egress and invasion of merozoites \(\mathrm{PfAC}\beta\) triggers cAMP levels fostering the EPAC pathway which in turn leads to phosphorylation of Rap-GDP to Rap-GTP [91] (Figure 1). Rap-GTP activates phospholipase C (PLC) to produce inositol triphosphate (IP\({}_{3}\)) that binds to the IP\({}_{3}\) receptor (IP\({}_{3}\)R) on the endoplasmic reticulum (Figure 1). This leads to a release of calcium ions. Calcium ions bind to calcineurin and activate calcium-dependent protein kinase 1 (CDPK1) (Figure 1) required for the discharge of microneme and rhoptry to facilitate invasion of the parasite. In addition to Plasmodium, a Rap-1 protein has been characterized in the Apicomplexan Babesia motasi which infects sheep [92] and horses [93]. To date, no chemical inhibitor was identified that inhibits Rap-1 from the parasites directly. Initially, Rap-1 was identified as a ras suppressor protein able to revert the morphological phenotype of Ras-transformed fibroblasts. Meanwhile, it was demonstrated that Rap-1 is activated by a variety of different receptors. Several messengers like calcium, diacylglycerol and cAMP mediate this effect. Inactivation is performed by specific GTPase-activating proteins (GAPs), one of which is regulated through an interaction with \(\mathrm{Galpha}_{\mathrm{i}}\). Currently, there have been no reports about Rap-1 orthologues in the database of _Toxoplasma gondii_ and _Cryptosporidium_ yet, although a putative adenylyl cyclase \(\beta\) from _T. gondii_ encodes a unique protein of 2400 amino acids with a conserved, catalytic adenylyl cyclase class III domain. Separate loci, encoding guanylyl cyclases have also been identified in the genome of _T. gondii_ with a typical, conserved domain for adenylyl- and guanyl cyclases. The amino acid identity corresponding to the _Cryptosporidium_ paralogues is approximately 48% and to the rodent _Plasmodium_ species 36%.