Candida albicans Cyr1, Cap1 and G‐actin form a sensor/effector apparatus for activating cAMP synthesis in hyphal growth

Abstract

SummaryA key virulence trait of Candida albicans is its ability to undergo the yeast-to-hyphal growth transition in response to environmental signals. This transition critically requires a rapid activation of the adenylyl cyclase Cyr1 to generate a cAMP spike. However, the identity of the signal sensors and mechanisms of signal processing and integration remain largely unclear. Recent evidence suggests that some sensors are embedded in Cyr1 itself. To test this hypothesis, we asked whether purified Cyr1 can respond to hyphal induction. Here, we report that Cyr1 co-purifies with Cap1 and G-actin as a tripartite complex which can increase cAMP synthesis in response to hyphal inducing signals in an actindependent manner. Cap1 binds Cyr1 and G-actin through its N-and C-terminus respectively. Deleting the G-actin binding sites or treating the complex with the actin toxin latrunculin A or cytochalasin A inhibits the activation of cAMP synthesis. Strains expressing Cap1 mutants lacking the G-actin binding site are impaired in both cAMP synthesis and hyphal morphogenesis. Thus, our findings reveal an essentially intact sensor/effector apparatus composed of Cyr1, Cap1 and G-actin. Furthermore, G-actin's regulatory role in this apparatus may prove to be the missing link whereby cellular actin status knowingly influences cAMP-mediated cellular processes.

The role of cyclase-associated protein in regulating actin filament dynamics – more than a monomer-sequestration factor

“…Interestingly, actin-anchored Ras is an active GTP-bound form and stimulates the production of cAMP, which results in the production of reactive oxygen species and is followed by apoptosis (Gourlay and Ayscough, 2006). The function of CAP as a link between adenylyl cyclase and actin has also been shown in other fungi such as Candida albicans Zou et al, 2010) and Magnaporthe oryzae (a rice pathogen) (Zhou et al, 2012). In Dictyostelium, although a direct binding between adenylyl cyclase and CAP has not been detected, CAP also regulates both intracellular cAMP signaling and chemotactic behaviors, and potentially links these two mechanisms Noegel et al, 1999;Sultana et al, 2009;Sultana et al, 2012).…”

Regulatory Circuitry Governing Fungal Development, Drug Resistance, and Disease

“…For instance, both CO 2 and peptidoglycans in serum directly stimulate Cyr1 to activate cAMP production and promote filamentous growth (276,635). Recently, Cyr1 was found to copurify with the cyclase-associated protein Srv2 as well as with monomeric actin (G-actin), which together increase cAMP synthesis in response to hyphal signals (659). The formation of this tripartite protein complex is essential for hyphal induction in response to serum and peptidoglycans, and a disruption of the complex leads to a reduction in the levels of cAMP production (659).…”

“…Recently, Cyr1 was found to copurify with the cyclase-associated protein Srv2 as well as with monomeric actin (G-actin), which together increase cAMP synthesis in response to hyphal signals (659). The formation of this tripartite protein complex is essential for hyphal induction in response to serum and peptidoglycans, and a disruption of the complex leads to a reduction in the levels of cAMP production (659). Considering that filamentous growth requires complex cytoskeletal restructuring, including the reorganization of actin filaments, this connection between G-actin, cAMP production, and filamentous growth provides an exciting and gratifying molecular link.…”

Fungal Morphogenesis

“…3) (Cottier and Muhlschlegel 2009). Many of the strong hypha-inducing signals are sensed and integrated by the adenylate cyclase Cyr1, which is indis-pensable for hyphal growth under all conditions (Bahn and Sundstrom 2001;Rocha et al 2001;Hogan and Sundstrom 2009;Zou et al 2009). CO 2 /HCO 3 2 directly stimulates Cyr1p activity by binding to the catalytic domain of Cyr1 (Klengel et al 2005).…”