Signaling through Lrg1, Rho1 and Pkc1 Governs Candida albicans Morphogenesis in Response to Diverse Cues

Abstract

The capacity to transition between distinct morphological forms is a key virulence trait for diverse fungal pathogens. A poignant example of a leading opportunistic fungal pathogen of humans for which an environmentally responsive developmental program underpins virulence is Candida albicans. C. albicans mutants that are defective in the transition between yeast and filamentous forms typically have reduced virulence. Although many positive regulators of C. albicans filamentation have been defined, there are fewer negative regulators that have been implicated in repression of filamentation in the absence of inducing cues. To discover novel negative regulators of filamentation, we screened a collection of 1,248 C. albicans homozygous transposon insertion mutants to identify those that were filamentous in the absence of inducing cues. We identified the Rho1 GAP Lrg1, which represses filamentous growth by stimulating Rho1 GTPase activity and converting Rho1 to its inactive, GDP-bound form. Deletion of LRG1 or introduction of a RHO1 mutation that locks Rho1 in constitutively active, GTP-bound state, leads to filamentation in the absence of inducing cues. Deletion of the Rho1 downstream effector PKC1 results in defective filamentation in response to diverse host-relevant inducing cues, including serum. We further established that Pkc1 is not required to sense filament-inducing cues, but its kinase activity is critical for the initiation of filamentous growth. Our genetic analyses revealed that Pkc1 regulates filamentation independent of the canonical MAP kinase cascade. Further, although Ras1 activation is not impaired in a pkc1Δ/pkc1Δ mutant, adenylyl cyclase activity is reduced, consistent with a model in which Pkc1 functions in parallel with Ras1 in regulating Cyr1 activation. Thus, our findings delineate a signaling pathway comprised of Lrg1, Rho1 and Pkc1 with a core role in C. albicans morphogenesis, and illuminate functional relationships that govern activation of a central transducer of signals that control environmental response and virulence programs.

The Candida albicans transcription factor Cas5 couples stress responses, drug resistance and cell cycle regulation

“…To confirm this shift was a consequence of cell wall stress, we employed a strain in which we could specifically inhibit the kinase activity of the cell wall integrity regulator Pkc1 35 . In this strain, the only allele of PKC1 carries an M850G gatekeeper mutation, rendering the kinase susceptible to inhibition by ATP analog 1-naphthyl-PP1 (1-NA-PP1), without affecting its kinase activity in the absence of the inhibitor 35 , 36 . In the presence of 1-NA-PP1, the gatekeeper mutant was hypersensitive to caspofungin, similar to a pkc1 Δ /pkc1 Δ mutant (Supplementary Fig.…”

Functional Genomic Analysis of Candida albicans Adherence Reveals a Key Role for the Arp2/3 Complex in Cell Wall Remodelling and Biofilm Formation

“…cerevisiae and C . albicans , Lrg1 is a Rho1 GAP that stimulates Rho1 GTPase activity and converts Rho1 to its inactive, GDP-bound form [21,22]. Thus, homozygous deletion of LRG1 should lock RHO1 in a constitutively active state.…”

The Hsp90 Chaperone Network Modulates Candida Virulence Traits

“…The PKC mitogen activated protein kinase (MAPK) cascade is required for tolerance to both the echinocandins [69] and azoles [70], which are the most widely deployed classes of antifungal drugs. Pkc1 is also a global regulator of C. albicans morphogenesis [71], and is required for virulence in a murine model of systemic infection [70]. Hsp90 is required for the stability of the terminal MAPK Mkc1 in both the basal and activated forms [50,70].…”

“…Notably, filamentation in response to Hsp90 inhibition does not require Efg1, the transcription factor downstream of PKA that is required for filamentation in response to serum and other cues [86]. Recent work implicated components of the cell wall integrity pathway in filamentation in response to diverse cues, including Hsp90 inhibition; key components include the GTPase Rho1 [87], the GTPase activator Lrg1, and the kinase Pkc1 [71]. Notably, downstream MAPK components are not required for filamentation in response to Hsp90 inhibition, implicating alternate downstream effectors [71].…”

“…Recent work implicated components of the cell wall integrity pathway in filamentation in response to diverse cues, including Hsp90 inhibition; key components include the GTPase Rho1 [87], the GTPase activator Lrg1, and the kinase Pkc1 [71]. Notably, downstream MAPK components are not required for filamentation in response to Hsp90 inhibition, implicating alternate downstream effectors [71]. In contrast to core circuitry required for filamentation in response to diverse cues, other pathways have more specific roles in filamentation in response to Hsp90 inhibition.…”