Functional connections between cell cycle and proteostasis in the regulation of Candida albicans morphogenesis

Abstract

Morphological plasticity is a key virulence trait for many fungal pathogens. For the opportunistic fungal pathogen Candida albicans, transitions among yeast, pseudohyphal, and hyphal forms are critical for virulence, because the morphotypes play distinct roles in the infection process. C. albicans morphogenesis is induced in response to many host-relevant conditions and is regulated by complex signaling pathways and cellular processes. Perturbation of either cell-cycle progression or protein homeostasis induces C. albicans filamentation, demonstrating that these processes play a key role in morphogenetic control. Regulators such as cyclin-dependent kinases, checkpoint proteins, the proteasome, the heat shock protein Hsp90, and the heat shock transcription factor Hsf1 all influence morphogenesis, often through interconnected effects on the cell cycle and proteostasis. This review highlights the major cell-cycle and proteostasis regulators that modulate morphogenesis and discusses how these two processes intersect to regulate this key virulence trait.

Figure 1.. Cyclin-dependent kinase (CDK) complexes control cell-cycle progression and play many critical roles during morphogenesis

CDK Cdc28 drives the cell cycle and maintains yeast-phase growth through its association with phase-specific cyclins with opposing activities. During early stages of the cell cycle, Cdc28 associates with the G1 cyclins Ccn1 and Cln3; as the cell cycle progresses, these are replaced by the G2 cyclins Clb2 and Clb4. The temporal binding of Cdc28 with these cyclins depends on their coordinated synthesis and degradation. Upon hyphal induction, the hyphal-specific G1 cyclin Hgc1 is expressed and associates with Cdc28 to target various cellular processes that promote morphogenesis, independently or in concert with other CDK complexes. Phosphorylation of the polarisome component Spa2 by Clb2-Cdc28 and Hgc1-Cdc28, the septin Cdc11 by Ccn1-Cdc28 and Hgc1-Cdc28, and the GTPase-activating protein Rga2 by Hgc1-Cdc28 promotes hyphal growth. Phosphorylation of the exocyst subunit Exo84 by Hgc1-Cdc28 regulates polarized secretion at the hyphal tip. The G1 CDK complexes, Ccn1-Cdc28 and Cln3-Cdc28, work together to phosphorylate the transcription factor Fkh2 to enhance expression of hyphal-specific genes.

Figure 2.. The ubiquitin-proteasome pathway influences proteostasis, the cell cycle, and filamentation

The polyubiquitin Ubi4 is cleaved to generate ubiquitin monomers. Ubiquitin is activated by a ubiquitin-activating enzyme (E1) in an ATP-dependent manner; transferred to a ubiquitin-conjugating enzyme (E2), such as the putative E2 Rad6; and subsequently covalently attached to substrate proteins by a ubiquitin ligase (E3). E3 is responsible for substrate selection and specificity. Addition of a single ubiquitin molecule to a substrate generates monoubiquitinated proteins, whereas multiple rounds of ubiquitination generate multi- or polyubiquitinated proteins. Polyubiquitinated proteins are preferentially targeted to the proteasome for degradation. The Skp1-Cullin-F-box (SCF) complex (consisting of the linker protein Skp1, scaffold protein Cdc53, and F-box protein) and the anaphase-promoting complex/cyclosome (APC/C) complex (including the coactivators Cdc20 and Cdh1) are important for proteasome-mediated turnover of proteins involved in the cell cycle and filamentation. UPP proteins and substrates with known roles in regulating C. albicans filamentation are indicated in bold.

Figure 3.. Hsp90 regulates C. albicans morphogenesis through its interactions with client proteins

(A) Hsp90 and its cochaperone Sgt1 physically interact with Cyr1, keeping it in an inactive conformation, repressing morphogenesis, and maintaining the yeast form of growth. Hsp90 also binds Hsf1, repressing Hsf1 activity. Hsp90, along with its cochaperone Cdc37, maintains the stability of Cdc28, ensuring proper cell-cycle progression. The proteasome maintains protein homeostasis through regulated degradation of polyubiquitinated proteins. (B) Conditions that compromise Hsp90’s function, such as elevated temperature or inhibition of Hsp90 or the proteasome, relieve repression of Hsf1 and the cAMP-PKA pathway, inducing the heat shock response and morphogenesis, respectively. Inhibition of Hsp90 also leads to Cdc28 destabilization and degradation by the proteasome, further affecting cell-cycle progression and influencing morphogenesis via Bub2, Pho85-Pcl1-Hms1 signaling, and the mitotic exit network. The role of Hsf1 in morphogenesis upon compromise of Hsp90 is unknown (indicated by dashed lines).

Figure 4.. Morphogenesis induced by perturbation of cell-cycle or proteostasis signals through the cAMP-PKA pathway

(A) Signaling through the cAMP-PKA pathway is required for filamentation in response to many cues. (B) Disrupting cell-cycle progression by treating with the DNA synthesis inhibitor hydroxyurea (in purple), depleting the checkpoint protein Rad52 (in blue), or loss of the polo-like kinase Cdc5 (in green) induces filamentation in a manner that depends on the adenyl-cyclase Cyr1 but is independent of the PKA transcription factor Efg1, implicating PKA targets that remain to be identified. Filaments induced by hydroxyurea treatment, but not Cdc5 depletion, also require Ras1. The dependence of filaments induced by depletion of Rad52 on Ras1 is unknown (indicated with dotted lines). Hyphal growth in response to depletion of Cln3 requires signaling through Cyr1 but has only moderate dependence on Ras1 and Efg1 (indicated by dashed lines). (C) Filamentation induced by inhibition of Hsp90 (in orange) or the proteasome (in yellow) or by depletion of Hsf1 (in red) requires signaling through Ras1, Cyr1, and the PKA subunits Tpk1 and Tpk2. Although filaments induced by Hsf1 depletion depend on Efg1, inhibition of Hsp90 or the proteasome induces filamentation through additional, unknown transcription factors downstream of PKA. Hsf1 overexpression (in pink) induces filamentation independently of cAMP-PKA signaling but requires Efg1.