Evolutionary Selection on Barrier Activity: Bar1 Is an Aspartyl Protease with Novel Substrate Specificity

ABSTRACTPeptide-based pheromones are used throughout the fungal kingdom for coordinating sexual responses between mating partners. Here, we address the properties and function of Bar1, an aspartyl protease that acts as a “barrier” and antagonist to pheromone signaling in multiple species.Candida albicansBar1 was purified and shown to exhibit preferential cleavage of native α pheromone over pheromones from related fungal species. This result establishes that protease substrate specificity coevolved along with changes in its pheromone target. Pheromone cleavage by Bar1 occurred between residues Thr-5 and Asn-6 in the middle of the tridecapeptide sequence. Surprisingly, proteolytic activity was independent of the amino acid residues present at the scissile bond and instead relied on residues at the C terminus of α pheromone. Unlike most aspartyl proteases, Bar1 also exhibited a near-neutral pH optimum and was resistant to the class-wide inhibitor pepstatin A. In addition, genetic analysis was performed onC. albicansBAR1and demonstrated that the protease not only regulates endogenous pheromone signaling but also can limit interspecies pheromone signaling. We discuss these findings and propose that the unusual substrate specificity of Bar1 is a consequence of its coevolution with the α pheromone receptor Ste2 for their shared peptide target.IMPORTANCEPheromones are important for intraspecies communication across the tree of life. In the fungal kingdom, extracellular proteases play a key role in antagonizing pheromone signaling in multiple species. This study examines the properties and function ofCandida albicansBar1, an aspartyl protease that cleaves and thereby inactivates α pheromone. We demonstrate that Bar1 plays important roles in regulating both intra- and interspecies pheromone signaling. The fungal protease shows preferential activity on the endogenous pheromone, but, surprisingly, cleavage activity is dependent on amino acid residues distal to the scissile bond. We propose that the unusual substrate specificity of Bar1 is a direct result of coevolution with Ste2, the receptor for α pheromone, for recognition of the same peptide target. The novel specificity of Bar1 reveals the complex forces shaping the evolution of mating pathways in fungi and uncovers a protease with potentially important applications in the biotechnology industry.

Pheromone-encoding mRNA is transported to the yeast mating projection by specific RNP granules

Association of messenger RNAs with large complexes such as processing bodies (PBs) plays a pivotal role in regulating their translation and decay. Little is known about other possible functions of these assemblies. Exposure of haploid yeast cells, carrying mating type “a,” to “α pheromone” stimulates polarized growth resulting in a “shmoo” projection; it also induces synthesis of “a pheromone,” encoded by MFA2. In this paper, we show that, in response to α pheromone, MFA2 mRNA is assembled with two types of granules; both contain some canonical PB proteins, yet they differ in size, localization, motility, and sensitivity to cycloheximide. Remarkably, one type is involved in mRNA transport to the tip of the shmoo, whereas the other—in local translation in the shmoo. Normal assembly of these granules is critical for their movement, localization, and for mating. Thus, MFA2 mRNAs are transported to the shmoo tip, in complex with PB-like particles, where they are locally translated.

Defining pheromone-receptor signaling inCandida albicansand related asexualCandidaspecies

Candida albicans is an important human fungal pathogen in which sexual reproduction is under the control of the novel white–opaque switch. Opaque cells are the mating-competent form, whereas white cells do not mate but can still respond to pheromones, resulting in biofilm formation. In this study, we first define the domains of the α-pheromone receptor Ste2 that are necessary for signaling in both white and opaque forms. Both cell states require the IC loop 3 (IC3) and the C-terminal tail of Ste2 for the cellular response, whereas the first IC loop (IC1) of Ste2 is dispensable for signaling. To also address pheromone-receptor interactions in related species, including apparently asexual Candida species, Ste2 orthologues were heterologously expressed in Candida albicans. Ste2 receptors from multiple Candida clade species were functional when expressed in C. albicans, whereas the Ste2 receptor of Candida lusitaniae was nonfunctional. Significantly, however, expression of a chimeric C. lusitaniae Ste2 receptor containing the C-terminal tail of Ste2 from C. albicans generated a productive response to C. lusitaniae pheromone. This system has allowed us to characterize pheromones from multiple Candida species and indicates that functional pheromone-receptor couples exist in fungal species that have yet to be shown to undergo sexual mating.

Self-Induction ofa/aor α/α Biofilms in Candida albicans Is a Pheromone-Based Paracrine System Requiring Switching

ABSTRACTLikeMTL-heterozygous (a/α) cells, whiteMTL-homozygous (a/aor α/α) cells ofCandida albicans, to which a minority of opaque cells of opposite mating type have been added, form thick, robust biofilms. The latter biofilms are uniquely stimulated by the pheromone released by opaque cells and are regulated by the mitogen-activated protein kinase signal transduction pathway. However, whiteMTL-homozygous cells, to which opaque cells of opposite mating type have not been added, form thinner biofilms. Mutant analyses reveal that these latter biofilms are self-induced. Self-induction ofa/abiofilms requires expression of the α-receptor geneSTE2and the α-pheromone geneMFα, and self-induction of α/α biofilms requires expression of thea-receptor geneSTE3and thea-pheromone geneMFa. In both cases, deletion ofWOR1, the master switch gene, blocks cells in the white phenotype and biofilm formation, indicating that self-induction depends upon low frequency switching from the white to opaque phenotype. These results suggest a self-induction scenario in which minority opaquea/acells formed by switching secrete, in a mating-type-nonspecific fashion, α-pheromone, which stimulates biofilm formation through activation of the α-pheromone receptor of majority whitea/acells. A similar scenario is suggested for a white α/α cell population, in which minority opaque α/α cells secretea-pheromone. This represents a paracrine system in which one cell type (opaque) signals a second highly related cell type (white) to undergo a complex response, in this case the formation of a unisexual white cell biofilm.

The White Cell Response to Pheromone Is a General Characteristic of Candida albicans Strains

ABSTRACT For Candida albicans, evidence has suggested that the mating pheromones activate not only the mating response in mating-competent opaque cells but also a unique response in mating-incompetent white cells that includes increased cohesion and adhesion, enhanced biofilm formation, and expression of select mating-related and white cell-specific genes. On the basis of a recent microarray analysis comparing changes in the global expression patterns of white cells in two strains in response to α-pheromone, however, skepticism concerning the validity and generality of the white cell response has been voiced. Here, we present evidence that the response occurs in all tested media (Lee's, RPMI, SpiderM, yeast extract-peptone-dextrose, and a synthetic medium) and in all of the 27 tested strains, including a/a and α/α strains, derivatives of the common laboratory strain SC5314, and representatives from all of the five major clades. The white cell response to pheromone is therefore a general characteristic of MTL-homozygous strains of C. albicans.

Unique Aspects of Gene Expression during Candida albicans Mating and Possible G1 Dependency

ABSTRACT Taking advantage of the high frequency of conjugation tube formation in mating mixtures and α-pheromone-treated a/a cells derived from saturation phase cultures of opaque cells of Candida albicans, 56 up-regulated and 30 down-regulated genes were identified employing microarray and Northern analyses. Combining these results with previous profiling studies of pheromone-induced cells, a more comprehensive transcript profile was developed for comparison with Saccharomyces cerevisiae. This comparison revealed the following: (i) that while a majority of mating-associated genes are regulated similarly between the two species, a significant minority are regulated dissimilarly; (ii) that filamentation genes are uniquely up-regulated and opaque-specific genes uniquely down-regulated during C. albicans mating; and (iii) that a newly identified class of genes is selectively down-regulated in opaque, but not white, cells that have entered saturation phase in a growth culture and then are up-regulated by pheromone. The observations that opaque cells are uniquely mating competent, that saturation phase facilitates mating, and that a newly identified group of genes is down-regulated only in opaque cells that have entered saturation phase led us to hypothesize that entering saturation phase may be requisite for mating. A test of this hypothesis revealed, however, that cells, whether in the exponential or saturation phase, may simply have to be in G1 of the cell cycle to respond to pheromone and that the response includes G1 arrest. These results add to the lists of similarities and dissimilarities between the mating processes of C. albicans and S. cerevisiae and underscore the unique regulation of filamentation and switching genes in the C. albicans mating process.

The Adhesin Hwp1 and the First Daughter Cell Localize to the a/a Portion of the Conjugation Bridge duringCandida albicansMating

The cell wall protein Hwp1 was originally demonstrated to be expressed exclusively in hyphae of Candida albicans and cross-linked to human epithelium by mammalian transglutaminase. Hwp1 is expressed on the walls of hyphae formed by a/α, a/a, and α/α cells. Hence, it is expressed on hyphae independently of mating type. However, Hwp1 is selectively expressed on the wall of conjugation tubes formed by a/a cells, but not α/α cells, in the mating process. This was demonstrated in all possible crosses between four unrelated natural a/a strains and four unrelated α/α strains. In zygotes, Hwp1 is restricted to that portion of the wall of the conjugation bridge contributed by the a/a parent cell. Hwp1 staining further revealed that the first daughter bud that emerges from the conjugation bridge does so from the a/a-contributed portion. Hwp1 expression and localization during the mating process is, therefore, mating type specific, opaque phase specific, and α-pheromone induced. These results indicate that the mating type-specific contributions to the conjugation bridge during the mating process in C. albicans are qualitatively and functionally distinct and that the a/a portion of the bridge, which selectively contains Hwp1, bears the first daughter cell in the mating process.