Sec15 links bud site selection to polarised cell growth and exocytosis in Candida albicans

Emerging Roles of Exocyst Complex in Fungi: A Review

The exocyst complex, an evolutionarily conserved octameric protein assembly, plays a central role in the targeted binding and fusion of vesicles at the plasma membrane. In fungal cells, this transport system is essential for polarized growth, morphogenesis, cell wall maintenance and virulence. Recent advances have greatly improved our understanding of the role and regulation of the exocyst complex in fungi. This review synthesizes these developments and focuses on the intricate interplay between the exocyst complex, specific fungal cargos and regulatory proteins. Insights into thestructure of the exocyst and its functional dynamics have revealed new dimensions of its architecture and its interactions with the cellular environment. Furthermore, the regulation of exocyst activity involves complex signaling pathways and interactions with cytoskeletal elements that are crucial for its role in vesicle trafficking. By exploring these emerging themes, this review provides a comprehensive overview of the multifaceted functions of the exocyst complex in fungal biology. Understanding these mechanisms offers potential avenues for novel therapeutic strategies against fungal pathogens and insights into the general principles of vesicle trafficking in eukaryotic cells. The review therefore highlights the importance of the exocyst complex in maintaining cellular functions and its broader implications in fungal pathogenicity and cell biology.

Septins function in exocytosis via physical interactions with the exocyst complex in fission yeast cytokinesis

Septins can function as scaffolds for protein recruitment, membrane-bound diffusion barriers, or membrane curvature sensors. Septins are important for cytokinesis, but their exact roles are still obscure. In fission yeast, four septins (Spn1 to Spn4) accumulate at the rim of the division plane as rings. The octameric exocyst complex, which tethers exocytic vesicles to the plasma membrane, exhibits a similar localization and is essential for plasma membrane deposition during cytokinesis. Without septins, the exocyst spreads across the division plane but absent from the rim during septum formation. These results suggest that septins and the exocyst physically interact for proper localization. Indeed, we predicted six pairs of direct interactions between septin and exocyst subunits by AlphaFold2 ColabFold, most of them are confirmed by co-immunoprecipitation and yeast two-hybrid assays. Exocyst mislocalization results in mistargeting of secretory vesicles and their cargos, which leads to cell-separation delay in septin mutants. Our results indicate that septins guide the targeting of exocyst complex on the plasma membrane for vesicle tethering during cytokinesis through direct physical interactions.

The Paxillin MoPax1 Activates Mitogen-Activated Protein (MAP) Kinase Signaling Pathways and Autophagy through MAP Kinase Activator MoMka1 during Appressorium-Mediated Plant Infection by the Rice Blast Fungus Magnaporthe oryzae

Paxillin is a focal adhesion-associated protein that functions as an adaptor to recruit diverse cytoskeleton and signaling molecules into a complex and plays a crucial role in several signaling pathways in mammal cells. However, paxillin-mediated signal pathways are largely unknown in phytopathogenic fungi. Previously, Pax1 of Magnaporthe oryzae (MoPax1), a paxillin-like protein, has been identified as a crucial pathogenicity determinant. Here, we report the identification of a mitogen-activated protein (MAP) kinase (MAPK) activator, Mka1 of M. oryzae (MoMka1), that physically interacts with MoPax1. Targeted gene deletion of MoMKA1 resulted in pleiotropic defects in aerial hyphal growth, conidiation, appressorium formation, and pathogenicity in M. oryzae. MoMka1 interacts with Mst50, an adaptor protein of the Mst11-Mst7-Pmk1 and Mck1-Mkk2-Mps1 cascades. Moreover, the phosphorylation levels of both Pmk1 and Mps1 in aerial hyphae of the ΔMomka1 mutant were significantly reduced, indicating that MoMka1 acts upstream from the MAPK pathways. Interestingly, we found that MoMka1 interacts with MoAtg6 and MoAtg13. Deletion of MoMKA1 led to impaired MoAtg13 phosphorylation and enhanced autophagic flux under nutrient-rich conditions, indicating that MoMka1 is required for regulation of autophagy in M. oryzae. Taken together, the paxillin MoPax1 may activate MAP kinase signaling pathways and autophagy through MAP kinase activator MoMka1 and play important roles during appressorium-mediated plant infection by the rice blast fungus. IMPORTANCE Paxillin, as an adaptor recruiting diverse cytoskeleton and signaling molecules into a complex, plays a crucial role in several signaling pathways in mammal cells. However, paxillin-mediated signal pathways are largely unknown in phytopathogenic fungi. Here, we identified that MoMka1 physically interacts with MoPax1. Furthermore, MoMka1 acts upstream from the MAPK pathways through interacting with Mst50, a key protein of the Mst11-Mst7-Pmk1 and Mck1-Mkk2-Mps1 cascades. Meanwhile, MoMka1 interacts with both MoAtg6 and MoAtg13 and controls autophagy initiation by influencing the phosphorylation level of MoAtg13. In summary, we describe a model in which MoPax1 activates MAP kinase signaling pathways and autophagy through MoMka1 during appressorium-mediated plant infection by M. oryzae.

From Jekyll to Hyde: The Yeast-Hyphal Transition of Candida albicans

Candida albicans is a major fungal pathogen of humans, accounting for 15% of nosocomial infections with an estimated attributable mortality of 47%. C. albicans is usually a benign member of the human microbiome in healthy people. Under constant exposure to highly dynamic environmental cues in diverse host niches, C. albicans has successfully evolved to adapt to both commensal and pathogenic lifestyles. The ability of C. albicans to undergo a reversible morphological transition from yeast to filamentous forms is a well-established virulent trait. Over the past few decades, a significant amount of research has been carried out to understand the underlying regulatory mechanisms, signaling pathways, and transcription factors that govern the C. albicans yeast-to-hyphal transition. This review will summarize our current understanding of well-elucidated signal transduction pathways that activate C. albicans hyphal morphogenesis in response to various environmental cues and the cell cycle machinery involved in the subsequent regulation and maintenance of hyphal morphogenesis.

Rab11 is required for lysosome exocytosis through the interaction with Rab3a, Sec15 and GRAB

Lysosomes are dynamic organelles, capable of undergoing exocytosis. This process is crucial for several cellular functions, namely plasma membrane repair. Nevertheless, the molecular machinery involved in this process is poorly understood. Here, we identify Rab11a and Rab11b as regulators of Ca2+-induced lysosome exocytosis. Interestingly, Rab11-positive vesicles transiently interact with lysosomes at the cell periphery, indicating that this interaction is required for the last steps of lysosome exocytosis. Additionally, we found that the silencing of the exocyst subunit Sec15, a Rab11 effector, impairs lysosome exocytosis, suggesting that Sec15 acts together with Rab11 in the regulation of lysosome exocytosis. Furthermore, we show that Rab11 binds the guanine nucleotide exchange factor for Rab3a (GRAB) as well as Rab3a, which we have previously described to be a regulator of the positioning and exocytosis of lysosomes. Thus, our study identifies new players required for lysosome exocytosis and suggest the existence of a Rab11-Rab3a cascade involved in this process.

The Role of Secretory Pathways in Candida albicans Pathogenesis

Candida albicans is a fungus that is a commensal organism and a member of the normal human microbiota. It has the ability to transition into an opportunistic invasive pathogen. Attributes that support pathogenesis include secretion of virulence-associated proteins, hyphal formation, and biofilm formation. These processes are supported by secretion, as defined in the broad context of membrane trafficking. In this review, we examine the role of secretory pathways in Candida virulence, with a focus on the model opportunistic fungal pathogen, Candida albicans.

Candida albicans Hyphae: From Growth Initiation to Invasion

Candida albicans is a commensal resident of the human gastrointestinal and genital tracts. Under conditions such as dysbiosis, host immune perturbances, or the presence of catheters/implanted medical devices, the fungus may cause debilitating mucosal or fatal systemic infections. The ability of C. albicans to grow as long filamentous hyphae is critical for its pathogenic potential as it allows the fungus to invade the underlying substratum. In this brief review, I will outline the current understanding regarding the mechanistic regulation of hyphal growth and invasion in C. albicans.

Genome-Wide Screen for Haploinsufficient Cell Size Genes in the Opportunistic Yeast Candida albicans

One of the most critical but still poorly understood aspects of eukaryotic cell proliferation is the basis for commitment to cell division in late G1 phase, called Start in yeast and the Restriction Point in metazoans. In all species, a critical cell size threshold coordinates cell growth with cell division and thereby establishes a homeostatic cell size. While a comprehensive survey of cell size genetic determinism has been performed in the saprophytic yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe, very little is known in pathogenic fungi. As a number of critical Start regulators are haploinsufficient for cell size, we applied a quantitative analysis of the size phenome, using elutriation-barcode sequencing methodology, to 5639 barcoded heterozygous deletion strains of the opportunistic yeast Candida albicans. Our screen identified conserved known regulators and biological processes required to maintain size homeostasis in the opportunistic yeast C. albicans. We also identified novel C. albicans-specific size genes and provided a conceptual framework for future mechanistic studies. Interestingly, some of the size genes identified were required for fungal pathogenicity suggesting that cell size homeostasis may be elemental to C. albicans fitness or virulence inside the host.