Protein Composition of Infectious Spores Reveals Novel Sexual Development and Germination Factors in Cryptococcus

Abstract

Spores are an essential cell type required for long-term survival across diverse organisms in the tree of life and are a hallmark of fungal reproduction, persistence, and dispersal. Among human fungal pathogens, spores are presumed infectious particles, but relatively little is known about this robust cell type. Here we used the meningitis-causing fungus Cryptococcus neoformans to determine the roles of spore-resident proteins in spore biology. Using highly sensitive nanoscale liquid chromatography/mass spectrometry, we compared the proteomes of spores and vegetative cells (yeast) and identified eighteen proteins specifically enriched in spores. The genes encoding these proteins were deleted, and the resulting strains were evaluated for discernable phenotypes. We hypothesized that spore-enriched proteins would be preferentially involved in spore-specific processes such as dormancy, stress resistance, and germination. Surprisingly, however, the majority of the mutants harbored defects in sexual development, the process by which spores are formed. One mutant in the cohort was defective in the spore-specific process of germination, showing a delay specifically in the initiation of vegetative growth. Thus, by using this in-depth proteomics approach as a screening tool for cell type-specific proteins and combining it with molecular genetics, we successfully identified the first germination factor in C. neoformans. We also identified numerous proteins with previously unknown functions in both sexual development and spore composition. Our findings provide the first insights into the basic protein components of infectious spores and reveal unexpected molecular connections between infectious particle production and spore composition in a pathogenic eukaryote.

Author Summary

Spores are a critical cell type that allow long-term survival of diverse organisms from bacteria to fungi to plants. Among fungi, spores are often formed when growth conditions are poor; spores can then disperse to more favorable environments and reinitiate growth. Spores of some environmental fungi can cause fatal disease in humans. Here we used the meningitis-causing yeast Cryptococcus neoformans to determine the roles of spore-enriched proteins in spore biology. Using a combined proteomics-genetics approach, we identified eighteen spore-enriched proteins, knocked out the genes encoding each of them, and assessed the resulting strains for phenotypes in a broad array of assays. We predicted that mutant strains would be likely to show defects in spore-specific processes, but instead, we discovered that the majority harbored defects in sexual development, the process by which spores are formed. Only one mutant exhibited a defect in a spore-specific process (germination). Our data reveal that many spore-represented proteins are associated with pre-spore developmental processes, rather than intrinsic spore-specific properties or processes. These findings indicate a previously unknown molecular connection between the developmental process that results in spore biogenesis and the composition of infectious spores.