Mitochondrial Sorting and Assembly Machinery Subunit Sam37 in Candida albicans: Insight into the Roles of Mitochondria in Fitness, Cell Wall Integrity, and Virulence

Abstract

f Recent studies indicate that mitochondrial functions impinge on cell wall integrity, drug tolerance, and virulence of human fungal pathogens. However, the mechanistic aspects of these processes are poorly understood. We focused on the mitochondrial outer membrane SAM (Sorting and Assembly Machinery) complex subunit Sam37 in Candida albicans. Inactivation of SAM37 in C. albicans leads to a large reduction in fitness, a phenotype not conserved with the model yeast Saccharomyces cerevisiae. Our data indicate that slow growth of the sam37⌬⌬ mutant results from mitochondrial DNA loss, a new function for Sam37 in C. albicans, and from reduced activity of the essential SAM complex subunit Sam35. The sam37⌬⌬ mutant was hypersensitive to drugs that target the cell wall and displayed altered cell wall structure, supporting a role for Sam37 in cell wall integrity in C. albicans. The sensitivity of the mutant to membrane-targeting antifungals was not significantly altered. The sam37⌬⌬ mutant was avirulent in the mouse model, and bioinformatics showed that the fungal Sam37 proteins are distant from their animal counterparts and could thus represent potential drug targets. Our study provides the first direct evidence for a link between mitochondrial function and cell wall integrity in C. albicans and is further relevant for understanding mitochondrial function in fitness, antifungal drug tolerance, and virulence of this major pathogen. Beyond the relevance to fungal pathogenesis, this work also provides new insight into the mitochondrial and cellular roles of the SAM complex in fungi.

Exploiting mitochondria as targets for the development of new antifungals

“…[33][34][35][36] Biogenesis usually includes both the SAM (Sorting and Assembly Machinery) and the ERMES (ER-Mitochondrial Encounter Structure) complexes of proteins that are vitally important in the insertion of proteins in mitochondria through the Transporters of the Outer Membrane (TOMs) (Fig. 1).…”

Masking of β(1-3)-Glucan in the Cell Wall of Candida albicans from Detection by Innate Immune Cells Depends on Phosphatidylserine

“…The mechanism by which PS controls ␤(1-3)-glucan masking could be related to effects of PS on cell wall signal transduction pathways, general endocytic trafficking, or alterations of the plasma membrane (35,36). A number of cell wall signaling pathways are directly affected by PS, including the protein kinase C (Pkc1p) and calcineurin pathways (37)(38)(39)(40)(41).…”

A model system for mitochondrial biogenesis reveals evolutionary rewiring of protein import and membrane assembly pathways

“…The wildtype strains were DAY185 (URA3 + ARG4 + HIS1 + ) and DAY286 (URA3 + ARG4 + ) (52). The C. albicans sam37ΔΔ mutant and the sam37ΔΔ mutant overexpressing SAM35 (sam37ΔΔ SAM35↑↑) under the strong constitutive promoter TEF1 have been described previously (35). The sam51ΔΔ mutant was constructed by standard methods using PCR and homologous recombination, with the URA3 and ARG4-based selection cassettes (48).…”

“…In S. cerevisiae, the import and assembly of VDAC depends largely on the SAM complex subunit Sam37 (33). In both S. cerevisiae and C. albicans, deletion of Sam37 leads to a concomitant reduction in levels of Sam35, and this reduction can be suppressed by overexpression of the SAM35 gene (34,35), With a sam37ΔΔ strain of C. albicans engineered to have one copy of the SAM35 gene under the control of the strong TEF1 promoter, a 9.5-fold up-regulation of SAM35 transcript levels was measured in the mutant (sam37ΔΔSAM35↑↑) (Fig. 3C).…”