Mrj1HA AP-MS in Cryptococcus neoformans

The opportunistic fungal pathogen Cryptococcus neoformans must adapt to the mammalian environment to establish an infection. Proteins facilitating adaptation to novel environments, such as chaperones, may be required for virulence. In this study, we identified a novel mitochondrial co-chaperone, Mrj1, necessary for virulence in C. neoformans. The mrj1∆ and J-domain inactivated mutants had general growth defects, and were deficient in two major virulence factors, thermotolerance and capsule elaboration. The latter phenotype was associated with cell wall changes and increased capsular polysaccharide shedding. Accordingly, the mrj1∆ mutant was avirulent in a murine model of cryptococcosis. Mrj1 has a mitochondrial localization and co-immunoprecipitated with Qcr2, a core component of complex III of the ETC (Electron Transport Chain). The mrj1 mutants were deficient in mitochondrial functions including growth on alternative carbon sources, growth without iron, and mitochondrial polarization. They were also insensitive to complex III inhibitors and hypersensitive to an alternative oxidase (AOX) inhibitor suggesting that Mrj1 functions in respiration. In support of this conclusion, mrj1 mutants also had elevated basal oxygen consumption rates which were completely abolished by the addition of the AOX inhibitor, confirming that Mrj1 is required for mitochondrial respiration through complexes III and IV. Furthermore, inhibition of complex III phenocopied the capsule and cell wall defects of the mrj1 mutants. Taken together, these results indicate that Mrj1 is required for normal mitochondrial respiration, a key aspect of adaptation to the host environment and virulence.