The role of nickel in the survival of the basidiomycete fungus Cryptococcus neoformans

Abstract

Cryptococcus neoformans is a basidiomycete fungus that is found throughout the world living as a saprophyte. It is an opportunistic pathogen in humans and prevalence has increased significantly with the spread of HIV-AIDS, particularly in sub-Saharan Africa. C. neoformans infects the body via inhalation into the lung and dissemination around the body, including the central nervous system. Infection of the brain is fatal if untreated. The virulence of this organism is dependent on the enzyme urease, which catalyses the degradation of urea to ammonia and carbamate. The molecular role of urease during infection is not clear, although there is evidence that it is involved in the ability of C. neoformans to cross the blood brain barrier. Urease enzymes require an iron or nickel cofactor to function, which suggests that acquisition and distribution of this cofactor is important for the virulence of this yeast. The homeostasis of transition metals in living organisms is tightly regulated by a number of mechanisms including; metallochaperones, selective metal permeases, metal-sensors and compartmentalisation. Cases of in vivo mis-population of metal proteins are extremely rare. In this project C. neoformans urease is identified as a nickel binding enzyme and regulation of urease activity in response to the available nitrogen source is characterised. The accumulation of nickel in C. neoformans is responsive to the available nitrogen source and a putative nickel-importer, CnNic1, is identified as the primary means of nickel accumulation. The urease accessory protein CnUreG is essential for urease maturation and binds two equivalents of nickel with high affinity. CnUreG is proposed to be a novel nickel chaperone for urease. Cobalt inhibits cryptococcal urease in vivo by binding to the protein and preventing nickel insertion into the active site. Urease inhibition by cobalt occurs at relatively low cobalt concentrations and therefore C. neoformans does not appear to have evolved effective mechanisms to protect against cobalt mis-population.