Copper-mediated reversal of defective laccase in a Δvph1 avirulent mutant of Cryptococcus neoformans

Abstract

Previous studies have shown that a Δvph1 Cryptococcus neoformans mutant defective in vesicular acidification lacked several important virulence factors including a copper-containing laccase and was avirulent in a mouse model. In the present studies, we characterized laccase transcription and protein production to obtain insights into the mechanism of the vph1 mutation in this pathogen. Although transcription and protein expression were somewhat reduced, laccase protein was found to be successfully translated and correctly targeted to the cell wall in the Δvph1 mutant as shown by Western blot and immunoelectron microscopy, despite a complete lack of laccase activity. Laccase activity was substantially restored in metabolically active Δvph1 cells at 30°C by addition of 100 μM copper sulphate. This restoration by copper was found to occur through both transcriptional and post-translational mechanisms. Laccase transcriptional induction by copper was found to be dependent on enhancer region II within the 5′-untranslated region of CNLAC1. Copper was also found to restore partial activity to Δvph1 cells at 0°C, suggesting that cell wall laccase was expressed in the mutant as an apo-enzyme. Apo-laccase restoration by copper was found to be facilitated by an acidic environment, consistent with a role for the vacuolar (H+)-ATPase proton pump in copper assembly of laccase in C. neoformans.