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Melanin is a major virulence factor in pathogenic fungi that enhances the ability of fungal cells to resist immune clearance. Cryptococcus neoformans is an important human pathogenic fungus that synthesizes melanin from exogenous tissue catecholamine precursors during infection, but the type of melanin made in cryptococcal meningoencephalitis is unknown. We analyzed the efficacy of various catecholamines found in brain tissue in supporting melanization using animal brain tissue and synthetic catecholamine mixtures reflecting brain tissue proportions. Solid-state NMR spectra of the melanin pigment produced from such mixtures yielded more melanin than expected if only the preferred constituent dopamine had been incorporated, suggesting uptake of additional catecholamines. Probing the biosynthesis of melanin using radiolabeled catecholamines revealed that C. neoformans melanization simultaneously incorporated more than one catecholamine, implying that the pigment was polytypic in nature. Nonetheless, melanin derived from individual or mixed catecholamines had comparable ability to protect C. neoformans against ultraviolet light and oxidants. Our results indicate that melanin produced during infection differs depending on the catecholamine composition of tissue and that melanin pigment synthesized in vivo is likely to accrue from the polymerization of a mixture of precursors. From a practical standpoint, our results strongly suggest that using dopamine as a polymerization precursor is capable of producing melanin pigment comparable to that produced during infection. On a more fundamental level, our findings uncover additional structural complexity for natural cryptococcal melanin by demonstrating that pigment produced during human infection is likely to be composed of polymerized moieties derived from chemically different precursors., Competing Interests: Conflict of interest Dr Arturo Casadevall reports financial support provided by the National Institutes of Health. The remaining authors declare no conflict of interest., (Copyright © 2021 The Authors. Published by Elsevier Inc. All rights reserved.)

Mog1 is a nuclear protein that interacts with Ran, the Ras family GTPase that confers directionality to nuclear import and export pathways. Deletion of MOG1 in Saccharomyces cerevisiae (Deltamog1) causes temperature-sensitive growth and defects in nuclear protein import. Mog1 has previously been shown to stimulate GTP release from Ran and we demonstrate here that addition of Mog1 to either Ran-GTP or Ran-GDP results in nucleotide release and formation of a stable complex between Mog1 and nucleotide-free Ran. Moreover, MOG1 shows synthetic lethality with PRP20, the Ran guanine nucleotide exchange factor (RanGEF) that also binds nucleotide-free Ran. To probe the functional role of the Mog1-Ran interaction, we engineered mutants of yeast Mog1 and Ran that specifically disrupt their interaction both in vitro and in vivo. These mutants indicate that the interaction interface involves conserved Mog1p residues Asp(62) and Glu(65), and residue Lys(136) in yeast Ran. Mutations at these residues decrease the ability of Mog1 to bind and release nucleotide from Ran. Furthermore, the E65K-Mog1 and K136E-Ran mutations in yeast cause temperature sensitivity and mislocalization of a nuclear import reporter protein, similar to the phenotype observed for the Deltamog1 strain. Our results indicate that a primary function of Mog1 requires binding to Ran and that the Mog1-Ran interaction is necessary for efficient nuclear protein import in vivo.

The base analog, 2-aminopurine (2AP), was used as a fluorescent reporter of the biochemical steps in the proofreading pathway catalyzed by bacteriophage T4 DNA polymerase. "Mutator" DNA polymerases that are defective in different steps in the exonucleolytic proofreading pathway were studied so that transient changes in fluorescence intensity could be equated with specific reaction steps. The G255S- and D131N-DNA polymerases can hydrolyze DNA, the final step in the proofreading pathway, but the mutator phenotype indicates a defect in one or more steps that prepare the primer-terminus for the cleavage reaction. The hydrolysis-defective D112A/E114A-DNA polymerase was also examined. Fluorescent enzyme-DNA complexes were preformed in the absence of Mg2+, and then rapid mixing, stopped-flow techniques were used to determine the fate of the fluorescent complexes upon the addition of Mg2+. Comparisons of fluorescence intensity changes between the wild type and mutant DNA polymerases were used to model the exonucleolytic proofreading pathway. These studies are consistent with a proofreading pathway in which the protein loop structure that contains residue Gly255 functions in strand separation and transfer of the primer strand from the polymerase active center to form a preexonuclease complex. Residue Asp131 acts at a later step in formation of the preexonuclease complex.