Relative Contributions of Prenylation and Postprenylation Processing in Cryptococcus neoformans Pathogenesis.


Prenyltransferase enzymes promote the membrane localization of their target proteins by directing the attachment of a hydrophobic lipid group at a conserved C-terminal CAAX motif. Subsequently, the prenylated protein is further modified by postprenylation processing enzymes that cleave the terminal 3 amino acids and carboxymethylate the prenylated cysteine residue. Many prenylated proteins, including Ras1 and Ras-like proteins, require this multistep membrane localization process in order to function properly. In the human fungal pathogen Cryptococcus neoformans, previous studies have demonstrated that two distinct forms of protein prenylation, farnesylation and geranylgeranylation, are both required for cellular adaptation to stress, as well as full virulence in animal infection models. Here, we establish that the C. neoformans RAM1 gene encoding the farnesyltransferase β-subunit, though not strictly essential for growth under permissive in vitro conditions, is absolutely required for cryptococcal pathogenesis. We also identify and characterize postprenylation protease and carboxyl methyltransferase enzymes in C. neoformans. In contrast to the prenyltransferases, deletion of the genes encoding the Rce1 protease and Ste14 carboxyl methyltransferase results in subtle defects in stress response and only partial reductions in virulence. These postprenylation modifications, as well as the prenylation events themselves, do play important roles in mating and hyphal transitions, likely due to their regulation of peptide pheromones and other proteins involved in development. IMPORTANCE Cryptococcus neoformans is an important human fungal pathogen that causes disease and death in immunocompromised individuals. The growth and morphogenesis of this fungus are controlled by conserved Ras-like GTPases, which are also important for its pathogenicity. Many of these proteins require proper subcellular localization for full function, and they are directed to cellular membranes through a posttranslational modification process known as prenylation. These studies investigate the roles of one of the prenylation enzymes, farnesyltransferase, as well as the postprenylation processing enzymes in C. neoformans. We demonstrate that the postprenylation processing steps are dispensable for the localization of certain substrate proteins. However, both protein farnesylation and the subsequent postprenylation processing steps are required for full pathogenesis of this fungus.





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Publication Info

Esher, Shannon K, Kyla S Ost, Lukasz Kozubowski, Dong-Hoon Yang, Min Su Kim, Yong-Sun Bahn, J Andrew Alspaugh, Connie B Nichols, et al. (2016). Relative Contributions of Prenylation and Postprenylation Processing in Cryptococcus neoformans Pathogenesis. mSphere, 1(2). 10.1128/mSphere.00084-15 Retrieved from

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James Andrew Alspaugh

Professor of Medicine

The focus of my research is to understand the ways in which microorganisms sense and respond to changes in their environment. As microbial pathogens enter the infected host, dramatic genetic and phenotypic events occur that allow these organisms to survive in this harsh environment. We study the model fungal organism Cryptococcus neoformans to define signal transduction pathways associated with systemic fungal diseases. This pathogenic fungus causes lethal infections of the central nervous system in patients with AIDS and other immunological disorders. In addition to being an important pathogen, C. neoformans displays well-characterized and inducible virulence determinants. It is an outstanding system for dissecting the signaling pathways associated with pathogenicity.

The main techniques used in the lab are those of molecular genetics. We are able to readily mutate C. neoformans genes by homologous recombination. Mutant strains with disruptions in targeted genes are then evaluated in vitro for various phenotypes including altered expression of polysaccharide capsule and melanin. The effects of gene disruption on pathogenicity are also evaluated in animal models of cryptococcal disease. Using these techniques, we have identified a novel G-alpha protein/cAMP-dependent signaling pathway associated with mating and pathogenicity.

This research is complemented by the other investigators in the Duke University Mycology Research Unit. The members of this research community are pursuing studies in fungal pathogenesis, identifying novel antifungal drug targets, and studying the ecology of several medically important fungi.

Keywords: Microbial Pathogenesis
Cryptococcus neoformans
Signal transduction
Fungal mating
G proteins

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