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Vol. 17, Issue 12, 5131-5140, December 2006

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A Eukaryotic Capsular Polysaccharide Is Synthesized Intracellularly and Secreted via Exocytosis

Department of Molecular Microbiology, Washington University School of Medicine, St. Louis, MO 63110

Submitted August 10, 2006; Revised September 18, 2006; Accepted September 26, 2006
Monitoring Editor: Sean Munro

Cryptococcus neoformans, which causes fatal infection in immunocompromised individuals, has an elaborate polysaccharide capsule surrounding its cell wall. The cryptococcal capsule is the major virulence factor of this fungal organism, but its biosynthetic pathways are virtually unknown. Extracellular polysaccharides of eukaryotes may be made at the cell membrane or within the secretory pathway. To test these possibilities for cryptococcal capsule synthesis, we generated a secretion mutant in C. neoformans by mutating a Sec4/Rab8 GTPase homolog. At a restrictive temperature, the mutant displayed reduced growth and protein secretion, and accumulated 100-nm vesicles in a polarized manner. These vesicles were not endocytic, as shown by their continued accumulation in the absence of polymerized actin, and could be labeled with anti-capsular antibodies as visualized by immunoelectron microscopy. These results indicate that glucuronoxylomannan, the major cryptococcal capsule polysaccharide, is trafficked within post-Golgi secretory vesicles. This strongly supports the conclusion that cryptococcal capsule is synthesized intracellularly and secreted via exocytosis.

This article was published online ahead of print in MBC in Press (http://www.molbiolcell.org/cgi/doi/10.1091/mbc.E06-08-0701) on October 4, 2006.

Address correspondence to: Tamara L. Doering (doering{at}borcim.wustl.edu)

Abbreviations used: GXM, glucuronoxylomannan; GalXM, galactoxylomannan.

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