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J. Biol. Chem., Vol. 281, Issue 4, 1868-1875, January 27, 2006

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The Physical Properties of the Capsular Polysaccharides from Cryptococcus neoformans Suggest Features for Capsule Construction^*

Diane C. McFadden1, Magdia De Jesus2, and Arturo Casadevall3

From the Department of Medicine, Division of Infectious Disease, and Department of Microbiology and Immunology, Albert Einstein College of Medicine, Bronx, New York 10461

The most distinctive feature of the human pathogenic fungus is a polysaccharide capsule that is essential for virulence and is composed primarily of glucuronoxylomannan (GXM) and galactoxylomannan (GalXM). GXM mediates multiple deleterious effects on host immune function, yet relatively little is known about its physical properties. The average mass of Cryptococcus neoformans GXM from four antigenically different strains ranged from 1.7 to 7 x 10⁶ daltons as calculated from Zimm plots of light-scattering data. GalXM was significantly smaller than GXM, with an average mass of 1 x 10⁵ daltons. These molecular masses imply that GalXM is the most numerous polysaccharide in the capsule on a molar basis. The radius of gyration of the capsular polysaccharides ranged between 68 and 208 nm. Viscosity measurements suggest that neither polysaccharide altered fluid dynamics during infection since GXM behaved in solution as a polyelectrolyte and GalXM did not increase solution viscosity. Immunoblot analysis indicated heterogeneity within GXM. In agreement with this, scanning transmission electron microscopy of GXM preparations revealed a tangled network of two different types of molecules. Mass per length measurements from light scattering and scanning transmission electron microscopy were consistent and suggested GXM molecules self-associate. A mechanism for capsule growth is proposed based on the extracellular release and entanglement of GXM molecules.

Received for publication, August 26, 2005 , and in revised form, November 3, 2005.

^* This work was supported in part by National Institutes of Health Grants AI33774, AI33142, and HL59842–01 (to A. C.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ A Burroughs Wellcome Fund Fellow of the Life Science Research Foundation.

² Supported by NCI, National Institutes of Health Training Grant 2T32CA009173-31.

³ To whom correspondence should be addressed: Albert Einstein College of Medicine, 1300 Morris Park Ave., Golding 701, Bronx, NY 10461. Tel.: 718-430-3665; Fax: 718-430-8701; E-mail: casadeva{at}aecom.yu.edu.

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