Structural features responsible for the biological stability of Histoplasma's virulence factor CBP

Moriah R. Beck, Gregory T. DeKoster, David M. Hambly, Michael L. Gross, David P. Cistola, William E. Goldman

Research output: Contribution to journalArticlepeer-review

15 Scopus citations


The virulence factor CBP is the most abundant protein secreted by Histoplasma capsulatum, a pathogenic fungus that causes histoplasmosis. Although the biochemical function and pathogenic mechanism of CBP are unknown, quantitative Ca2+ binding measurements indicate that CBP has a strong affinity for calcium (KD = 6.45 ± 0.4 nM). However, no change in structure was observed upon binding of calcium, prompting a more thorough investigation of the molecular properties of CBP with respect to self-association, secondary structure, and stability. Over a wide range of pH values and salt concentrations, CBP exists predominantly as a stable, noncovalent homodimer in both its calcium-free and -bound states. Solution-state NMR and circular dichroism (CD) measurements indicated that the protein is largely α-helical, and its secondary structure content changes little over the range of pH values encountered physiologically. ESI-MS revealed that the six cysteine residues of CBP are involved in three intramolecular disulfide bonds that help maintain a highly protease resistant structure. Thermally and chemically induced denaturation studies indicated that unfolding of disulfide-intact CBP is reversible and provided quantitative measurements of protein stability. This disulfide-linked, protease resistant, homodimeric α-helical structure of CBP is likely to be advantageous for a virulence factor that must survive the harsh environment within the phagolysosomes of host macrophages.

Original languageEnglish
Pages (from-to)4427-4438
Number of pages12
Issue number15
StatePublished - Apr 15 2008


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