The Yeast Phase of Histoplasma Capsulatum Secretes: A Low Molecular Weight Calcium Binding Protein
Abstract
Histoplasma capsulatum is a dimorphic fungus that is able to survive and grow
within macrophages. It is the causative agent of histoplasmosis, one of the opportunistic
infections associated with the Acquired Immune Deficiency Syndrome. Little is known
about the mechanism of pathogenicity of this organism. Recently, it has been reported
that phagosomes have a very low concentration of calcium (Pollack et al., 1986). Other
organisms, such as Toxoplasma gondii, have adapted to life in this environment by
secreting proteins which bind calcium. Does H. capsulatum use a similar adaptive
strategy? To answer this question the calcium binding protein detection method
developed by Maruyama et ale (1984) was utilized. The supernatant from cultures of H.
capsulatum was concentrated by ultrafiltration to remove salts and other contaminants.
After subjecting the proteins to either sodium dodecyl sulfate polyacrylamide gel
electrophoresis or non-denaturing gel electrophoresis, a Western blot analysis was
performed. A calcium binding protein with a molecular weight of about 6kd was
apparent on the autoradiograph of the denaturing gel. This calcium binding protein was
seen in ten different strains of H. capsulatum that correspond to four evolutionary
classes. The protein appears to be highly conserved throughout these evolutionarily
diverse strains and may be important in the virulence of the organism.