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.