Cbp1, a rapidly evolving fungal virulence factor, forms an effector complex that drives macrophage lysis

Intracellular pathogens secrete effectors to manipulate their host cells. Histoplasma capsulatum (Hc) is a fungal intracellular pathogen of humans that grows in a yeast form in the host. Hc yeasts are phagocytosed by macrophages, where fungal intracellular replication precedes macrophage lysis. The most abundant virulence factor secreted by Hc yeast cells is Calcium Binding Protein 1 (Cbp1), which is absolutely required for macrophage lysis. Here we take an evolutionary, structural, and cell biological approach to understand Cbp1 function. We find that Cbp1 is present only in the genomes of closely related dimorphic fungal species of the Ajellomycetaceae family that lead primarily intracellular lifestyles in their mammalian hosts (Histoplasma, Paracoccidioides, and Emergomyces), but not conserved in the extracellular fungal pathogen Blastomyces dermatitidis. We determine the de novo structures of Hc H88 Cbp1 and the Paracoccidioides americana (Pb03) Cbp1, revealing a novel "binocular" fold consisting of a helical dimer arrangement wherein two helices from each monomer contribute to a four-helix bundle. In contrast to Pb03 Cbp1, we show that Emergomyces Cbp1 orthologs are unable to stimulate macrophage lysis when expressed in the Hc cbp1 mutant. Consistent with this result, we find that wild-type Emergomyces africanus yeast are able to grow within primary macrophages but are incapable of lysing them. Finally, we use subcellular fractionation of infected macrophages and indirect immunofluorescence to show that Cbp1 localizes to the macrophage cytosol during Hc infection, making this the first instance of a phagosomal human fungal pathogen directing an effector into the cytosol of the host cell. We additionally show that Cbp1 forms a complex with Yps-3, another known Hc virulence factor that accesses the cytosol. Taken together, these data imply that Cbp1 is a rapidly evolving fungal virulence factor that localizes to the cytosol to trigger host cell lysis. Author SummaryThe members of the Ajellomycetaceae fungal family are human pathogens that are responsible for a rising number of mycoses around the world. Calcium binding protein 1 (Cbp1) is a rapidly evolving virulence factor that is present in the genomes of the Ajellomycetaceae species that lead primarily intracellular lifestyles, including Histoplasma, Paracoccidioides, and Emergomyces but not Blastomyces, which remains largely extracellular during infection. Both Paracoccidioides and Histoplasma Cbp1 homologs are able to cause lysis of macrophages whereas Emergomyces homologs cannot. This result is consistent with Emergomyces africanus natural infection of macrophages, during which the yeast cells can replicate but cannot actively lyse the host cell. Despite divergence of the primary sequence of Histoplasma and Paracoccidioides Cbp1 homologs, their protein structures are remarkably similar and reveal a novel fold. During infection, Cbp1 enters the cytosol of the host macrophage, making it the first known virulence factor from an intracellular human fungal pathogen that localizes to the cytosol of the host cell. We also show that Cbp1 forms a complex with another cytosolic virulence factor, Yps-3. Taken together, these studies significantly advance our understanding of Histoplasma virulence.