Septin localization in the dimorphic fungus Paracoccidioides brasiliensis

Septins are evolutionarily conserved proteins that contain a GTPase domain and are capable of forming filaments at the cell periphery. Septins are involved in many essential cellular processes, such as cytokinesis and cell polarization, and are used as markers of morphogenesis in several fungi. Dimorphism in fungi enables cells to switch between morphologies (yeast or filament forms), due to changes in the temperature of the environment. We analysed the localization of septin proteins in yeast and filamentous cells of the dimorphic fungus Paracoccidioides brasiliensis, a common cause of granulomatous mycosis. In order to determine septin localization, we first cloned Cdc12p, a septin homolog from P. brasiliensis, and expressed it in Escherichia coli. Following PbCdc12p purification, specific serum against PbCdc12p were raised for use in immunofluorescence assays. We observed the hourglass and ring forms of septin filaments during cell division in yeast. Septin filaments were also simultaneously localized in the necks of multiple budding cells. A distinctive pattern of punctuate and/or diffuse localization was also seen in the periphery of multinucleate yeast cells and at the tips and septa of filamentous cells. A more diffuse and punctuate pattern of localization observed in P. brasiliensis cells seems to be unique to filamentous and dimorphic fungi and may be related to their specialization in cell wall deposition, morphogenesis and cell cycle control.