A potential phosphorylation site for an A-type kinase in the Efg1 regulator protein contributes to hyphal morphogenesis of Candida albicans

Efg1p in the human fungal pathogen Candida albicans is a member of the cons erved APSES class of proteins regulating morphogenetic processes in fungi. We have analyzed the importance for hyphal morphogenesis of a putative phos phorylation site for protein kinase A (PKA), threonine-206, within an Efg1p domain highly conserved among APSES proteins. Alanine substitution of T206 , but not of the adjacent T207 and T208 residues, led to a block of hypha f ormation on solid and in liquid media, while a T206E exchange caused hyperf ilamentation. The extent of the morphogenetic defect caused by the T206A mu tation depended on hypha-induction conditions. Extragenous suppression of m utations in signaling components, including tpk2 and cek1 mutations, was ac hieved by wild-type- and T206E-, but not high production levels supported p seudohyphal formation. The results are consistent with a role of Efg1p as a central downstream component of a PKA-signaling pathway including Tpk2p or other PKA isoforms. Threonine-206 of Efg1p is essential as a putative phos phorylation target to promote hyphal induction by a subset of environmental cues.