Identification and functional characterization of a novel Candida albicans gene CaMNN5 that suppresses the iron-dependent growth defect of Saccharomyces cerevisiae aft1Δ mutant

In Saccharomyces cerevisiae, the transcription factor Aft1p plays a central role in regulating many genes involved in iron acquisition and utilization. An aft1Δ mutant exhibits severely retarded growth under iron starvation. To identify the functional counterpart of AFT1 in Candida albicans, we transformed a C. albicans genomic DNA library into aft1Δ to isolate genes that could allow the mutant to grow under iron-limiting conditions. In the present paper, we describe the unexpected discovery in this screen of CaMNN5. CaMnn5p is an α-1,2-mannosyltransferease, but its growth-promoting function in iron-limiting conditions does not require this enzymatic activity. Its function is also independent of the high-affinity iron transport systems that are mediated by Ftr1p and Fth1p. We obtained evidence suggesting that CaMnn5p may function along the endocytic pathway, because it cannot promote the growth of end4Δ and vps4Δ mutants, where the endocytic pathway is blocked at an early and late step respectively. Neither can it promote the growth of a fth1Δ smf3Δ mutant, where the vacuole–cytosol iron transport is blocked. Expression of CaMNN5 in S. cerevisiae specifically enhances an endocytosis-dependent mechanism of iron uptake without increasing the uptake of Lucifer Yellow, a marker for fluid-phase endocytosis. CaMnn5p contains three putative Lys-Glu-Xaa-Xaa-Glu iron-binding sites and co-immunoprecipitates with ⁵⁵Fe. We propose that CaMnn5p promotes iron uptake and usage along the endocytosis pathway under iron-limiting conditions, a novel function that might have evolved in C. albicans.