Three Cell Wall Mannoproteins Facilitate the Uptake of Iron in Saccharomyces cerevisiae

Abstract

Analysis of iron-regulated gene expression in Saccharomyces cerevisiae using cDNA microarrays has identified three putative cell wall proteins that are directly regulated by Aft1p, the major iron-dependent transcription factor in yeast. FIT1, FIT2, and FIT3 (for facilitator of iron transport) were more highly expressed in strains grown in low concentrations of iron and in strains in which AFT1-1 up , a constitutively active allele of AFT1, was expressed. Northern blot analysis confirmed that FIT1, FIT2, and FIT3 mRNA transcript levels were increased 60 -230-fold in response to iron deprivation in an Aft1p-dependent manner. Fit1p was localized exclusively to the cell wall by indirect immunofluorescence. Deletion of the FIT genes, individually or in combination, resulted in diminished uptake of iron bound to the siderophores ferrioxamine B and ferrichrome, without diminishing the uptake of ferric iron salts, or the siderophores triacetylfusarinine C and enterobactin. FITdeletion strains exhibited increased expression of Aft1p target genes as measured by a FET3-lacZ reporter gene or by Arn1p Western blotting, indicating that cells respond to the absence of FIT genes by up-regulating systems of iron uptake. Aft1p activation in FIT-deleted strains occurred when either ferrichrome or ferric salts were used as sources of iron during growth, suggesting that the FIT genes enhance uptake of iron from both sources. Enzymatic digestion of the cell wall resulted in the release of significant amounts of iron from cells, and the relative quantity of iron released was reduced in FIT-deletion strains. Fit1p, Fit2p, and Fit3p may function by increasing the amount of iron associated with the cell wall and periplasmic space.

Aft1p and Aft2p Mediate Iron-responsive Gene Expression in Yeast through Related Promoter Elements

“…These include individual genes that are activated by the AFT1-1 up allele, genes within the iron regulon that are activated indirectly in response to cobalt stress and genes that are activated in a strain that lacks a functional Yfh1p, a protein that is involved in mitochondrial iron-sulfur cluster formation (10,12,13,14,25,26). There are, however, differences between these data sets.…”

“…These include genes that encode siderophore transporters (ARN1-4), iron reductases (FRE1-6), iron permeases (FTR1, FET4), and multicopper oxidases that are involved in the coordinated oxidation and transport of iron across membranes (FET3, FET5) (8 -12). In addition, three related genes (FIT1-FIT3) encode proteins that localize within the yeast cell wall and whose function is partially related to siderophore uptake (13).…”

A Specific Role of the Yeast Mitochondrial Carriers Mrs3/4p in Mitochondrial Iron Acquisition under Iron-limiting Conditions

“…The transition metals occurring at much lower concentrations were analyzed using an elemental mass spectrometer (ICP-MS VG Elemental Plasmaquad 2 Turboϩ STE, Thermo, Winsford, UK). The following isotopes were used for quantification: 26 Mg, 55 Mn, 57 Fe, 59 Co, 60 Ni, 63 Cu, 65 Cu, 64 Zn, and 66 Zn. Typical concentrations of the elements in solution were low to mid ng/ml for the metals and g/ml for phosphorus.…”

Iron restriction-induced adaptations in the wall proteome of Candida albicans

“…Saccharomyces cerevisiae cannot utilize haemoglobin-bound iron for growth (Weissman & Kornitzer, 2004;Weissman et al, 2002), thus excluding a role for ScCcw14 in this process and making a role for its homologue in C. albicans (CaSsr1) in this process also less likely. Interestingly, the wall of Saccharomyces cerevisiae contains three GPI-modified proteins (Fit1, Fit2 and Fit3) that are involved in siderophore binding, hence facilitating iron uptake in a different way consistent with its different natural environment (Protchenko et al, 2001).…”