A PCBP1–BolA2 chaperone complex delivers iron for cytosolic [2Fe–2S] cluster assembly

Abstract

Hundreds of cellular proteins require iron (Fe) cofactors for activity, and cells express systems for their assembly and distribution. Molecular details of the cytosolic iron pool used for iron cofactors are lacking, but iron chaperones of the poly rC-binding protein (PCBP) family play a key role in ferrous ion distribution. Here we show that, in cells and in vitro , PCBP1 coordinates iron via conserved cysteine and glutamate residues and a molecule of non-covalently bound glutathione (GSH). Proteomics analysis of PCBP1-interacting proteins identified BolA2, which functions, in complex with Glrx3, as a cytosolic [2Fe–2S] cluster chaperone. The Fe–GSH-bound form of PCBP1 complexes with cytosolic BolA2 via a bridging Fe ligand. Biochemical analysis of PCBP1 and BolA2, in cells and in vitro , indicates that PCBP1–Fe–GSH–BolA2 serves as an intermediate complex required for the assembly of [2Fe–2S] clusters on BolA2–Glrx3, thereby linking the ferrous iron and Fe–S distribution systems in cells.

The Role of GSH in Intracellular Iron Trafficking

“…It has been suggested that the labile iron pool is directed to some intracellular sites via protein chaperones [ 53 , 54 ], but a related possibility is direction via low molecular weight ternary glutathione complexes. Both mechanisms will be considered below.…”

“…Poly r C-binding protein 1 (PCBP1) binds iron and has been reported to deliver iron to specific proteins, for instance ferritin [ 53 ] and the cytosolic [2Fe-2S] cluster assembly [ 54 ]. In the latter interaction, iron is presented to PCBP1 as iron(II)glutathione, forming a ternary complex.…”

The battle for iron in enteric infections

“…The moonlighting RNA-binding proteins PCBP1 and PCBP2 have been proposed to play a role in chaperoning cytoplasmic iron. 31,32 Labile iron can then be: (i) placed into cytosolic iron-containing proteins; (ii) transported to the mitochondria and incorporated into haem and Fe-S clusters; 2,33 (iii) sequestered inside ferritin (Ft); or (iv) exported to the extracellular space by FPN. 30 Nanocages of Ft, formed from 24 light (FtL) and heavy (FtH) subunits, provide a long-term storage site for iron, shielding the cell from potential redox activity of free iron.…”

“…Free iron in the cytosol might partake in dangerous biochemical reactions; however, the factors that control trafficking of the cellular labile iron pool are only beginning to be understood. The moonlighting RNA‐binding proteins PCBP1 and PCBP2 have been proposed to play a role in chaperoning cytoplasmic iron 31,32 . Labile iron can then be: (i) placed into cytosolic iron‐containing proteins; (ii) transported to the mitochondria and incorporated into haem and Fe–S clusters; 2,33 (iii) sequestered inside ferritin (Ft); or (iv) exported to the extracellular space by FPN 30 .…”

PCBP2 post‐transcriptionally regulates sortilin expression by binding to a C‐rich element in its 3′ UTR

“…A recent study demonstrated that Pcbp1-deficient mouse embryos die at the peri-implantation stage [7], indicating an essential role of PCBP1 in mouse embryonic development. PCBPs are also important for iron metabolism, acting as iron chaperones [8][9][10][11][12][13][14]. Moreover, PCBP1 mediates proinflammatory cytokine production via stabilization of mRNA in iron-promoted CD4 + T-cell pathogenicity [15].…”