Fe–S cluster binding versatility and structural dynamics of GciS proteins from Megavirinae giant viruses

In 2003, mimivirus, initially mistaken for an intracellular parasitic bacterium, was identified as the first giant virus, with a large particle size of around 700 nm in diameter and a complex DNA genome (1.2 Mb) encoding approximately 1000 proteins¹. Mimivirus is the first representative and prototype member of the Mimiviridae family. In Megavirinae, a sub-family of Mimiviridae², we recently identified a new family of small proteins of about 60 amino acids, enriched in glycine, cysteine, and aromatic residues, with highly unusual Fe–S cluster binding properties³. This family was coined GciS, for Glycine/Cysteine-rich Iron–Sulfur proteins³. The gene encoding GciS is among the most highly transcribed at the late stage of infection, and proteomic analyses further indicate that GciS is one of the most abundant proteins within the viral particle, suggesting an important role during the early steps of host–virus interaction. Nevertheless, the function of GciS remains unknown, as this protein shares no homology with any viral or cellular protein, and its structure cannot be reliably predicted even using AI-based structural modeling approaches.
By combining biochemical, structural, and spectroscopic approaches, we demonstrated that GciS proteins possess an intrinsic ability to accommodate multiple types of Fe–S clusters, including a mixture of a [2Fe–2S]²⁺ cluster and an uncommon linear [3Fe–4S]¹⁺ cluster in the recombinant as-purified form³, as well as a [4Fe–4S]²⁺ cluster under anaerobic conditions following in vitro chemical Fe–S reconstitution. This unusual and conserved Fe–S binding versatility across all clades suggests that it is an intrinsic property of the GciS protein family and is reminiscent of Fe–S cluster carrier or chaperone proteins involved in cluster assembly and transfer. Accordingly, GciS may participate in Fe–S cluster trafficking or homeostasis of the host cell, as supported by the ability of GciS to exchange in vitro FeS clusters with model recipient Fe-S proteins (ferredoxins, mono-thiol glutaredoxins). In parallel, we are investigating the structural properties of GciS proteins, which display a tendency to oligomerize and can further assemble into β-amyloid-like fibrillar structures.