Evidence for [2Fe-2S]<sup>2+</sup> and Linear [3Fe-4S]<sup>1+</sup> Clusters in a Unique Family of Glycine/Cysteine-Rich Fe-S Proteins from <i>Megavirinae</i> Giant Viruses
Alejandro Villalta, Batoul Srour, Audrey Lartigue, Martin Clémancey, Deborah Byrne, Florence Chaspoul, Antoine Loquet, Bruno Guigliarelli, Geneviève Blondin, Chantal Abergel, Bénédicte Burlat
Abstract
We have discovered a protein with an amino acid composition exceptionally rich in glycine and cysteine residues in the giant virus mimivirus. This small 6 kDa protein is among the most abundant proteins in the icosahedral 0.75 μm viral particles; it has no predicted function but is probably essential for infection. The aerobically purified red-brownish protein overproduced in Escherichia coli contained both iron and inorganic sulfide. UV/vis, EPR, and Mössbauer studies revealed that the viral protein, coined GciS, accommodated two distinct Fe-S clusters: a diamagnetic S = 0 [2Fe-2S] 2+ cluster and a paramagnetic S = 5/2 linear [3Fe-4S] 1+ cluster, a geometry rarely stabilized in native proteins. Orthologs of mimivirus GciS were identified within all clades of Megavirinae, a Mimiviridae subfamily infecting Acanthamoeba, including the distantly related tupanviruses, and displayed the same spectroscopic features. Thus, these glycine/cysteine-rich proteins form a new family of viral Fe-S proteins sharing unique Fe-S cluster binding properties.