The first viral cytochrome b5

A unicellular green alga Ostreococcus tauri is the smallest (less than 1 μm in diameter) free-living eukaryote yet described. Viruses that can infect high-light and low-light adapted strains of O. tauri have been isolated and their genomes sequenced. Interestingly, low-light-strain infecting virus (OtV-2) differ from the high-light-strain infecting viruses by encoding a potential cytochrome b₅ [1]. This protein was cloned, biochemically characterised and its three-dimensional structure resolved [2, 3].

The absorption spectra of oxidised and reduced recombinant OtV-2 haemoprotein are almost identical to those of purified human cytochrome b₅.

Absorbance spectra of purified recombinant human cytochrome b₅ and OtV-2_201.

It was also shown that the protein can substitute for yeast cytochrome b₅ in the CYP51-mediated sterol 14α-demethylation. Structurally, the viral protein is similar to other known cytochromes b₅ but lacks a hydrophobic C-terminal anchor. Thus, the first virally encoded cytochrome b₅ is also the first cytosolic cytochrome b₅ characterised. However, the physiological role of viral cytochrome b₅ remains unknown.

1. UniProt:E4WM77
2. Reid, E.L., Weynberg, K.D., Love, J., Isupov, M.N., Littlechild, J.A., Wilson, W.H., Kelly, S.L., Lamb, D.C. and Allen, M.J. (2013) Functional and structural characterisation of a viral cytochrome b₅. FEBS Letters, 587, 3633—3639.
3. PDB:4B8N

A structural representation of the OtV-2 cytochrome b₅ (OtV-2_201) protein shown as a ribbon diagram.