Tail Tip Proteins Related to Bacteriophage λ gpL Coordinate an Iron-Sulfur Cluster

The assembly of long non-contractile phage tails begins with the formation of the tail tip complex. Tail tip complexes are multi-functional protein structures that mediate host cell adsorption and genome injection. The tail tip complex of phage λ is assembled from multiple copies of eight different proteins, including gpL. Purified preparations of gpL and several homologues all displayed a distinct reddish colour, suggesting the binding of iron by these proteins. Further characterization the gpL homologue from phage N15, which was most amenable to in vitro analyses, showed that it contains two domains. The C-terminal domain was demonstrated to coordinate an iron-sulphur cluster, providing the first example of a viral structural protein binding to this type of metal group. We characterized the iron-sulphur cluster using inductively coupled plasma-atomic emission spectroscopy, absorbance spectroscopy, and electron paramagnetic resonance spectroscopy and found that it is an oxygen-sensitive [4Fe-4S]2+ cluster. Four highly conserved cysteine residues were shown to be required for coordinating the iron-sulphur cluster, and substitution of any of these Cys residues with Ser or Ala within the context of λ gpL abolished biological activity. These data imply that the intact iron-sulphur cluster is required for function. The presence of four conserved Cys residues in the C-terminal regions of very diverse gpL homologues suggest that utilization of an iron-sulphur cluster is a widespread feature of non-contractile tailed phages that infect Gram-negative bacteria. In addition, this is the first example of a viral structural protein that binds an iron-sulphur cluster.