A member of the immunoglobulin superfamily in bacteriophage T4

We report a prediction that the highly immunogenic outer capsid (Hoc) protein of the prokaryotic phage T4 contains three tandem immunoglobulin-like domains. Immunoglobulin-like folds have previously been identified in prokaryotic proteins but these share no recognizable sequence similarity with eukaryotic immunoglobulin superfamily (IgSF) folds, and may represent products of convergent evolution. In contrast, the Hoc immunoglobulin-like fords are proposed, based on immunoglobulin-like sequence consensus matches detected by hidden Markov modeling. We propose that the Hoc immunoglobulin-like domains and eukaryotic immunoglobulin-like domains are likely to be related by divergence from a common ancestor.