NMR assignments for the telokin-like domain of bacteriophage P22 coat protein.

The bacteriophage P22 virion is assembled from identical coat protein monomers in a complex reaction that is generally conserved among tailed, double-stranded DNA bacteriophages and viruses. Many coat proteins of dsDNA viruses have structures based on the HK97 fold, but in some viruses and phages there are additional domains. In the P22 coat protein, a "telokin-like" domain was recently identified, whose structure has not yet been characterized at high-resolution. Two recently published low-resolution cryo-EM reconstructions suggest markedly different folds for the telokin-like domain that lead to alternative conclusions about its function in capsid assembly and stability. Here we report (1)H, (15)N, and (13)C NMR resonance assignments for the telokin-like domain. The secondary structure predicted from the chemical shift values obtained in this work shows significant discrepancies from both cryo-EM models but agrees better with one of the models. In particular, the functionally important "D-loop" in one model shows chemical shifts and solvent exchange protection more consistent with β-sheet structure. Our work will set the basis for a high-resolution NMR structure determination of the telokin-like domain that will help improve the cryo-EM models, and in turn lead to a better understanding of how coat protein monomers assemble into the icosahedral capsids required for virulence.