Bacteriophage T5 capsid is formed of 775 copies of the major head protein pb8 arranged as pentamers at the vertices and hexamers on the faces of the icosahedron. After the termination of DNA packaging into the DNA-free procapsid and expansion of the capsid shell, the outer surface is decorated with 120 copies of the monomeric 17.3 kDa protein pb10. This decoration protein is localized in the center of each hexamer of the major head protein pb8. The decoration protein exists in all T5-like phages, indicating its significance, but its function remains unknown.
Purified empty procapsids can be expanded in vitro, yielding highly stable icosahedral DNA-free shells, with the same size (940 Å in diameter) and shape as DNA-filled mature capsids (Preux et al. (2013) J Mol Biol 425:1999). Purified pb10 binds to these capsids with a very high affinity (KD ≈ 10-12 M), as demonstrated by Surface Plasmon Resonance.
The 3D structure of the two-domain protein pb10 was solved by Nuclear Magnetic Resonance. The N-terminal binding domain is composed of small alpha-helices. The C-terminal domain adopts an immunoglobulin-like fold and is displayed to the solvent. The strong affinity of pb10 for T5 capsids results from a tight network of electrostatic and hydrophobic interactions between the binding domain and the capsid shell. Thermal resistance experiments suggest a role of pb10 in the capsid stability.
Variants of pb10 modified in the C-terminal domain bind to in vitro expanded capsids with the same affinity as the native decoration protein. T5 capsids can therefore be functionalized with proteins of interest fused to pb10. Such nanoshells can be of interest in many fields, including imaging and targeting virus-like particles.