Phage_tail_NK is the globular tip protein of some tailed bacteriophages. Tailed bacteriophage virions deliver DNA to susceptible cells after adsorbing to specific receptors on the surface of the bacteria. In the Gram-negative bacteria these receptors are surface proteins or polysaccharides. In the phage Sf6-type needle, this distal tip folds into a knob with a TNF-like fold, similar to the fibre knobs of bacteriophage PRD1 and Adenovirus. It contains three bound L-glutamate molecules that are bind tightly in the crevices between the trimers of this trimeric tip [1,2].

1: Atomic structure of bacteriophage Sf6 tail needle knob. Bhardwaj A, Molineux IJ, Casjens SR, Cingolani G; J Biol Chem. 2011;286:30867-30877. PMID:21705802

2: The tip of the tail needle affects the rate of DNA delivery by bacteriophage P22. Leavitt JC, Gogokhia L, Gilcrease EB, Bhardwaj A, Cingolani G, Casjens SR; PLoS One. 2013;8:e70936. PMID:23951045