Structural component of the distal part of the tail. Mediates T5 binding to its host receptor, the E.coli outer membrane ferrichrome transporter fhuA protein. Upon binding to host FhuA, pb5 undergoes conformational changes that are probably the key to the transmission of the signal through the tail to the capsid, triggering DNA release. Monomer. Interacts with the host FhuA receptor; this interaction is necessary for the entry of the viral genome into the host cell, although pb5 alone is not sufficient for the conversion of FhuA into an open channel. Localizes at the tip of the central tail fiber. Only small regions of the protein may be involved in protein-protein interactions in the tail structure, the major part being used in receptor binding.