1.A.44 The Pore-forming Tail Tip Pb2 Protein of Phage T5 (T5-Pb2) Family
The Bb2 protein of phage T5 is a 1219 aa protein. It can exist in a soluble state but exhibits a hydropathy profile revealing four peaks of hydrophobicity near the N-terminus (one low peak at residues 80-100 and three higher but sharper peaks between residues 200-300) plus a large and very hydrophobic peak at about residue position 1050. These putative α-helical TMSs may be important for insertion into the membrane and pore formation.
When phage T5 tails bind to the T5 receptor, FhuA, Bb2 becomes extremely protease sensitive, but when T5 tails are bound to FhuA integrated into liposomes, it becomes much less sensitive. This is believed to be because Bb2 inserts into the membrane (Feucht et al., 1990). Purified monomeric Bb2 is also extremely protease sensitive but less so when incorporated into lipid bilayers. Further, it forms non-selective ion pores with an average single channel conductance of 4.6 nanosiemens in 1M KCl. Thus, Bb2 forms large water-filled transmembrane channels that are believed to allow import of the phage DNA into the bacterial cell cytoplasm (Feucht et al., 1990; Poranen et al., 2003).
Multimeric Bb2 that forms the phage T5 straight fiber and is implicated in DNA transfer into the host. Bb2 consists of three domains. Region I (residues 1-1030) is mainly organized in coiled coil and shares features of tape measure proteins. Region II (residues 1030-1076) contains two alpha-helical transmembrane segments. Region III (residues 1135-1148) includes a metallopeptidase motif. A truncated version of Bb2 (Bb2-Cterm, residues 964-1148) shares common features with fusogenic membrane polypeptides (Boulanger et al. 2008). It forms oligomeric structures and inserts into liposomes, triggering fusion. It causes beta-galactosidase release from Escherichia coli cells. Binding of phage T5 to its receptor may thus trigger large conformational changes in Bb2. The coiled coil region may serve as a sensor for triggering the opening of the head-tail connector. The C-terminal region would gain access to the host envelope, permitting the local degradation of the peptidoglycan and the formation of the DNA pore by fusion of the two membranes.
The transport reactions catalyzed by Bb2 are:
(1) Ions (in) ions (out)
(2) DNA (out) → DNA (in)