FIGURE 3.

Structure and organization of the translocation domain in bacteriocins. A, surface representation of Pst shown in sand color with the T domain shown in schematic form as partially random coil in magenta with the N terminus (NT) marked. B, for comparison, the surface representation of colicin M (Cma) in the same schematic drawing as Pst showing the T domain wrapped around the R domain. C, model of the translocation of Pst through the FyuA receptor (the structure of FhuA, PDB code 1BY3, was taken as model for FyuA due to the similarity). The distance of the entire membrane-spanning protein is ∼9 nm, although the diameter of the membrane spanning part is 5–6 nm. Assuming the T domains of Pst are extended by an artificial force, the terminus could be elongated to ∼12 nm. If this terminus is reduced by five residues (∼1.5 nm), it would be sufficient to span the membrane and to attach to the periplasmic domain of TonB. A further reduction in length of this domain by another 10 residues (∼3 nm) would presumably not allow binding of the bacteriocin to TonB and thereby abolish uptake and in vivo activity. The model illustrates the effect of truncation of the T domain on Pst uptake. A schematic representation of the Pst constructs used to determine the importance of the T domain in uptake and activity is given on the right side. AA, amino acids; TBB, TonB box.