FIGURE 8.
Model for oligomeric complex assembly. Our data indicate that membrane-bound PFO monomers initially collide and form weak, reversible interactions (A), similar to what we observed for PFOW165T. Monomers collide many times, and a fraction of these collisions result in the formation of a stable dimer if the β4-β5 interaction of monomer 1 happens to be in the open conformation (B), as a result of normal random thermal fluctuations in the D3 structure. β4 of the core β-sheet of monomer 1 can then form a stable interaction with β1 of the core β-sheet of a second monomer (C). Once this interaction is formed its reversal is less likely because β1 is capable of forming eight backbone hydrogen bonds with β4 whereas only four are formed between β4 and β5 in the monomer (11). Also, Tyr-181 in β1 and Phe-318 in β4 form a π-stacking interaction, which further stabilizes this interaction and results in a SDS-stable prepore complex (7, 13). The formation of the stable β1-β4 interaction induces structural changes in D3 that also disrupt the hydrogen bonds between the β4-β5 pair of monomer 2 (D), similar to that observed when PFOPPL is incubated with PFOW165T (Fig. 6B). β4 on the open interface of the dimer pair (D) is then free to interact with β1 of another monomer. The D3 structural changes are propagated to each successive monomer as it is added, thus exposing β4 on the growing end of the oligomer. Because β4 is already exposed in the elongating oligomer the interaction of β4 with the β1 of each new monomer is not subject to the same energy barrier that exists during the formation of a stable dimer (B). Therefore, the addition of free monomers to established oligomers is preferred over the initiation of new oligomers.