D Pfo with regard to surface binding. Previous research conducted by Ramachandran et al. has shown that all 4 homologous domain 4 loops of Pfo interact with the lipid environment during cholesterol binding, and Soltani et al. observed that when the loop 22948146 residues of Pfo are individually mutated to aspartate, then surface binding to cholesterol containing membranes is almost completely abolished [26,46]. Our surface binding results for Ply and HCECs are unique from the 1934-21-0 findings observed for Pfo. When the domain 4 loops of Ply are mutated to glutamate, then surface binding to HCECs is unaffected as all mutants bind with the same efficiency as PlyWT. This difference from the observed findings for Pfo indicates one of three possibilities: 1) aspartate and glutamate result in two different outcomes when substituted at the loop residues, 2) Pfo and Ply have different binding behaviors which react differently to the presence of a charged polar amino acid in the loops of domain 4, or 3) the observed difference is due to the use of HCECs as the target cell. A recent study by Farrand et al. reported that the CDC cholesterol recognition motif for several CDCs including Pfo and Ply is a threonine-leucine amino acid pair found in domain 4, corresponding to T459 and L460 in Ply [34]. They found that double glycine substitutions of these residues dramatically reduced cholesterol binding on RBCs, and this threonine-leucine pair is conserved across all CDCs. Interestingly, our results indicate that when PlyL460 is substituted with glutamate, it still retains its ability to bind to the surface of HCECs at an undiminished capacity when compared to PlyWT. This binding behavior was not expected due to the previous results showing that T459 and L460 comprised the cholesterol recognition motif for Ply when exposed to cholesterol-rich liposomes. Our results indicate that it is unlikely that L460 is part of the cholesterol recognition motif of Ply when targeting HCECs, since the addition of a polar charged residue or removal of the R-group has no observed effect on surface binding to HCECs. Likewise, in addition to PlyL460E, flow cytometryrevealed that the other glutamate substitution mutants, PlyA370E, PlyA406E, and PlyW433E were also capable of binding to the surface of HCECs with no significant differences when compared to PlyWT. These results indicate that cholesterol recognition and binding by Ply is likely carried out not by a single loop structure, but rather a concerted effort between 2 or more of the loops. The oligomerization behaviors of our Ply variants also yielded some unique results when compared to other CDCs. We observed that PlyW433G was unable to form oligomeric complexes under our experimental conditions. However, a previous study that examined the oligomerization behavior of Ily found that IlyW491A, the Ily mutant corresponding to same position as PlyW433G, was able to form high molecular 58-49-1 web weight oligomeric complexes [33]. Interestingly, PlyW433F was also found to be capable of 
oligomerization indicating that W433 is likely involved in a molecular interaction required for oligomerization to occur, since a conservative substitution, tryptophan to phenylalanine, resulted in the retention of oligomerization ability. The same study by Soltani et al. observed that IlyL518D was able to oligomerize, although at a markedly reduced capacity when compared to IlyWT. Our Ply mutant with a similar mutation, PlyL460E, was unable to oligomerize at any detec.D Pfo with regard to surface binding. Previous research conducted by 
Ramachandran et al. has shown that all 4 homologous domain 4 loops of Pfo interact with the lipid environment during cholesterol binding, and Soltani et al. observed that when the loop 22948146 residues of Pfo are individually mutated to aspartate, then surface binding to cholesterol containing membranes is almost completely abolished [26,46]. Our surface binding results for Ply and HCECs are unique from the findings observed for Pfo. When the domain 4 loops of Ply are mutated to glutamate, then surface binding to HCECs is unaffected as all mutants bind with the same efficiency as PlyWT. This difference from the observed findings for Pfo indicates one of three possibilities: 1) aspartate and glutamate result in two different outcomes when substituted at the loop residues, 2) Pfo and Ply have different binding behaviors which react differently to the presence of a charged polar amino acid in the loops of domain 4, or 3) the observed difference is due to the use of HCECs as the target cell. A recent study by Farrand et al. reported that the CDC cholesterol recognition motif for several CDCs including Pfo and Ply is a threonine-leucine amino acid pair found in domain 4, corresponding to T459 and L460 in Ply [34]. They found that double glycine substitutions of these residues dramatically reduced cholesterol binding on RBCs, and this threonine-leucine pair is conserved across all CDCs. Interestingly, our results indicate that when PlyL460 is substituted with glutamate, it still retains its ability to bind to the surface of HCECs at an undiminished capacity when compared to PlyWT. This binding behavior was not expected due to the previous results showing that T459 and L460 comprised the cholesterol recognition motif for Ply when exposed to cholesterol-rich liposomes. Our results indicate that it is unlikely that L460 is part of the cholesterol recognition motif of Ply when targeting HCECs, since the addition of a polar charged residue or removal of the R-group has no observed effect on surface binding to HCECs. Likewise, in addition to PlyL460E, flow cytometryrevealed that the other glutamate substitution mutants, PlyA370E, PlyA406E, and PlyW433E were also capable of binding to the surface of HCECs with no significant differences when compared to PlyWT. These results indicate that cholesterol recognition and binding by Ply is likely carried out not by a single loop structure, but rather a concerted effort between 2 or more of the loops. The oligomerization behaviors of our Ply variants also yielded some unique results when compared to other CDCs. We observed that PlyW433G was unable to form oligomeric complexes under our experimental conditions. However, a previous study that examined the oligomerization behavior of Ily found that IlyW491A, the Ily mutant corresponding to same position as PlyW433G, was able to form high molecular weight oligomeric complexes [33]. Interestingly, PlyW433F was also found to be capable of oligomerization indicating that W433 is likely involved in a molecular interaction required for oligomerization to occur, since a conservative substitution, tryptophan to phenylalanine, resulted in the retention of oligomerization ability. The same study by Soltani et al. observed that IlyL518D was able to oligomerize, although at a markedly reduced capacity when compared to IlyWT. Our Ply mutant with a similar mutation, PlyL460E, was unable to oligomerize at any detec.
