m of ECAM could correspond to a change in the position of the TED domain, which, in C3b, is located between 75 and 100 A away from its position in C3. In order “7901789 to gain further insight into this possibility, we manually fitted the structure of C3b onto the electron microscopy 3D model of methylamineactivated ECAM. This analysis reveals two important points. First, it corroborates the location the TED domain in the activated form of the bacterial protein. In addition, this analysis suggests that the C-terminal region of C3b could be fitted into two different regions of density; only one was modeled, but the other potential conformation of the C-terminus of ECAM is indicated with red arrows. Thus, both SAXS and EM techniques point to the fact that the C-terminus of ECAM is potentially solvent-exposed and flexible. In eukaryotic a2Ms, the C-terminal, receptor-binding domain is exposed when the molecules are reacted with either methylamine or proteases, thus requiring a conformational modification for solvent accessibility . This is also confirmed by the elegant docking of the structure of C3 and C3b onto electron microscopy maps of eukaryotic a2M, performed by Janssen and coworkers, as well as the recent 4.3 A crystal structure of methylamine-activated human a2M. This suggests that proteins of the a2M family share a number of overall structural similarities that include overall conformational modifications upon activation. It is of interest that inhibition of C3b by a Staphylococcal inhibitor protein occurs through the generation of an `open’ conformer of the former, which subsequently blocks formation of the C3 convertase, underlining the importance of complex conformational changes not only for C3 518303-20-3 manufacturer function but also for its targeting by pathogens. The level of circulating a2M-protease complexes in humans is low, as a consequence of the recognition of the C-terminus of a2M by lipoprotein receptors and their subsequent internalization and degradation. Thus, the C-terminal region of eukaryotic a2M plays a key role in its recognition of partner macromolecules, leading to its eventual clearance. The flexible C-terminal end of ECAM, described here, could also potentially serve as a binding region for partners. This could include PBP1c, whose gene cooccurs with that of a-macroglobulin in a number of bacterial species. PBP1c is a periplasmic molecule that is anchored to the inner membrane through a single transmembrane region. The concerted action of PBP1c and ECAM could favor protection of cell integrity in the presence of foreign proteases, potentially through the involvement of a direct interaction between the PBP and the C-terminal region of the a-macroglobulin. This could reflect a novel bacterial defense mechanism that implicates the action of both protease inhibition and cell wall biosynthesis processes. On the other hand, pathogens have also been shown to encode proteins that mimic components of the complement system in order to manipulate the host inflammatory response; thus, due to their similarity to “9357531 C3/C3b, it is conceivable that bacterial a-macroglobulins could also play yet undefined roles in the disruption of the complement amplification pathway in situations where the outer cell wall is weakened. Either one of these potential mechanisms could represent unexplored targets for the development of novel antibacterials. Materials and Methods Materials Porcine pancreatic elastase was dissolved in 0.2 M Tris-HCl pH 8.0. HisTrap HP, Superd