ve recognition of proteases from the infected host, or secreted by competing bacterial species, through steps that are associated to a vast structural rearrangement. Results and Discussion Activated bacterial a2M highly resembles eukaryotic C3b The a2M from E. coli is a 1653-residue molecule that carries a signal peptide, a lipoprotein box immediately following this sequence, and a multi-protease recognition region Structural Studies of 
a Bacterial a2-Macroglobulin . Sequence analyses using SMART suggest the presence of multiple macroglobulin-like domains as well as a thioester-containing domain, which are hallmarks of eukaryotic proteins of the a2M superfamily, including the well-studied C3 molecule. In order to obtain the first structural information of a bacterial a2M, we expressed ECAM in its soluble form and activated it by treating with methylamine. This procedure yielded homogeneous samples of ECAM that were subsequently analyzed by negative staining electron microscopy employing sodium silico tungstate. In total, 51,700 individual particles were selected and aligned against the re-projections of a 30 A-filtered model of C3. This projection matching procedure yielded, after 50 cycles, a stable 3D model of ECAM with an estimated resolution between 15 and 20 A. Notably, this 3D model showed clear similarities to the original SB-590885 web images obtained by negative staining. In order to confirm that our 3D reconstruction was not model-biased, we performed image analysis by a reference-free classification. Comparison of the final ECAM activated 3D structure with that of C3b, filtered to 15 A, allows for the recognition of a number of key similarities, and one notable difference. Methylamine-activated ECAM is an elongated molecule with overall dimensions of 140 A680 A680 A, thus being reminiscent of the structure of C3b, whose dimensions are approximately 140 A680 A670 A. Analysis of both the raw images and the reprojections of the 3D reconstructions suggest a molecule presenting 3 to 4 main regions of density, which could represent groups of domains, and a considerable level of flexibility. The latter point is also visible in the 3D models of ECAM shown in Fig. 2B, in which the top of the ECAM structure clearly shows two individual regions of electron density. It is of note that only one of these protrusions is present in the filtered structure of C3b; it is possible that this region, which corresponds to the C345C domain of C3b’s a chain, is highly flexible in ECAM, and is positioned with two different conformations on the carbon grid, with both conformations being detected in the final structure. Attempts to individually characterize the two conformations were not successful, probably due to the relatively limited number of particles used in the 3D reconstruction. An alternative explanation to the existence of the two protrusions would be that one of them represents an additional domain present in ECAM but not in its eukaryotic counterparts; this seems unlikely, since sequence comparisons do not indicate the insertion of any large stretches of amino acids that would be required to generate a domain of this size. Negative staining electron microscopy experiments of the native form were also performed, but a stable 3D model could not be obtained, probably due to a higher flexibility than for the activated form. Thus, in order to expand our study of the conformational changes undertaken by a bacterial a-macroglobulin during activation, we studied
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