Model of the PTX3 octamer: arrangement of protomer subunits. (a) Schematic representation of the PTX3 protomer showing the N-terminal domain in yellow, followed by the globular pentraxin domain in red. Positions of the Cys residues, the N-glycosylation site at Asn220, and the pentraxin signature motif are indicated (see text for details). (b) Disulfide bond organization of the PTX3 octamer. The N-terminal domain (yellow) is comprised of an N-terminal segment followed by three α-helices (see text). The α-helical regions of the N-terminal domains, predicted to form coiled-coil-like structures, are hypothesized to adopt two distinct structural arrangements: either an extended conformation (right) in which four protomers associate through interchain interactions (i.e., to form tetramers) or a compact organization (left) where each protomer self-associates to form an antiparallel three-helix bundle (i.e., to form dimer of dimers). Cys317 and Cys318 in the C-terminal pentraxin domains (denoted in red) link the protomers (i.e., a tetramer and two dimers) into octamers. (c) A comparison of the SAXS envelope with a schematic model for PTX3-based on the two different organizations proposed for the N-terminal domain above; the α-helical segments of the N-terminal domain are depicted as yellow rods. The C-terminal pentraxin domains are in red.
