ac plane view of crystal form II. fZP3-N and fZP3-C of complexes contained in adjacent asymmetric units interact head-to-tail, bringing the C- and N-terminal ends of cleaved fZP1 IDLs close to one another (red arrows). This regenerates pseudo-infinite helical egg coat filaments, only one of which is shown here, that run parallel along the c axis. Asymmetric unit boundaries are indicated by vertical dashed lines; the second of the three complexes that make up a complete helical turn, according to the parameters reported on top, is enclosed by a red rectangle and depicted as in (B). Protein subunits, colored in alternating yellow and gold (fZP3) or cyan and pale cyan (fZP1), are numbered according to their order in the filament. At the bottom, a black horizontal arrow indicates the filament polarity, with vertical black, white, and gray double arrows highlighting the alternation of homomeric ZP-C/ZP-N interfaces, heteromeric IDL/ZP(-N/C) interfaces, and heteromeric ZP-N/ZP-C interfaces, respectively.
