[2] Functional and crystallographic studies revealed that the "ZP domain" region common to all these proteins is a protein polymerization module that consists of two distinct but structurally related immunoglobulin-like domains, ZP-N and ZP-C, separated by an interdomain linker (ITD).
[3][4][5][6][7][8][9] The ZP module is located in the C-terminal portion of the extracellular region and – with the exception of non-polymeric family member ENG[10] – contains 8 or 10 conserved Cys residues involved in disulfide bonds.
[11] The first 3D structure of a homopolymeric ZP module protein filament, native human uromodulin (UMOD), was determined by cryo-EM.
[12][13] Additional copies of isolated ZP-N domains are found in the N-terminal region of egg coat protein subunits involved in fertilization in both vertebrates and invertebrates, with the human zona pellucida components ZP1, ZP2 and ZP4 being the best understood.
[4][14] The mollusc "vitelline envelope receptor for egg lysin" (VERL, Q8WR62) is found in the vitelline envelope of mollusc eggs and consists of 22 VERL repeats followed by a ZP module.