Prt/ZP predicted binding. (A) Prt/ZP2 model; (B) Prt/ZP3 model; (C) Prt/ZP4 model. ZPs and Prt are shown respectively in surface and cartoon representations. Residues 36–150 of bovine ZP2 are colored in orange. ZP2 (36–635), ZP3 (23–346) and ZP4 (18–461) chains, are colored in light green. Polar contacts are colored in red (corresponding ZP aminoacids are indicated with one-letter code). N-terminal signal peptide, and C-terminal propeptide (both removed in mature form), are colored in light gray. Ovine Prt model (cartoon) is colored in blue. Based on UniProtKB/Swiss-Prot sequences [Prt (A4ULE2); ZP2 (Q9BH10); ZP3(P48830); ZP4(Q9BH11)].