Structures of representative class P Drosophila XC domains mainly involved in protein-protein interactions. (a)DmelToll receptor extracellular region (4lxr) with LRR(g)(green), LRRNT (blue), and LRRCT (red) domains. The Spatzle ligands are shown in yellow and cyan. (b) A Ldl_recept_bbeta-propeller domain (3sov, A20-A281) with the sidechains of the YWTD motif in the third blade shown. (c) AnActivin_recp domain (2h62, C34-C117) belonging to the uPAR_Ly6_toxin(g) XC domain. (d) A tetraspanin XC domain(1g8q, A113-A202). (e) A CUB(g) domain (3kq4, B932-B1044). (f) the XC domain (ASC) of an EnaC channel (2qts, A72-A423). (g) a CAP domain (4ifa, A212-A344). Disulfide bonds are shown in magenta.