Dimer interface of TTR-52 (M5). (A) The interface of the TTR-52 (M5) dimer shown as a cartoon. The proposed PtdSer- and CED-1-binding sites are indicated as blue and black arrows, respectively, in the left panel. The key residues on the surface (D58, R63, W65, Q73, and T75) are shown as stick models in the right panel. The interactions between D58/R63′, R63/R63′, W65/T75′, and Q73/Q73′ are labeled (residues from monomer B are labeled with a prime). (B) Cross-linking of TTR-52 (M5) and TTR-52 (M5)-R63D/W65Q/Q73A. Cross-linker EGS was added to recombinant proteins TTR-52 (M5) and TTR-52 (M5)-R63D/W65Q/Q73A purified from E. coli. TTR-52 (M5), but not TTR-52 (M5)-R63D/W65Q/Q73A, was cross-linked in a concentration-dependent manner and appeared as a dimer-sized band on a 15% SDS-PAGE gel.