Promotion of Ub chain synthesis by TRAF6 RING hybrid dimer. (A) The construct used for generating TRAF6 wild-type/L74K hybrid. L74K is a previously identified Ubc13-binding deficient mutant of TRAF6 (6). (B) Promotion of polyubiquitin chain synthesis by wild-type, hybrid, and L74K TRAF6 RING dimers in the presence of E1, the E2 complex Ubc13/Uev1A, and ubiquitin. (C) Promotion of polyubiquitin chain synthesis by wild-type, hybrids of R125E/L74K, S129A/L74K, and E144A/L74K TRAF6 RING dimers in the presence of E1, the E2 complex Ubc13/Uev1A, and ubiquitin.