(A) C-terminally 3×Flag-tagged MTBP-WT, deletion (Δ), or point mutants (m) in the MTBP-phyre2 region were transiently transfected into 293T cells before analysis by anti-Flag IP and immunoblotting using antibodies against MTBP (12H7) and Treslin/TICRR (30E7). Δphyr2, amino acids V295-T329 deleted; 5m, amino acid exchanges V306D, I309D, D313A, L314D, P315D. (B) Experiment as in (A) except lanes 1–3: C-terminally Flag-GFP-tagged MTBP was immunoprecipitated using anti-GFP nanobodies. *, different Treslin/TICRR phosphorylation forms visible in some immunoblots; point mutations: m1–6, C413A, A415Q, Q423A, I424Q, N425A, G426Q; m3–6, Q423A, I424Q, N425A, G426Q; m1/2, C413A, A415Q; m1, C413A, m2, A415Q. Cdk8, cyclin-dependent kinase 8; GFP, green fluorescent protein; IP, immunoprecipitation; m, point mutant; MTBP, Mdm2 binding protein; Sld7, Synthetic lethal with Dpb11; TICRR, TopBP1-interacting checkpoint and replication regulator; WT, wild-type.