How the replicative Mcm2-7 helicase is activated during replication origin firing remains largely unknown. Our biochemical and structural studies reported here, demonstrate that the helicase activator GINS interacts with TopBP1 through two separate binding surfaces, the first involving a stretch of highly conserved amino acids in the TopBP1-GINI region, the second a surface located on TopBP1-BRCT4. The two surfaces bind to opposite ends of the A domain of the GINS subunit Psf1, and their cooperation is required for a biochemically stable TopBP1-GINS interaction. The GINI and BRCT4 domains also cooperate during replication origin firing, as revealed by immuno-replacement experiments in Xenopus egg extracts using different sets of mutations in either interface. Furthermore, the TopBP1-GINS interaction is incompatible with simultaneous binding of DNA polymerase epsilon to GINS when bound to Mcm2-7-Cdc45. Our TopBP1-GINS model predicts the coordination of three molecular processes, DNA polymerase epsilon arrival, TopBP1 ejection and GINS integration into Mcm2-7-Cdc45.