RESUMO
Tim (Timeless) and Tipin (Tim-interacting protein) form a stable heterodimeric complex that influences checkpoint responses and replication fork progression. We report that the Tim-Tipin complex interacts with essential replication fork proteins and affects their biochemical properties. The Tim-Tipin complex, reconstituted and purified using the baculovirus expression system, interacts directly with Mcm complexes and inhibits the single-stranded DNA-dependent ATPase activities of the Mcm2-7 and Mcm4/6/7 complexes, the DNA unwinding activity of the Mcm4/6/7 complex, and the DNA unwinding and ATPase activity of Cdc45-Mcm2-7-GINS complex, the presumed replicative DNA helicase in eukaryotes. Although stable interactions between Tim-Tipin and DNA polymerases (pols) were not observed in immunoprecipitation experiments with purified proteins, Tim was shown to interact with DNA pols α, δ, and ε in cells. Furthermore, the Tim-Tipin complex significantly stimulated the pol activities of DNA pols α, δ, and ε in vitro. The effects of Tim-Tipin on the catalytic activities of the Mcm complexes and DNA pols are mediated by the Tim protein alone, and distinct regions of the Tim protein are responsible for the inhibition of Mcm complex activities and stimulation of DNA pols. These results suggest that the Tim-Tipin complex might play a role in coupling DNA unwinding and DNA synthesis by directly affecting the catalytic activities of replication fork proteins.
