The Replication Initiation Protein Sld2 Regulates Helicase AssemblyThis work was supported by National Science Foundation Grant 1265431 (to D. L. K.)

Assembly of the Cdc45-Mcm2-7-GINS (CMG) replicative helicase complex must be regulated to ensure that DNA unwinding is coupled with DNA synthesis. Sld2 is required for the initiation of DNA replication in budding yeast. We identified a mutant of Sld2, Sld2-m1,4, that is specifically defective in Mcm2-7 binding. When this sld2-m1,4 mutant is expressed, cells exhibit severe inhibition of DNA replication. Furthermore, the CMG complex assembles prematurely in G1 in mutant cells, but not wild-type cells. These data suggest that Sld2 binding to Mcm2-7 is essential to block the inappropriate formation of a CMG helicase complex in G1. We also study a mutant of Sld2 that is defective in binding DNA, sld2-DNA, and find that sld2-DNA cells exhibit no GINS-Mcm2-7 interaction. These data suggest that Sld2 association with DNA is required for CMG assembly in S phase. Background: Sld2 is required for the initiation of DNA replication. Results: An Sld2-Mcm2-7 binding mutant exhibits premature helicase assembly, and an Sld2-DNA binding mutant displays no helicase assembly. Conclusion: Sld2-Mcm interaction is critical to prevent the premature assembly of the replicative helicase, and Sld2-DNA interaction is required for proper helicase assembly. Significance: Sld2 regulates assembly of the replicative helicase.