The PS1 hairpin of Mcm3 is essential for viability and for DNA unwinding in vitro

Area of Science:

• Molecular Biology
• Biochemistry
• Genetics

Background:

• The pre-sensor 1 (PS1) hairpin is a motif found in AAA+ ATPases, crucial for DNA translocation in archaeal MCM and viral helicases.
• The eukaryotic replicative helicase Mcm2-7 is essential for DNA replication and is composed of AAA+ proteins.

Purpose of the Study:

• To investigate the role of the PS1 hairpin in the function of the eukaryotic Mcm2-7 replicative helicase.
• To determine if the PS1 hairpin is essential for Mcm2-7 subunit function and overall viability.

Main Methods:

• Site-directed mutagenesis of the conserved lysine residue in the PS1 hairpin motif of each Saccharomyces cerevisiae Mcm2-7 subunit to alanine.
• Assessing the viability and growth phenotypes of mutated strains.
• Reconstitution of Mcm2-7 complexes with mutated subunits.
• Enzymatic assays (helicase and ATPase activity) and electrophoretic mobility shift assays (EMSA) to evaluate DNA binding and complex stability.

Main Results:

• Mutation of the PS1 hairpin in Mcm3 (Mcm3(K499A)) was essential for viability, causing severely decreased helicase activity.
• Mutations in other Mcm subunits showed minimal phenotypes, except for Mcm7 which exhibited slow growth.
• Viable mutant alleles were synthetically lethal with each other, indicating functional interdependence.
• Mcm2-7(3K499A) showed wild-type ATPase activity but subtle alterations in single-stranded DNA binding and cellular associations.

Conclusions:

• The PS1 hairpin of Mcm3 is essential for the helicase activity and viability of Saccharomyces cerevisiae.
• PS1 hairpins in Mcm2-7 subunits have distinct and important functions, particularly Mcm3's role in DNA unwinding.
• The findings provide insights into the specific contributions of PS1 hairpins to the complex mechanism of eukaryotic DNA replication.