Novel DNA binding properties of the Mcm10 protein from Saccharomyces cerevisiae

The Mcm10 protein is essential for chromosomal DNA replication in eukaryotic cells. We purified the Saccharomyces cerevisiae Mcm10 (ScMcm10) and characterized its DNA binding properties. Electrophoretic mobility shift assays and surface plasmon resonance analysis showed that ScMcm10 binds stably to both double strand (ds) DNA and single strand (ss) DNA. On short DNA templates of 25 or 50 bp, surface plasmon resonance analysis showed a approximately 1:1 stoichiometry of ScMcm10 to dsDNA. On longer dsDNA templates, however, multiple copies of ScMcm10 cooperated in the rapid assembly of a large, stable nucleoprotein complex. The amount of protein bound was directly proportional to the length of the DNA, with an average occupancy spacing of 21-24 bp. This tight spacing is consistent with a nucleoprotein structure in which ScMcm10 is aligned along the helical axis of the dsDNA. In contrast, the stoichiometry of ScMcm10 bound to ssDNA of 20-50 nucleotides was approximately 3:1 suggesting that interaction with ssDNA induces the assembly of a multisubunit ScMcm10 complex composed of at least three subunits. The tight packing of ScMcm10 on dsDNA and the assembly of a multisubunit complex on ssDNA suggests that, in addition to protein-DNA, protein-protein interactions may be involved in forming the nucleoprotein complex. We propose that these DNA binding properties have an important role in (i) initiation of DNA replication and (ii) formation and maintenance of a stable replication fork during the elongation phase of chromosomal DNA replication.