INTERACTIONS OF HUMAN NUCLEAR PROTEINS P1MCM3 AND P1CDC46

Human nuclear proteins P1Mcm3 and P1Cdc46 have high sequence similarit ies with the corresponding yeast proteins known to be required for the initiation of genome replication. Nuclei of proliferating HeLa cells contain relatively high amounts of P1Mcm3 (about 10(6) molecules/nucle us) of which only a small fraction is bound to a nuclear structure, mo st probably chromatin. At 0.5 M NaCl, the structure bound nuclear prot ein can be partially solubilized as a dimer composed of P1Mcm3 and the related protein P1Cdc46. However, most protein P1Mcm3 is not bound to a nuclear structure and appears in the nucleoplasm. About 10% of prot ein P1Mcm3 in the soluble fraction is free and uncomplexed, and the re maining P1Mcm3 forms stable complexes with protein P1Cdc46. These P1Mc m3/Cdc46 complexes occur as dimers and in high-molecular-mass complexe s (approximate to 500 kDa). The high-molecular-mass complexes dissocia te in 0.5 M NaCl and release P1Mcm3/Cdc46 dimers. It has frequently be en proposed that the Mcm proteins may function as licensing factors fo r genome replication. Our data imply that the active form of an Mam pr otein is not a monomer, but a protein complex that includes an Mcm3/Cd c46 dimer. DNA polymerase a is not a component of this complex.