123 Mini-chromosome maintenance complexes form a filament to remodel DNA structure and topology

Abstract

Deregulation of mini-chromosome maintenance (MCM) proteins are associated with genomic instability and cellular abnormality. MCM complexes are recruited to replication origins for S phase genome duplication. Paradoxically, MCM proteins are expressed in large number of origins and are associated with unreplicated chromatin regions away from the origins during G1 and S phases. We report an unusually wide left-handed filament structure for an archeal MCM, as determined by X-ray and electron microscopy. The crystal structure reveals that an α-helix bundle formed between two neighboring subunits plays a critical role in filament formation which we show through mutation and electron microscopy. The filament interior has a remarkably strong electro-positive surface spiraling along the inner filament channel which we establish as the binding site for double-stranded DNA. We find that MCM filament binding to DNA causes dramatic topological changes which negatively supercoil circular DNA through inducing loosening of the double helix. This newly identified biochemical activity of MCM may imply a wider functional role for MCM in DNA metabolism beyond helicase function, for the non-origin bound MCMs. Finally, using yeast genetics, we show that the inter–subunit interactions important for MCM filament formation play a role for cell growth and survival.