Stimulation of MCM helicase activity by a Cdc6 protein in the archaeon Thermoplasma acidophilum

Replicative DNA helicases are ring-shaped hexam-ers that play an essential role in chromosomal DNA replication. They unwind the two strands of the duplex DNA and provide the single-stranded (ss) DNA substrate for the polymerase. The minichro-mosome maintenance (MCM) proteins are thought to function as the replicative helicases in eukarya and archaea. The proteins of only a few archaeal organisms have been studied and revealed that although all have similar amino acid sequences and overall structures they differ in their biochemi-cal properties. In this report the biochemical properties of the MCM protein from the archaeon Thermoplasma acidophilum is described. The enzyme has weak helicase activity on a substrate containing only a 30-ssDNA overhang region and the protein requires a forked DNA structure for efficient helicase activity. It was also found that the helicase activity is stimulated by one of the two T.acidophilum Cdc6 homologues. This is an inter-esting observation as it is in sharp contrast to observations made with MCM and Cdc6 homo-logues from other archaea in which the helicase activity is inhibited when bound to Cdc6