The Cdc6 protein is required to load a complex of Mcm2-7 family members (th
e MCM complex) into prereplicative complexes at budding yeast origins of DN
A replication. Cdc6p is a member of the AAA(+) superfamily of proteins, whi
ch includes the prokaryotic and eukaryotic clamp loading proteins. These pr
oteins share a number of conserved regions of homology and a common three-d
imensional architecture. Two of the conserved sequence motifs are the Walke
r A and B motifs that are involved in nucleotide metabolism and are essenti
al for Cdc6p function in vivo. Here, we analyse mutants in the other conser
ved sequence motifs. Several of these mutants are temperature-sensitive for
growth and are unable to recruit the MCM complex to chromatin at the restr
ictive temperature. In one such temperature-sensitive mutant, a highly cons
erved asparagine residue in the sensor I motif was changed to alanine. Over
expression of this mutant protein is lethal. This phenotype is very similar
to the phenotype previously described for a mutation in the Walker B motif
, suggesting a common role for sensor I and the Walker B motif in Cdc6 func
tion. (C) 2001 Academic Press.