The dnaA protein of Escherichia coli. Abundance, improved purification, and membrane binding

Immunoassays of dnaA protein in extracts from five strains showed a rather constant abundance relative to cell mass, with a variation of 800-2100 molecules/cell; overproducing cells contained 100-fold that number. About half of the dnaA protein in wild type cells was solubilized by a lysis procedure. Within the insoluble fractions, dnaA protein was identified by its characteristic high-affinity binding of ATP. An improved, rapid procedure for purifying dnaA protein from overproducing cells appears to depend on its coprecipitation with phospholipids and depends on solubilization by guanidine HCl. The procedure, with a 5-fold increased yield, also eliminates a potent ATPase contaminant. Purified dnaA protein, unlike dnaB and dnaC proteins, binds to phospholipid vesicles as judged by analysis on sucrose gradient centrifugation.