Membrane association of active plasmid partitioning protein A in Escherichia coli

QsopA and SopA, proteins essential for stable maintenance of low copy number plasmids and encoded on plasmid QpH1 of Coxiella burnetii and the F plasmid of Escherichia coli, respectively, are shown to be membrane associated using three independent approaches: isolation of hybrid protein A-PhoA proteins that display PhoA (bacterial alkaline phosphatase) activity indicating a periplasmic location, biochemical fractionation by flotation gradient centrifugation, and subcellular localization by immunoelectron microscopy. These data provide insight into the mechanism by which partitioning protein A spatially directs plasmids into daughter cells at bacterial division.