Putative Structure and Functions of a Poly-β -hydroxybutyrate/calcium Polyphosphate Channel in Bacterial Plasma Membranes

A poly-β -hydroxybutyrate complex extracted from the plasma membranes of genetically competent Escherichia coli contained polyhydroxybutyrate:polyphosphate:calcium in moral ratios approximating 1:1:0:5. The chain length of the polyhydroxybutyrate was estimated as 120-200 subunits, and that of the polyphosphate was estimated as 130-170 subunits. The extracted complex, when incorporated into liposomes, exhibited a lipid phase transition in the same temperature range as that of the membrane complex in whole cells as well as the same properties of irreversibility, lability, and sensitivity to chelating buffers. Space-filling molecular models and molecular energy minimization methods (Charmm) were used to develop and evaluate a plausible structure for the complex. It is proposed that the polyhydroxybutyrate forms an exolipophilic--endopolarophilic helix around an inner framework helix of calcium polyphosphate. The calcium ions link the two polymers by forming ionic bonds with phosphoryl oxygens of the polyphosphate and ion-dipole bonds with the ester carbonyl oxygens of the polyhydroxybutyrate. This symmetrical structure forms a channel through the membrane and may play a role in the transport of calcium, phosphate, and DNA.