1.D.6 The Complexed Poly 3-Hydroxybutyrate Ca2+ Channel (cPHB-CC) Family
Low molecular weight poly 3-hydroxybutyrate (PHB) (<15,000 Da) can complex with Ca2+ and polyphosphate to form Ca2+ channels in the bacterial cytoplasmic membrane. These structures have been proposed to facilitate DNA import after Ca2+ treatment to render bacterial cells artificially competent for DNA uptake (transformation). Genetic competence of E. coli by Ca2+ treatment correlates with the cPHB content of the cytoplasmic membrane. The molar ratio of PHB to polyphosphate to Ca2+ was reported to be 1:1:0.5. These isolated PHB complexes formed Ca2+ channels when introduced into liposomes and voltage-activated Ca2+ channels when present in lipid bilayers. Similar Ca2+ channels can be reconstituted from Ca2+, polyphosphate and synthetically prepared (R)-3-hydroxybutyrate oligomers when present. A model for the structure of the channel has been proposed in which an outer ring of PHB and an inner core of polyphosphate complex Ca2+. In this model, the methyl side chains of the PHB helix face outwardly to interact with the phospholipid side chains, and the carboxyl groups face inwardly to ligand the Ca2+ (Madison and Huisman, 1999). PHB and polyphosphate have been shown to associate with the KcsA K+ channel of Streptomyces lividans (see 1.A.1.1.1) (30 PHBs and 15 polyPs per KcsA tetramer). The KcsA tetramer, PHB and polyP have thus been suggested to cooperate in the selective transport of K+ (Reusch, 1999).