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).