Polyphosphate/poly-(R)-3-hydroxybutyrate) ion channels in cell membranes.

Abstract

Among the most intriguing structures formed by inorganic polyphosphates (polyP) are their complexes with poly-(R)-3-hydroxybutyrates) (PHB) that form ion channels in lipid bilayers. All peptide and protein ion channels, as well as synthetic ion channels, are amphiphilic structures with an outer coat of non-polar residues and a lining of polar and charged residues (Urry 1985; Christensen et al. 1988; Nakano et al. 1990; Sansom 1991; Kobuke et al. 1992; Epand 1993; Fyles et al. 1993). These attributes are provided in a cooperative fashion by the two structurally distinct homopolymers, polyP and PHB. The polymeric anion, polyP, forms a ladder of cation binding sites that stretches across the bilayer, shielded from the hydrophobic environment by the amphiphilic solvating polyester, PHB. Complexes of polyP and PHB, located in the plasma membranes of diverse bacteria (Reusch and Sadoff 1983; Reusch et al. 1986, 1987), are the first nonproteinaceous, ion-selective channels discovered in biological cells. Their capacity to form voltage-activated, calcium-selective channels in planar lipid bilayers has been established (Reusch et al. 1995; Das et al. 1997). As yet, there is no direct evidence of their in vivo functions; however, a number of studies point to calcium involvement in important cellular functions in bacteria, such as chemotaxis (Matsushita et al. 1989; Tisa and Adler 1992) and cell division (Chang 1986; Smith 1995; Norris 1996). In addition, there is substantial evidence that polyP/PHB complexes may serve as calcium pumps and DNA channels. Here we will discuss the singular molecular characteristics of polyP and PHB that relate to their roles in ion transport, and consider how the two polymers act in synergy to form these interesting transmembrane ion channels.